Hot water concrete tank to store solar generated …...Mohamed Attia Mohamed Abd Elrahman aus...

HOT WATER CONCRETE TANK TO STORE

SOLAR GENERATED ENERGY

vorgelegt von

Master of Science ‐ M.Sc.

Mohamed Attia Mohamed Abd Elrahman

aus Ägypten

von der Fakultät VI – Planen Bauen Umwelt

der Technischen Universität Berlin

Institut für Bauingenieurwesen

zur Erlangung des akademischen Grades

Doktor der Ingenieurwissenschaften

Dr.‐Ing.

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr.‐Ing. Frank U. Vogdt Gutachter: Prof. Dr.‐Ing. Bernd Hillemeier Gutachter: Prof. Dr. rer. nat. Dietmar Stephan Gutachter: Prof. Dr.‐Ing. Alexander Taffe

Tag der wissenschaftlichen Aussprache: 31. März 2014

Berlin 2014

D 83

Acknowledgments iii

Acknowledgements

I would like to express my deepest appreciation to all those who provided me with the possibility to

complete this work with their suggestions, encouragement and advices. First of all, I would like to

express my heartfelt thanks to Prof. Dr.‐Ing. Bernd Hillemeier for his help, valuable time,

encouragement and guidance during all stages of my research work. His outstanding personality as well

as his enthusiasm support, advice, detailed comments and innovative views contributed very much both

to the quality of the research work and to the improvement of my professional skills. I am sincerely

grateful to Prof. Hillemeier, for the confidence he gave me during this study and for the time he has

spent reading and careful review of the manuscript of this thesis. Without his personal interest in the

research work and his great support to overcome all obstacles, this investigation would have not been

materialized.

I would like to express my sincere acknowledgement for the support and help from Prof. Dr. rer.

Nat. Dietmar Stephan, who was abundantly helpful and offered invaluable assistance and provided

me with excellent informational resources and laboratory facilities. I want to thank him for giving

me the chance and space to complete and finish this thesis in his department. I also want to express

him my appreciation for the valuable discussions, beneficial comments and continuous support

during my research.

I am indebted to all the staff and PhD students of the department of building materials and

construction chemistry at TU Berlin for their help and cooperation in my research. It is my pleasure,

however, to mention some of my colleagues: Sameena Kamaruddin, Veronika Märkl and Kai Foth for

their help. I would like to address special thanks to my roommate, Kasra Shafiei, for his support,

advice and for his friendly relationship. Moreover, I wish to express my appreciation to Dr. Roland

Herr for his valuable comments and advices regarding C‐S‐H phase’s transformation. Additionally, I

would like to convey thanks to Dr. Ralf Röben from KIWA for his kindness, friendship and support during

the measurement of porosity and chloride diffusion of concrete.

I also extend my gratitude to German Academic Exchange Service (DAAD) and Egyptian government

for providing financial support for the project during the period of this research. Furthermore, I wish

also to express my thanks for all the great opportunities and professional development I have had at

Berding Beton Firma in Linthe, especially from Mr. Axel Munke and Mrs. Angela Mehlhase for

supporting our experiments.

Finally, I wish to avail myself of this opportunity, and express a never ending sense of gratitude to

my family, my parents, wife, and sons (Omar, Amal and Alaa) for their blessings, understanding,

endless patience, and encouragement at all times.

Acknowledgments iv

Abstract v

Abstract

The solar energy flux reaching the earth equals about 6000 times the current global energy

consumption. Although solar energy is abundant, clean and safe, its supply is intermittent and

irregular. Therefore, a considerable amount of energy should be stored during the day to cover the

demand during the night or from summer to winter times. Hot water storage tank is considered one

of the best technologies for seasonal energy storage because of the high specific heat of water and

the high capacity rate of charging and discharging. Most of these tanks are built of prestressed

concrete with internal lining to prevent the water leakage. The economic studies indicated that the

costs of liner are very high. Moreover, the storing temperature is low (up to 95 °C) which limits

the storing capacity and therefore, the stored energy is used for heating and hot water supply

only. However, to make a practical use of the stored energy such as industrial processes and

steam generation, the storing temperature must be increased above 100 °C such as in the case of

steam accumulator.

The main objective of this work is to study the possibility of optimizing a concrete mixture to be

used in a hot water concrete tank to store solar generated energy at temperatures up to 200 ° C and

pressures up to 15.5 bars. This investigation is divided into three main parts. In the first one, high

density concrete mixtures have been optimized in order to prevent the leakage of water and vapour

through concrete and to ensure high sustainability under hydrothermal conditions. Increasing the

packing density is considered an excellent strategy to get an optimized concrete mix. In this concern,

the Ideal grading curve according to Fuller has been used to achieve the maximum packing density

of the solid materials and to reduce the required binder content. In addition, various cementitious

materials were examined to select the most suitable mixture with high resistance to hydrothermal

attack. Siliceous fillers have been added also to close the gaps between aggregate and fine materials

on one hand and to enhance the stability of C‐S‐H phases under hydrothermal conditions on the

other hand. The second part of this investigation focuses on the effect of autoclaving with 200 °C

and 15.5 bars on concrete properties. Mechanical properties including compressive strength, tensile

strength and rebound number have been measured at normal conditions and after autoclaving with

numerous cycles. In addition, concrete porosity is measured before and after autoclaving for several

cycles. Regarding concrete durability, the main three mechanisms of ingress of gases and liquids

through concrete; permeability, absorption and diffusion, have been measured before and after

autoclaving for several cycles. In the third part, 10 different cement pastes have been prepared and

tested in order to deeply understand the effect of hydrothermal conditions on characteristics and

stability of C‐S‐H phases. In this concern, EDX, TGA and SEM measurements have been used to study

the changes in pastes morphology and properties due to autoclaving for 50 cycles. In addition, the

changes in compressive strength and porosity after hydrothermal treatment have been determined.

Abstract vi

The investigation results showed that the optimized mixes exhibited very low porosity and high

mechanical properties as well as high durability at normal conditions where the grading and particle

size distribution are the main factors influencing the results. However after hydrothermal exposure,

mixes with low C/S ratio showed very stable performance regarding mechanical properties and

durability. The addition of silica‐rich materials such as fly ash, slag and quartz reduces the C/S ratio

of the system and consequently strong product with low porosity and high stability is produced. On

the other hand, mixes with OPC and with high C/S ratio suffered poor durability, high porosity as

well as strength retrogression.

Kurzfassung vii

Kurzfassung

Die auf die Erde treffende Strahlungsenergie der Sonne übertrifft den heutigen

Gesamtenergieverbrauch um die 6000 Fächer. Obwohl die solare Energie unbegrenzt zur Verfügung

steht und sauber und sicher ist, ist ihre Nutzung schwankend und häufig ungewiss. Deshalb sollte

eine beträchtliche Energiemenge tagsüber gespeichert werden, um entsprechend nachts zur

Verfügung zu stehen. Die Speicherung von Energie in heißem Wasser wird als eine der effektivsten

Technologien gesehen, um saisonal Energie zu speichern, weil die spezifische Wärme von Wasser

hoch ist und hohe Beladungs‐ und Entladungsraten erreicht werden können. Um Wasserverluste zu

vermeiden, werden die meisten Speichertanks aus Spannbeton mit einem Inliner hergestellt.

Wirtschaftlichkeitsstudien zeigen, dass die Kosten für den Liner hoch sind. Zudem sind die

Speichertemperaturen verhältnismäßig niedrig (bis zu 95 °C), was die Speicherkapazität wesentlich

einschränkt. Um jedoch die gespeicherte Energie z.B. industriell und bei der Dampferzeugung zu

nutzen, muss die Speichertemperatur deutlich über 100 °C angehoben werden.

Das vorrangige Ziel der vorliegenden Arbeit ist die theoretische und technische Erforschung eines

speziellen Betons für einen Heißwassertank, um solare Energie zu speichern bei Temperaturen bis

zu 200 °C und Drücken bis zu 15.5 bar. Die Forschungsarbeit ist in drei Abschnitte unterteilt.

Zunächst wird ein hochdichter Beton entwickelt, der unter den geschilderten hydrothermalen

Bedingungen zuverlässig wasser‐ und dampfdicht ist. Als spezielle strategische Maßnahme erwies

sich dabei die Erhöhung der Packungsdichte der festen Bestandteile des Betons als besonders

wirksam. Bezüglich dieser Forderung wurde die Idealsieblinie nach Fuller zum Erreichen einer

extrem hohen Packungsdichte und damit zur Reduzierung des Zementleimgehalts angestrebt.

Zusätzlich wurden verschiedene Bindemittelkombinationen untersucht, um die geeignetste

Mischung mit der höchsten Widerstandsfähigkeit gegenüber hydrothermalen Einwirkungen zu

finden. Füller auf Siliziumbasis wurden dem Beton hinzugefügt, um die Lücke innerhalb des

Kornaufbaus zwischen den groben und feinen Materialanteilen zu schießen und die Stabilität der C‐

S‐H Phasen unter hydrothermalen Bedingungen zu erhöhen. Der zweite Teil der Arbeit geht auf die

Analyse der Betoneigenschaften ein, die sich während der Belastung im Autoklaven bei 200 °C und

15.5 bar einstellen. Die mechanischen Eigenschaften wie Druckfestigkeit, Zugfestigkeit und

Rückprallwert wurden bei Normalbedingungen und nach dem Autoklavieren nach verschieden

hohen Zyklen zahlen gemessen. Entsprechend wurden bezüglich der Dauerhaftigkeit des Betons die

drei wesentlichen Mechanismen des Eindringens von Gasen und Flüssigkeiten wie Permeabilität,

Absorption und Diffusion messtechnisch überprüft. In dem dritten Teil der Arbeit wurden 10

verschiedene Zementleime untersucht, um unabhängig von den Einflüssen einer Gesteinskörnung

den Effekt der hydrothermalen Behandlung auf die Charakteristik und die Stabilität der C‐S‐H

Phasen zu erforschen. Dabei wurden die Verfahren EDX, TGA und SEM angewandt, um die

Kurzfassung viii

morphologischen Veränderungen bei den Zementsteinen nach 50 Autoklav Zyklen festzustellen.

Zusätzlich wurden die Veränderungen der Druckfestigkeit und der Porosität gemessen.

Die Versuchsergebnisse zeigen, dass die optimierten Mischungen sich günstig durch eine sehr

niedrige Porosität und hohe mechanische Eigenschaften für die Dauerhaftigkeit auszeichnen. Die

optimierte Kornabstufung wird dabei als der wesentliche Faktor angesehen. Nach der

hydrothermalen Behandlung wird zusätzlich das Calcium/Silizium (C/S) Verhältnis des Systems als

wesentlicher Einflussfaktor auf das Verhalten und die Eigenschaften eines Betons erkannt. Die

Zugabe siliziumreicher Stoffe wie Flugasche, Hüttensand oder Quarzsand reduzieren das C/S

Verhältnis des Systems und führen konsequent zu einem hoch widerstandsfähigen Beton mit

niedriger Porosität bei hoher Zuverlässigkeit. Sie übertreffen in allen ihren Eigenschaften jene

Mischungen deutlich, die ausschließlich aus Portlandzement als Bindemittel hergestellt wurden.

Table of contents ix

Table of contents

Acknowledgements …………………………………………………………………………………….……......….iii

Abstract …………………………………………………………………………………………………………..….……..v

Kurzfassung ……………………………………………………………………………………………………………...vii

Notations and symbols ………………………………………………………………………………….….………xv

1. Introduction ................................................................................................................ 1

1.1 General ..................................................................................................................... 1

1.2 Research Objectives ................................................................................................. 3

1.3 Research strategy ..................................................................................................... 4

1.4 Outline ...................................................................................................................... 4

2. Literature review ......................................................................................................... 9

2.1 General ..................................................................................................................... 9

2.2 Solar energy ............................................................................................................. 9

2.2.1 Introduction ................................................................................................. 9

2.2.2 Concentration of solar power (CSP) .......................................................... 11

2.2.3 Solar Energy storage .................................................................................. 11

2.2.4 Some applications of sensible heat storage systems ................................ 12

2.2.4.1 Central seasonal heat storage .................................................... 12

2.2.4.2 Hot water tank ............................................................................ 13

2.2.4.3 Steam accumulator ...................................................................... 14

2.3 Packing density as a key for concrete mix design .................................................. 17

2.3.1 Introduction ............................................................................................... 17

2.3.2 History of packing density theory .............................................................. 18

2.3.3 The work of Fuller ...................................................................................... 20

2.3.4 State of the art of particle packing modeling ............................................ 23

2.4 Effect of heat on concrete properties .................................................................... 25

2.4.1 Introduction ............................................................................................... 25

2.4.2 Effect of high temperature on properties of cement based materials ..... 26

2.4.3 Explosive thermal spalling ......................................................................... 29

2.4.4 Effect of autoclaving on concrete properties ............................................ 30

Table of contents x

2.4.5 Stability of C‐S‐H phases at hydrothermal conditions ............................... 32

3. Materials and methods ............................................................................................. 39

3.1 Introduction ........................................................................................................... 39

3.2 Materials ................................................................................................................ 39

3.2.1 Aggregates ................................................................................................. 39

3.2.2 Cement ...................................................................................................... 40

3.2.3 Fly ash ........................................................................................................ 41

3.2.4 Silica fume .................................................................................................. 42

3.2.5 Filler ........................................................................................................... 43

3.2.6 Superplasticizer ......................................................................................... 44

3.3 Specimens preparation and curing ........................................................................ 44

3.4 Tests ....................................................................................................................... 45

3.4.1 Fresh concrete properties. ........................................................................ 45

3.4.1.1 Consistency ................................................................................. 45

3.4.1.2 Air content .................................................................................. 46

3.4.1.3 Fresh density ............................................................................... 46

3.4.2 Mechanical properties. .............................................................................. 46

3.4.2.1 Compressive strength ................................................................. 46

3.4.2.2 Splitting tensile strength ............................................................. 46

3.4.2.3 Modulus of elasticity ................................................................... 46

3.4.2.4 Density of hardened concrete .................................................... 47

3.4.2.5 Rebound Hammer ....................................................................... 47

3.4.3 Durability of concrete ................................................................................ 48

3.4.3.1 Water permeability ..................................................................... 48

3.4.3.2 Air permeability .......................................................................... 49

3.4.3.3 Absorption .................................................................................. 50

3.4.3.4 Chloride diffusion test................................................................. 50

3.4.4 Concrete microstructure ........................................................................... 51

3.4.4.1 Effective porosity ........................................................................ 51

3.4.4.2 Helium pycnometry .................................................................... 52

3.4.4.3 Mercury intrusion porosimetry (MIP) ......................................... 52

3.4.4.4 Thermal gravimetric analysis (TGA) ............................................ 53

Table of contents xi

4. Optimizing a high dense concrete mixture................................................................. 55

4.1 Introduction ........................................................................................................... 55

4.2 Maximizing the packing density of solid particles ................................................. 55

4.3 Designing a dense cement matrix .......................................................................... 58

4.3.1 Introduction ............................................................................................... 58

4.3.2 Dense packing of cement matrix ............................................................... 59

4.3.3 Hydration products characteristics ........................................................... 60

4.3.4 Optimizing a dense cement matrix ............................................................ 61

4.3.5 Optimization of water/binder ratio ........................................................... 62

4.4 Densifying the interfacial transition zone .............................................................. 63

4.5 Mix design and mixes composition ........................................................................ 66

4.6 Measuring the packing density .............................................................................. 68

4.7 Measuring the actual packing density αj................................................................ 69

4.8 Results of packing density ...................................................................................... 70

4.8.1 Packing density of fine materials. .............................................................. 70

4.8.2 Packing density of dry concrete mixtures. ................................................ 71

4.8.3 Results of fresh concrete properties ......................................................... 73

4.8.3.1 Air content .................................................................................. 73

4.8.3.2 Workability .................................................................................. 74

4.8.3.3 Density of fresh concrete ............................................................ 75

5. Results and discussion of the optimized concrete properties ........................................ 77

5.1 Introduction ........................................................................................................... 77

5.2 Mechanical properties ........................................................................................... 78

5.2.1 Compressive strength ................................................................................ 78

5.2.2 Splitting tensile strength............................................................................ 79

5.2.3 Modulus of elasticity ................................................................................. 80

5.3 Porosity .................................................................................................................. 82

5.3.1 Porosity measured with helium pycnometry ............................................ 82

5.3.2 Water porosity (effective water porosity). ................................................ 82

5.3.3 Porosity and pore size distribution measured with MIP ........................... 83

5.4 Durability ................................................................................................................ 84

5.4.1 Permeability ............................................................................................... 84

Table of contents xii

5.4.1.1 Water penetration depth ............................................................ 84

5.4.1.2 Air permeability .......................................................................... 85

5.4.2 Absorption (capillary suction) .................................................................... 86

5.4.3 Chloride diffusion ...................................................................................... 88

5.5 Discussion ............................................................................................................... 89

5.5.1 Mechanical properties ............................................................................... 89

5.5.2 Porosity ...................................................................................................... 91

5.5.3 Durability ................................................................................................... 94

5.6 Porosity & mechanical properties relationship ..................................................... 96

5.7 Permeability & capillary porosity relationship ....................................................... 97

5.8 Absorption & capillary porosity relationship ......................................................... 98

5.9 Chloride diffusion & capillary porosity relationship. ............................................. 99

6. Effect of hydrothermal conditions on the properties of densely packed concrete ... 101

6.1 Introduction ......................................................................................................... 101

6.2 Methods ............................................................................................................... 102

6.3 Mechanical properties results.............................................................................. 103

6.3.1 Compressive strength .............................................................................. 103

6.3.2 Rebound number ..................................................................................... 104

6.3.3 Splitting tensile strength.......................................................................... 107

6.4 Porosity measured with MIP ................................................................................ 107

6.5 Durability .............................................................................................................. 109

6.5.1 Permeability ............................................................................................. 109

6.5.1.1 Water penetration depth. ......................................................... 109

6.5.1.2 Air permeability ........................................................................ 110

6.5.2 Absorption (Capillary suction) ................................................................. 111

6.5.3 Chloride diffusion .................................................................................... 113

6.6 Discussion ............................................................................................................. 114

6.6.1 Mechanical properties. ............................................................................ 114

6.6.2 Porosity .................................................................................................... 117

6.6.3 Durability ................................................................................................. 121

6.6.3.1 Permeability .............................................................................. 121

6.6.3.2 Absorption (Capillary suction) .................................................. 122

Table of contents xiii

6.6.3.3 Chloride diffusion ...................................................................... 123

7. Studying the influence of autoclaving on the properties of cement paste................ 125

7.1 General ................................................................................................................. 125

7.2 Mixes and tests .................................................................................................... 125

7.3 Hydration of cement paste .................................................................................. 126

7.3.1 Hydration heat ......................................................................................... 126

7.3.2 Discussion ................................................................................................ 128

7.4 The influence of hydrothermal treatment on cement pastes ............................. 130

7.4.1 Compressive strength .............................................................................. 130

7.4.2 Porosity .................................................................................................... 132

7.4.3 Density ..................................................................................................... 133

7.4.4 Calcium hydroxide content ...................................................................... 134

7.4.5 C‐S‐H phases transformation ................................................................... 135

7.5 Discussion ............................................................................................................. 136

8. Applying the optimized concrete in hot water tank ................................................. 143

8.1 Introduction ......................................................................................................... 143

8.2 Concrete mixture ................................................................................................. 143

8.3 Concrete tank model ............................................................................................ 145

8.4 First experiment ................................................................................................... 145

8.5 Second experiment .............................................................................................. 147

8.6 Future work .......................................................................................................... 149

9. Conclusions and recommendations ......................................................................... 151

9.1 Conclusion ............................................................................................................ 151

9.2 Recommendations ............................................................................................... 154

10. References .............................................................................................................. 157

11. Appendices ............................................................................................................. 171

Table of contents xiv

Notations and Symbols xv

Notations and symbols

Roman

a horizontal axis of the ellipse in the Ideal Fuller curve

aij loosening effect exerted by the particles in size class j on the packing density of the

particles in size class i

A cross section area of the concrete sample (cm2)

b vertical axis of the ellipse in the Ideal Fuller curve

bij wall effect exerted by the particles in class j on the packing of the particles in class i

co chloride concentration of the potassium hydroxide solution (mol.l‐1)

cd chloride concentration at which the colour changes, cd = 0.07 (mol.l‐1)

Cp specific heat of water (J/kg.K)

d diameter of the current sieve (mm)

di diameter of dominant size class i

dj diameter of particle class j

dmin minimum particle size

ds diameter of concrete sample (mm)

D maximum aggregate size (mm)

Dcl migration coefficient of chloride (m2/s)

erf ‐1 inverse error function

Ec modulus of elasticity (MPa)

fc compressive strength of concrete (MPa)

fcm average compressive strength (MPa)

ft splitting tensile strength (MPa)

Ft splitting force (KN)

F Faraday constant, F = 9.649 x 104 (Joule./Volt.mol)

h height of the concrete sample (cm)

Kair air permeability coefficient (cm2)

K compaction index

K absorption water absorption coefficient (kg/m2 hr0.5)

L specimen length in mm

mdry mass of the dried sampled

msat mass of saturated sample

msub mass of sample under water

M mass of the storing material (kg)

Mc mass of concrete sample (kg)

Notations and Symbols xvi

Mp mass of the powder (kg)

Mw mass of water (kg)

n number of size classes in a mixture

pm applied pressure in mercury intrusion porosimetry test (bar)

P applied pressure in air permeability test (N/cm2)

P total per cent passing through a sieve (%)

q gradation ratio (distribution factor)

Q amount of stored heat (Joule)

r radius of the intruded pore in mercury intrusion porosimetry test (nm)

rj volume fraction of size class j

R gas constant, R = 8.315 Joule/(K.mol)

t duration of the chloride diffusion test (s)

T absolute mean temperature of the solutions during the chloride diffusion test (K)

tf the testing period of water absorption (hours)

ta passing time of the air in air permeability test (seconds)

U absolute potential difference in chloride diffusion test (V)

V the volume of concrete sample (m3)

Vf flow volume (cm3)

xo the horizontal value of x axis between D/10 to D (mm)

x1 value where ellipse and the straight line are intersected, equals one‐tenth the

maximum size

xd: mean penetration depth of the chloride ions in each half of the test specimen (m)

y1 vertical value at x1

z valency, for chloride ions, z= 1

Greek

αj actual packing density of each concrete components

αt calculated packing density of the mixture

i virtual packing of size class i

j virtual packing density of size class j

ti calculated virtual packing density of a mixture when size class i is dominant

γc density of hardened concrete (k/m3)

γ surface tension of mercury (N/m)

ε total porosity measured with helium pycnometry (%)

εo free water porosity (%)

η dynamic viscosity of air in (N.s/cm2)

θ contact angle between mercury and the pore walls (degrees)

Notations and Symbols xvii

ρ powder density (kg/m3)

ρb bulk density, calculated by dividing the sample dry mass by the volume (kg/m3)

ρs specific density, determined by using helium pycnometer (kg/m3)

σ splitting tensile strength (MPa)

Abbreviations

αC2SH Alpha dicalcium silicate hydrates

C2S Dicalcium silicates

C3S Tricalcium silicates

C‐S‐H Calcium silicate hydrate gel

C/S Calcium/silicon ratio

CH Calcium hydroxide

CIPM Compaction‐interaction packing model

CPM Compressible packing model

CSP Concentrating solar power

EDX Energy dispersive x‐ray spectroscopy

FA Fly ash

GGBS Ground granulated blast furnace slag

HPC High performance concrete

HSC High strength concrete

ITZ Interfacial transition zone

LPM Linear packing model

M10 Fine fly ash with specific surface area of 6400 cm2/g

M20 Fine fly ash with specific surface area of 6000 cm2/g

MIP Mercury intrusion porosimetry

NSC Normal strength concrete

OPC Ordinary Portland cement

P.D Packing density of each materials

PCE Polycarboxylate‐based superplasticizer

QP Quartz powder

QS Quartz sand

SEM Scanning electron microscopy

SF Silica fume

SP Superplasticizer

TGA Thermogravimetric analysis

UHPC Ultra high performance concrete

w/b Water/binder ratio

w/c Water/cement ratio

Notations and Symbols xviii

Introduction 1

1. Introduction

1.1 General

Nowadays, the rapid growths of population along with the quick developments in all fields lead to a

significant increase in the energy demand. Nevertheless, fossil fuels which are the main energy

resource are limited and subjected to depletion in the next few decades. In addition to the expected

depletion in the near future, their use has a harmful impact on the environment. They are

responsible for the climate changes and environmental pollution; more than 95 % of CO2 emission is

produced by fossil fuel combustion [IEA, 2012]. To cope with these challenging conditions, it is

imperative to develop new energy resources or to enhance the efficiency of the existing resources.

Renewable energy provides an optimum solution for these problems. However, its contribution in

the world energy consumption is very limited (< 1 %). Solar energy can be considered as one of the

most promising alternative energy resource options because it is abundant, clean, safe and

available. The solar energy flow arrived to the earth equals about 6000 times the global energy

consumption [GCEP, 2006]. Nevertheless, the use of this energy is still small compared to the

potential of this resource. Unreliability is the major problem hinders the excessive use of solar

energy. In addition, solar technologies suffer some drawbacks that make them poorly competitive

in the energy market such as high cost, low efficiency and intermittency [Medrano, 2010]. The solar

energy supply is variable during the day and zero at night. On the other hand, the energy demand

also is irregular. So, the energy supply and energy demand, in general, do not match each other.

Therefore, a considerable amount of the produced energy should be stored during the day to cover

the demand at night or from summer to winter times. Solar radiation cannot be stored as such. It

should be transformed into a suitable energy type to be stored. The conversion into thermal energy

is the easiest and the most used method. Solar thermal energy can be stored for long time such as

seasonal energy storage where the stored energy is used only for hot water supply and domestic

heating. Hot water storage tank is considered one of the best technologies for seasonal energy

storage because of the high specific heat of water and the high capacity rate of charging and

discharging. Most of these storage tanks are built of prestressed concrete with internal lining to

prevent the water leakage. The economic studies indicated that the costs of liner represent about

20 % of the total cost of the projects. Recently in Hanover, a concrete tank has been built of dense

high performance concrete without internal lining to store solar energy at temperature up to 95 °C.

However, to make a practical use of the stored energy such as industrial processes and steam

generation, the storing temperature must be increased above 100 °C such as in the case of steam

accumulator. In PS 10 project, in Spain, steam accumulators have been used to store solar thermal

energy at temperature up to 250 °C and pressure of 40 bars [Solucar, 2006]. Nevertheless, the stored

energy covers the energy demands for only one hour with capacity of 50 %. So, improvements are

Introduction 2

required in order to enhance the performance and efficiency of the system, particularly in the

pressurized tank construction materials.

When a concrete wall subjected to hydrothermal cycles, water and vapour try to penetrate the wall.

The penetration rate depends mainly on the temperature and pressure of the water and vapour as

well as on the microstructure, durability and porosity of concrete. In such aggressive conditions,

extremely dense concrete should be optimized to ensure high durability and sustainability. One

possibility of optimizing the mixture is the selection of concrete constituents in such a way that the

packing density of the whole granulometric assemblage is maximized. Proper proportioning of

constituents is important for concrete in order to achieve the homogeneity and uniformity which

cannot be attained without paying attention to the grading and proportions of all components. In

addition to the importance of granulometric optimization, chemical composition of cement paste

has a significant influence on concrete properties. All transport processes depend primarily on the

structure of the hydrated cement paste. During the hydration process, the size and continuity of the

pores control the permeability of the hardening concrete. If the hydration process of cement is too

fast, large amounts of hydration products with capillary pores are generated on the surface of

cement particles at early age and the microstructure is not dense as desired. In contrast, if the

hydration rate is slow, a denser microstructure is formed. The type of cementitious materials has a

crucial role on concrete properties. Portland cement hydration produces about one‐fourth of its

mass of calcium hydroxide (CH), which is associated with greater permeability and lower durability.

The use of pozzolans improves the durability through the pore refinement and the reduction in the

CH content. Concrete durability can be improved significantly by using cements containing blast

furnace slag. These cements, in contrast to ordinary Portland cement (OPC), shows lower

permeability, lower hydration heat, lower effective alkali content, and lower steel corrosion. On the

other hand, the transition zone between cement and aggregates is considered the weakest link in

concrete under mechanical action and it is also the locus of micro‐cracking. It plays a critical role

in controlling the bulk transport properties. The incorporation of mineral admixture significantly

enhances the transition zone properties from several aspects depending on its reactivity and

particle size.

When exposed to elevated temperature, concrete chemical and physical characteristics are severely

changed, which affect its overall performance. The most important influences are the significant

increase of porosity, moisture content alteration, pore pressure change, compressive strength

retrogression (strength loss) and thermal cracking. Concrete elements can be implemented in many

applications in which different types of heat exposure can be expected. It can subject to pure heat as

in the case of fire or to hydrothermal conditions where it subjected to both heat and saturated vapour

Introduction 3

pressure. The behaviour of concrete under hydrothermal conditions (autoclaving) is complex

because it exposed to repeated cycles of heating and cooling as well as repeated cycles of

vaporization and condensation (autoclaving cycles). The chemistry of autoclaving is differing from

that at normal conditions or at heat only and it depends mainly on the chemical composition of the

bulk mixture. The dominant process on pure heat exposure of concrete is the loss of various forms

of water (free, adsorbed and chemically bound). However, the dominant process in the autoclaving

process is hydrothermal chemical reactions and phases transformation which may result in stronger

or weaker products depending on the C/S ratio of the bulk [Khoury, 2000]. When C‐S‐H gel of OPC is

subjected to autoclaving with temperature higher than 110 °C in the absence of reactive silica, it is

quickly converted to lime‐rich phases with high crystallinity [Hewlett, 2003; Mindess, 2003]. Phases

such as αC2SH, hillebrandite and ‐tricalcium silicate are the most stable phases in the system with

high lime content. The lime‐rich phases formation is accompanied by an increase in the density

which causes shrinkage of the solid products and therefore the porosity increases and the strength

is markedly decreased. The loss of strength due to elevated temperature exposure is commonly

known as strength retrogression. With sufficient addition of silica, pozzolanic reaction is accelerated,

portlandite is consumed, C/S is considerably reduced and the formation of lime‐rich C‐S‐H phases is

prevented [Nelson, 1990; Glasser, 2003]. Instead, numerous silica‐rich phases such as gyrolite,

xonotlite and 1.1 nm tobermorite are formed, which lead to high strength and low permeability.

1.2 Research Objectives

Worldwide, several hot water concrete tanks have been constructed to store solar thermal energy.

Most of these tanks use an internal lining to ensure leak‐tightness. The working temperature is

limited below the boiling point of water on most tanks. However, for efficient energy usage in

industrial applications using steam generation, the storing temperature must be increased above

100 °C while water and steam become pressurized. The main objective of this project is to study the

possibility of optimizing a concrete mixture to be used in a hot water concrete tank to store solar

generated energy at a temperature of 200 °C and a pressure of 15.5 bars without internal lining. In

the targeted tank, the leakage of water and vapour is prevented by implementing a very dense high

performance concrete. The optimized concrete should exhibited high efficiency and stability after

exposing to various autoclaving cycles. An important subject in this respect is the stability of C‐S‐H

phases after hydrothermal exposure with several cycles. Many factors influence the behavior of

concrete under these hydrothermal conditions. Regarding transport properties through the

optimized concrete, the mineralogical properties of concrete constituents are of more concern.

However, chemical composition of the concrete mixture is considered the basic parameter with high

impact on the stability of C‐S‐H phases.

Introduction 4

1.3 Research strategy

In order to achieve the objectives of this investigation, the experimental work is divided into three

main parts. In the first part, a high density concrete mixture has been optimized in order to prevent

the leakage of gases and liquids through concrete and to ensure high sustainability under

hydrothermal conditions. Increasing the packing density is considered an excellent strategy to get an

optimized concrete mix. In this concern, the Ideal grading curve according to Fuller has been used to

achieve the maximum packing density of the solid materials and to reduce the required binder

content. In addition, various cementitious materials were examined to select the most suitable

mixture with high resistance to hydrothermal attack. Furthermore, siliceous fillers have been added

to close the gaps between aggregate and fine materials on one hand and to enhance the stability of

C‐S‐H phases under hydrothermal conditions on the other hand. Moreover, Rene LCPC program

based on compressible packing model has been used to compare the effect of grading and

proportions of concrete components on the packing density of the mixture. The second part of this

investigation focuses on the effect of autoclaving with 200 °C and 15.5 bars on concrete properties.

Mechanical properties including compressive strength, tensile strength and rebound number have

been measured at normal conditions and after autoclaving with numerous cycles. In addition,

concrete porosity is measured before and after autoclaving for several cycles. Regarding concrete

durability, the main three mechanisms of ingress of gases and liquids through concrete;

permeability, absorption and diffusion, have been measured before and after autoclaving for several

cycles. In the third part, 10 different cement pastes have been prepared and tested in order to

deeply understand the effect of hydrothermal conditions on characteristics and stability of C‐S‐H

phases. In this concern, EDX, TGA and SEM measurements have been used to study the changes in

pastes morphology and properties due to autoclaving for 50 cycles. In addition, compressive

strength and porosity after hydrothermal treatment have been determined.

1.4 Outline

The research work presented in this investigation covers several fields, actually different but

nevertheless closely related to the topic of this thesis. Two main topics represent the main body of

this thesis. First, optimization of dense concrete mixture (Chapter 3, 4 and 5), second; studying the

behaviour of cement based materials after hydrothermal exposure (Chapter 6 and 7). This PhD

thesis consists of 9 Chapters, and its structure is given in Figure 1.1.

In the first chapter (Introduction), basic facts are given on solar energy, optimization of concrete

mixture and behavior of concrete under hydrothermal treatment, on the objectives of this

investigation and on research methodology.

Chapter 2 is a literature survey and divided into three main parts. In the first part, a literature

overview on solar energy technologies is given. In this part, a description of different methods of

Introduction 5

concentrating solar energy is introduced. This part also presented the modern technologies of solar

energy storage techniques either for seasonal terms for domestic heating and hot water supply or

for short terms for electricity and power generation. The second part of this chapter presented a

general historical background of the dense packing. In addition, some important formulas of

particles grading which assumed to achieve the maximum packing density are presented. Several

packing models to predict the packing density of concrete mixtures are also given. The last part of

this chapter points out the influence of elevated temperature on concrete properties. In addition,

the effect of hydrothermal treatment on concrete characteristics and stability of different C‐S‐H

phases are also discussed.

Chapter 3 describes the properties of the used materials and test methods. Chemical properties of

the used materials as received from the manufactures are given in this chapter. Physical properties

of aggregates and cementitious materials which have been determined in the laboratory are

introduced. In addition, particle size distribution of all materials measured either by sieve analysis

(aggregate) or by laser granulometry is given. In addition, a description of the experimental tests is

given. In this concern, measurement procedures of properties of fresh and hardened concrete

according to the standards are demonstrated.

Chapter 4 focuses on optimizing a high density concrete mixture. The mixture optimization is based

on three fundamental mechanisms: enhancing the packing density of solid particles, designing of

dense cement matrix, and densifying the interfacial transition zone. Enhancing the packing density

of solid particles is ensured by applying the Ideal Fuller curve for proportioning aggregates and fines.

However, the designing of dense cement matrix and enhancing the properties of transition zone are

achieved by incorporation of several fine materials and by adjusting the water/binder ratio. Twenty

three mixes were prepared and tested. At the end of this chapter the results of packing density

measurement and properties of fresh concrete are given.

Chapter 5 contains the results and discussions of the experimental investigations. These

experiments are conducted to evaluate the mechanical properties and durability of the optimized

mixes. Mechanical properties including compressive strength, tensile strength and modulus of

elasticity are measured. However for durability, permeability, capillary suction and chloride diffusion

are determined. In addition, the results of porosity measured with three different mediums; water,

helium and mercury are presented. At the end, relationships between mechanical properties and

durability with porosity are drawn in order to declare the effect of porosity on concrete properties.

In chapter 6 the results of the effect of hydrothermal conditions on concrete properties are

presented. The experiments aim to highlight the effect of autoclaving with 200 °C and 15.5 bars for

various cycles on the mechanical properties and durability of concrete. These results demonstrate

the suitability and sustainability of concrete mixtures to be used in the targeted conditions. In

addition to durability and mechanical properties, a comparison between concrete porosity and pore

Introduction 6

size distribution before and after autoclaving with 50 cycles are provided by using mercury intrusion

porosimetry.

In chapter 7, the experimental test results of 10 different cement pastes are given. The aim of this

experimental part is to study the stability and characteristics of C‐S‐H phases after hydrothermal

exposure. Strength retrogression and permeability increase due to phase’s transformation are the

main focus of this chapter. Therefore, compressive strength and porosity tests of different cement

pastes are performed. In addition, in order to deeply understand the effect of hydrothermal

treatment on the stability and transformation of different C‐S‐H phases SEM, EDX, TGA

measurements have been implemented.

Chapter 8 presents the results of the preliminary experiments of hot water concrete tank model

made of the optimized concrete to store hot water and vapor at temperature above 100 °C.

Chapter 9 summarizes the achievements and final conclusions of this study. Based on the results of

this research project, recommendations and ideas for future research are also provided.

Introduction 7

Figure 1.1: Outline of the thesis

General

Concrete mixture

optimization

Effect of hydrothermal

conditions of cement

based materials

Chapter 4: Optimizing a high dense concrete mixture

Chapter 5: Results and discussion of the optimized

concrete properties

Chapter 1: Introduction

Chapter 2: Literature review

Chapter 3: Materials and Methods

Chapter 6: Effect of hydrothermal conditions on the

properties of densely packed concrete

Chapter 7: Studying the influence of autoclaving on

the properties of cement paste

Chapter 8: Applying the optimized concrete in

hot water concrete

Chapter 9: Conclusions and recommendations

Applications

Introduction 8

Literature review 9

2. Literature review

2.1 General

This chapter gives a literature survey on three different main themes which are required as

foundations of this research work. The first part of this chapter, solar energy storage, discusses the

new technologies in solar energy applications. It points out the different methods of concentrating

sun radiation. In addition, it refers to various storing systems using hot water and steam as working

medium, either with low temperature (< 100 °C) or with high temperature (> 100 °C). In the second

part of this chapter, a historical background about packing density is presented. It includes a

demonstration of the ideal grading curves and formulas which can be applied to achieve the

maximum dense packing of concrete mixtures. Moreover, it introduces some packing density

models which can be used to predict the maximum packing density of any concrete mixture. In the

last part, the effect of high temperature exposure on concrete mechanical properties and durability

is discussed. In this section, three topics are of main interest: effect of pure heat, explosive spalling

and hydrothermal conditions up to 200 °C. At the end of this part, a literature review about C‐S‐H

phases and their stability and properties after hydrothermal exposure are given.

2.2 Solar energy

2.2.1 Introduction

Fast worldwide, economic development leads to a quickly increase in energy demand. In recent

years, many attentions have been paid to renewable energy because of the environmental harmful

effects of the conventional energy resources in addition to the expected depletion of them in the

next few decades. Nevertheless, the contribution of the renewable resources in the world energy

consumption is very limited (< 1 %) as it is obvious in Figure 2.1. Solar energy can be considered as

one of the most promising alternative energy resource options because it is abundant, clean, safe

and available. The sun energy potential is the greatest energy resource on the earth and by far

exceeds those of other renewable energy sources [Philibert, 2011]. The solar energy flow arrived to

the earth equals about 6000 times the current total global energy consumption [GCEP, 2006].

Despite the high abundance of solar energy, its use is still small compared to the potential of this

resource. Solar technologies suffer from some drawbacks that make them poorly competitive in the

energy market such as high cost, low efficiency and intermittency [Medrano, 2010]. Unreliability is

the major problem hinders the excessive use of solar energy. From a scientific and technological

viewpoint, the great challenge is to find a new solution for solar energy systems to become less

intermittent, more efficient and reliable.

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2.2.2 Concentration of solar power (CSP)

Concentrating solar thermal power provides a possibility for using sun energy to generate electricity

with producing very low levels of greenhouse gas [IEA, 2010]. Thus, it has strong benefits to be the

new technology for mitigating global warming. It is a power generation technology by using lenses

or mirrors to concentrate a large area of sun rays onto a small area, either point or line. Most

systems use glass mirrors with flat or curved surfaces because of their high reactivity [Trieb, 2009].

The concentrated heat is absorbed on the receiver which is designed to reduce the heat losses. The

collected energy is used to heat a fluid flowing through the receiver and to produce a steam with

high pressure [IRENA, 2012]. The produced steam drives a turbine and generates energy as in the

traditional power plants. The heat transfer fluids could be air, water, oil or molten salt [Trieb, 2005].

There are four main technologies for concentrating solar rays: parabolic trough, solar tower, linear

fresnel and parabolic dish as shown in Figure 2.2.

2.2.3 Solar Energy storage

Although the solar energy is abundant, clean and safe, the supply of this energy is intermittent and

irregular. The solar energy supply is variable during the day and zero at night. On the other hand,

the energy demand is also irregular. So, the energy supply and energy demand, in general, do not

match each other. Therefore, a considerable amount of the produced energy should be stored

during the day to cover the demand at night. The thermal energy storage is essential for solar

energy system in order to enhance the reliability, efficiency and competitiveness of this system. The

need for solar energy storage could be linked to the following causes:

1. To adjust the mismatch between energy supply and demand.

2. To provide a continuous and reliable energy source.

3. To reduce the greenhouse gas emission by substituting fossil fuels with clean energy.

4. To compensate the times of fluctuation in solar energy systems.

5. To serve as a reservoir to store the excess energy (buffering) when the energy supply higher

than energy demand.

Solar radiation cannot be stored as such; it should be transformed into a suitable energy type to be

stored. The conversion into thermal energy is the easiest and the most used method. Thermal

energy storage is one of the advantages of concentration of solar power (CSP) compared to other

renewable energy technologies such as photovoltaic or wind [Trieb, 2009]. Many parameters affect

the storage system efficiency such as maximum working temperature and specific heat of the

storing material, storage capacity, chemical and mechanical stability, compatibility between storing

material and transfer fluid, thermal losses and ease of control [GCEP, 2006]. Mainly, there are three

methods for thermal energy storage: sensible heat, phases change materials and chemical reactions.


2.2.4 Some applications of sensible heat storage systems

In sensible heat storage method, the storage is based on increasing or reducing the temperature of

the material without changing its phase during the process. The storage capacity depends upon the

heat capacity of the medium and the temperature increase. Other parameters are significant for

sensible heat storage such as diffusivity, working temperature, thermal conductivity and heat loss

per unit volume. A sensible heating system consists mainly of the container, inlet/outlet device and

storage medium [GCEP, 2006]. The container (tank) should be able to resist the loads, temperature

differences and also to prevent thermal losses. Several materials could be used as storage medium

such as solids (rocks, concrete…) or liquid (water, oil…). Because of its very low heat capacity, gases

cannot be used for cold or hot storage [Paksoy, 2007]. Sensible heat technology is cheap compared

to other mechanisms, but the energy density is low. The stored energy can be calculated according

to the following formula:

Q= M.Cp. T (1.1)

Where Q is the amount of stored heat (J), M is the mass of the storing material (kg), Cp is the specific

heat (J/kg.K) and T is the temperature range of operation (K).

2.2.4.1 Central seasonal heat storage

Domestic water heating represents about 30 % of the total energy consumed by buildings [IEA,

2010]. The primary function of this system is the use of solar collector to capture the sun rays and

transfer it directly to heat. The heat is transferred from the collectors by using thermal transfer fluid,

which carries the heat for direct use or to heat storage (sensible heat storage medium). The heat

storage is charged when the solar collectors gather solar heat more than the required by the

network. During the summer, charging takes place, whereas in the winter the stored energy is used

for domestic hot water and space heating. In addition, a heat pump is integrated in the system in

order to achieve high efficiency [Schmidt, 2006]. The long term (seasonal) heat storage can provide

heat up to about 70 % of the annual heating demand [Novo, 2010]. In all types of seasonal heat

storage, the maximum temperature is limited below 100 °C. Four main technologies are developed

and used for storing solar thermal energy as can be seen in Figure 2.3.

Aquifer thermal energy storage. Borehole thermal energy storage.

Pit storage. Hot water tank

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2.3 Packing density as a key for concrete mix design

2.3.1 Introduction

In recent years, progress in science and technology in the field of construction materials and usage

of new materials have resulted in the use of reinforced concrete in special structures, which need

high durability and high resistance to chemical attack such as cooling towers of power plants,

sewage systems, nuclear power containments, etc… In these aggressive conditions, the transport of

contaminants through concrete is of more concerns. Movement of fluids and gases in concrete

occurs through the network of continuous pores, which exist in cement matrix as well as in the

interfacial region with the aggregate. Concrete is a heterogeneous material with complex

microstructure consists mainly of several components and phases with various properties.

Aggregate occupies between 60 ‐ 80 % of the total volume of concrete [Rached, 2009]. It represents

the main body of concrete. Therefore, proper selection of aggregate type and content directly

influence the main properties of concrete. On the other hand, cement paste takes around 15 ‐ 30 %

of the whole volume. It fills the voids between the aggregate, binds them together and renders the

fluidity of fresh concrete. Although increasing cement paste has a positive influence on the properties

of fresh concrete, it has some negative effects on the hardened state, especially in the early ages. It is

responsible for some of the problems in concrete such as shrinkage and hydration heat in addition to

high porosity.

Optimization of a concrete mixture is most important and complicated as there are a variety of

variables influencing the results. One possibility of optimizing the mixture is the selection of

concrete constituents in such a way that the packing density of the whole granulometric assemblage

is maximized. Granular materials can be packed together in several arrangements, from dense

packing to loose packing [Alexander, 2005]. The type of arrangement relies mainly on particle size

distribution, particles shape and particles interactions. If a high volume of solid materials can be

packed efficiently in a certain volume, then the voids in the mixture are minimized, and thereby the

required paste to fill this void is significantly reduced. As a consequence, more paste is available to

maintain the required workability and increasing the strength [Grutzeck, 1993]. In addition, mixes

with high density exhibited lower segregation and bleeding [Quiroga, 2003]. For hardened concrete,

the hydration heat, creep and shrinkage are significantly reduced due to minimizing cement content.

An additional benefit of maximizing packing density is to block and lengthen the access path of

contaminants into concrete.

The packing density can be defined as the ratio of fraction volume occupied by the solids to the

volume of the surrounding container. Goltermann defined the aggregate packing degree as the ratio

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The optimum packing density of the system could be attained only if spheres with smaller sizes are

added to the assemblage. The small size spheres can fill the voids between the large spheres, and

thereby the packing density is significantly increased. In 1961, McGeary reported that it is possible

to achieve a packing density of 95.1 using four different sizes of spheres which have diameter ratios

of 1, 7, 38 and 316 with fraction volumes of 6.1, 10.2, 23 and 60.7 % respectively. However, the

maximum density of infinite differences in sizes can be attained is 97.5 % [McGeary, 1961].

For concrete, the situation is more complex since the system composed of various particle sizes with

different shapes. Effective packing can be attained by selecting a proper proportion and size of small

particles to fill in the voids between the bigger particles. The important effects of aggregate grading

on the properties of concrete have been emphasized in very early reports. Several early studies

were concentrated on solving the problem of establishing the ideal grading curves to achieve the

maximum packing density by combining fine and coarse aggregate. In 1892, Rene Feret studied the

relationship between the packing of concrete components and the properties of concrete

[Newlands, 2001]. He concluded that the maximum strength can be achieved when the porosity of

the mixture is minimal [Hüsken, 2008]. Fuller (1907) was one of the first researchers deeply studied

the maximum density of aggregate and a mixture of aggregate and cement [Fuller, 1907]. After a lot

of experimental works, he reported that the best grading of aggregate to get the maximum density

is a parabolic shape. Both Feret and Fuller confirmed that concrete properties can be significantly

improved by using continuous grading [Hüsken, 2008]. In 1923, Talbot developed the well‐known

equation:

P (2.2)

Where P is the total percent passing through a sieve, d is the diameter of the current sieve, D is the

maximum aggregate size and q is the gradation ratio [Rached, 2009]. The maximum packing density

can be achieved when q = 0.5, which is close to the Fuller curve [Powers, 1968; Brouwers, 2005;

Fennis‐Huijben, 2010], but the resulting concrete is harsh and unsuitable. In 1930, Andreasen and

Andersen tried to improve the Fuller curve. They suggested using the exponent q in the range of

0.33 ‐ 0.5 because fine particles are not able to pack similar to bigger particles [Andreasen, 1930;

Zheng, 1990; Hunger, 2010]. The factor q has to be experimentally determined and it depends upon

the properties of particles, considering the fact that no theoretical determination of this factor has

been attempted [Zheng, 1990]. For angular particles, low q is preferred in order to increase the fine

materials to fill the voids between coarse particles, to keep the workability at the required level and

to enhance the packing of the system [Fennis‐Huijben, 2010]. Hummel found that Fuller curve (q =

0.5) does not give the maximum density at any case. He recommended q = 0.3 for angular

aggregate, and 0.4 for round aggregate in order to get the optimum packing density [Hunger, 2010].


Shilston suggested that taking q = 0.45 provides the optimum packing density [Shilstone, 1990;

Rached, 2009]. However, according to equation 2.2, infinite small particles are required to achieve

the maximum theoretical packing density [Zheng, 1990]. Alexander and Mindess also pointed out

the important role of including the overall grading of all mixture components in the packing theory

in order to achieve the high performance characteristics [Alexander, 2005]. Funk and Dinger stated

that any size distribution must have minimum particle size (dmin). The ideal particle size distribution

depends not only on the maximum aggregate size, but also on the minimum particle size as can be

seen in equation 2.3 [Zheng, 1990; Hüsken, 2008; Hunger, 2010].

P (2.3)

Higher values of q (> 0.5) lead to coarse mixtures with low fine material content. However lower

values (< 0.25) lead to mixture with high fine material content which considerably reduces the

packing density [He, 2010]. Generally, in order to achieve the maximum packing and keep the

workability at an acceptable level, it is suggested to take q = 0.37 [Fennis‐Huijben, 2010; Vogt, 2010].

For self‐compacting, it is recommended to take q in the range of 0.22 – 0.25 in order to achieve the

required workability [Hüsken, 2008].

2.3.3 The work of Fuller

The American William Fuller can be considered as the father of granulometric optimization in

concrete technology [Puntke, 1990]. At the beginning of the last century, he studied grading analysis

for a wide variety of aggregate types and mixtures for the sake of deriving the optimum grading that

achieves the maximum packing density [Lees, 1967]. In his famous book, The laws of proportioning

concrete, he mentioned that, in 1901 it was found that the best density of aggregate can be attained

when the particle size distribution of aggregate is continuously graded and the grading curve takes a

parabolic shape when plotted in natural scale as can be seen in Figure 2.9. Despite of its historical

value, Fuller curve is used as the base for proportioning of aggregate in many national concrete

standards [Vogt, 2010]. This curve is still known until now as Fuller parabola and can be applied for

calculating the optimum grading of aggregate only as he himself mentioned later [Fuller, 1907]. The

reason for that lies on the fact that the mixture of aggregate which gives the maximum density in

the dry state does not necessarily achieve the greatest density when combined with cement and

water. This is because the low void content between the aggregate particles prevents the cement

and water to fit in perfectly [Fuller, 1907; Richardson, 2005; Rached, 2009].

In 1903, Fuller began an intensive work to achieve the greatest packing density for a mixture of

aggregate and fine materials. He reported that the best curve was obtained by trial mixes without


referring to the mathematical basis of the curve. It was concluded from this work that there is a

certain ideal grading curve to all solid materials, composed basically of an ellipse at the lower part,

and merging into a straight line tangent to the elliptical part [Powers, 1968]. He stated that “The

curve which gave the best result when using a graded coarse aggregate with the cement was found

to be one resembling a parabola in appearance, but, more strictly, consisting of a curve having for

the lower portion the form of an ellipse, and above this a straight line running to 100 % on the

maximum diameter of the stone”. After finding the best grading, equations were fitted to these

curves. The ellipse begins from 0.0029 inch (sieve No 200) and runs to a value of x equal to one‐tenth

of the maximum grain size. At this point, the straight line begins and continues to y = 100 percent and

x = D (where D is the maximum grain size of aggregate) as can be seen in Figure 2.10. The equation

covering this curve was divided into two parts:

For the elliptical part:

71 (2.3)

For the straight line part:

100

(2.4)

Where a and b are the axis of the ellipse and their values depend mainly on the shape of the particles

and the maximum aggregate size and x0 = D/10 to D, y1 = yel at D/10 and x1 = D/10 [Puntke, 1990].

The linear scale for the Ideal curve is not satisfactory because the lines in the small size zone are too

crowded for defining. In 1990, Puntke referred to all these notes and redraw the Ideal Fuller curve in

a semi‐logarithmic scale for the sake of simplicity as can be seen in Figure 2.11 [Puntke, 1990]. The

Ideal Fuller curve has been used for designing concrete mixes for some applications, particularly

those need high density and high resistance to acid attack. For example, in 2000, the highest cooling

tower in the world (200 m, Niederaußem, Germany) has been constructed of acid resistant

concrete. The used concrete has been designed on the basis of the Ideal Fuller curve. Because of its

high density and resistance to acid attack, the concrete did not need any additional protective layer

[Busch, 1999; Hüttl, 2000]. Furthermore, it showed very low chloride diffusion as well as high frost

resistance without air entraining agents. In addition to the technical benefits, some economic

benefits were gained by reducing the cement content to about 50 % compared to conventional high

performance concrete with similar strength.

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Figure 2.11: Ideal Fuller grading curve for aggregate and fine materials in logarithmic scale According to [Fuller, 1907]

Many authors did not differentiate between Fuller parabola and Ideal Fuller curve [Powers, 1968;

Newlands, 2001; Mindess, 2003; Alexander, 2005; Brouwers, 2005; Brandt, 2009; Rached, 2009;

Hunger, 2010]. They did not consider the elliptical section of the Ideal Fuller curve and assumed

equation 2.2 as synonymous with this curve. Furthermore, they deal with Fuller parabola as the

optimum grading curve for aggregate and binder mixture. This assumption tends to produce lean

mixes with lower fines content [Lees, 1967]. Therefore, the use of this curve in concrete mix design

is limited due to the poor workability and probability of segregation of the produced concrete.

However, compared to Fuller parabola, the Ideal grading curve has higher content of fines and lower

coarse aggregate content as can be seen in Figure 2.11. For example, for mixture with maximum size

of 16 mm, about 8.8 % of solid materials pass from the sieve size 0.125 mm according to Fuller

parabola, whereas according to Ideal Fuller curve, 14.9 % passes from the same sieve size.

2.3.4 State of the art of particle packing modeling

In the previous section, some theories about the appropriate selection of concrete components in

order to get the ideal grading of particles have been mentioned and discussed. However in this

section, theoretical particle packing models to estimate the packing density ratio of solid

combination will be presented. Packing model provides an important tool to design concrete

mixture with minimum cement and water contents and with maximum solids content [Jones, 2002].

One of the big problems for engineers is to achieve high packing density and ensure the desirable

workability. By developing packing models, it is possible to select the mixture which provides the

maximum packing density and minimizes the remaining voids. The particle packing models explain

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the way of which the particles are packed together based on mathematical hypothesis. Most of

these models calculate the minimum voids ratio on the basis of the particle geometry and size

distribution.

In 1929, Furnas developed a model for predicting the packing density of two monosized particles in

two cases; the small particles are the dominant or the large particles are the dominant [Fennis‐

Huijben, 2010]. Two years later, he developed a method to estimate the maximum packing density

of multi‐size particles taking into account the effect of interaction between particle groups [Furnas,

1931]. The main idea of his approach is that the maximum packing density will be achieved if the

small size particles fill the voids between the large particles [Vogt, 2010]. Ben Aiim and Goff

modified the Furnace model by implementing the wall effect into the model [Andersen, 1993; Nehdi,

1998; Fennis‐Huijben, 2010]. In 1976, Toufar et al developed a model based on Furnace equations.

Afterwards, Toufar model was modified by implementing diameter and packing density of each

material to predict the packing density of the mixture [Jones, 2002]. In 1999, Dewar developed a

packing model based on the particle size distribution. It can calculate the theoretical packing density

of a concrete mixture by knowing voids ratio and the mean size of each single constituent [Dewar,

2002]. Stoval et al developed linear packing model (LPM) based on discretely sized particles [Vogt,

2010]. These models take into account the wall effect, particle shape and effect of fine particles on

the packing of larger particles. With some modification on LPM, De Larrard developed compressible

packing model (CPM) in 1999. This model is based on two new concepts: virtual packing density and

compaction index [Larrard, 1999]. Jones et al made a comparison of four packing models (modified

Toufar, Dewar, LPM and CPM models) by making some laboratory experiments. It is found that all

models give similar output and suggest similar materials combination to achieve the minimum

porosity [Jones, 2002]. However, in the conclusion he mentioned that much work is still required in

the development of modeling, in particular, in the way in which the particle shape and mean particle

sizes are considered. Fennis also studied several models and concluded that the compressible

packing model is the most accurate model with the highest ability for modification to cover various

interactions. She developed compaction‐interaction packing model taking into account the influence

of inter‐particle forces in addition to wall effect and loosening effect [Fennis‐Huijben, 2010]. In this

model, packing of fine materials and binders should be experimentally determined and included as

inputs. It is concluded that no direct relation between maximum packing density and either

compressive strength or hydration heat of cement was found. A good relation was found between

compressive strength and the distance between cement particles. It was mentioned also that, more

than 50 % of cement can be saved and about 25 % of CO2 emission can be reduced while the

concrete is still achieve the requirements for the appropriate use by applying packing density

models.


Over the last few years, a number of computer programs based on the packing equations and

models have been developed. The development of computer analysis and simulation provides a

powerful tool for modeling the packing of particles. These programs give the engineers the ability to

determine the optimum combination of concrete constituents that ensures a maximum packing

density and minimum voids. For example, EMMA is commercial computer software based on the

Andreassen equation for packing density, and can be used for concrete mix design

(www.concrete.elkem.com). In addition, MixSim (www.mixsim.net) is a mix design program based

on Dewar model [Dewar, 2002]. On the other hand, Rene‐LCPC is software based on CPM and can

be used to determine the maximum packing density (lcpc.fr). Similarly, Europack is a software based

on the modified Toufar model to estimate the packing density of dry mixtures [Alexander, 2005;

Rached, 2009].

2.4 Effect of heat on concrete properties

2.4.1 Introduction

Compared to other construction materials, concrete can be considered one of the best materials

regarding resistance to elevated temperature. It is incombustible and has low thermal diffusivity. It

has good fire resistance and provides a protection layer for reinforcement. Various structures can be

subjected to elevated temperature, which must be taken into consideration when designing and

detailing of reinforced concrete structures. Tunnels, high rise building, underground parks, nuclear

power plants and jet aircraft engine blasts are examples of such structures where the probability of

elevated temperature exposure is high and very dangerous. Studying the behaviour of concrete at

high temperature is very complex not only because it is composed of several constituents with

different properties, but also because the porosity and moisture content play important roles in the

concrete performance at these aggressive conditions. At high temperature, chemical and physical

characteristics of concrete are severely changed, which affect the overall properties of concrete.

Chemical changes can be beneficial (further hydration of unhydrated cement particles) or

detrimental (decomposition of hydrated cement paste), and it relates to the cement paste and

aggregate. However, the physical effects are related to dimension compatibility between aggregate

materials and cement paste.

The moisture content of concrete affects its performance at high temperature dramatically. At high

temperature (> 100 °C), water changes its state and as a result some influences take place at certain

temperatures. The capillary and gel water are evaporated at temperature of 100 ‐ 150 °C. However,

above 250 °C, the chemically bound water starts to evaporate and the compressive strength starts

to decrease. Dehydration of calcium hydroxide takes place at approximately 450 °C. At higher


temperature, the C‐S‐H begins to decompose and significant decrease in compressive strength

occurs [Rashad, 2012]. Concrete can be implemented in many applications in which different types

of heat exposure can be expected. It can subject to pure heat as in the case of fire. However,

hydrothermal conditions are another case in which the concrete can be subjected to both heat and

saturated vapour pressure such as in nuclear power plants and water tanks for storing thermal energy.

2.4.2 Effect of high temperature on properties of cement based materials

Under high temperature exposure, heat is transferred by convection and conduction through the

concrete, and the free water starts to evaporate depending on the working temperature. Concrete

durability and mechanical properties are markedly changed due to deterioration of aggregate,

decomposition of hydrated cement paste and thermal incompatibility between paste and aggregate

which resulted in microcracking and stress concentration [Mindess, 2003]. The literature search

results on the behaviour of concrete at elevated temperature give widely varying results [Neville,

2004]. As Neville reported, "globally valid generalizations are difficult" [Neville, 2004]. This can be

attributed to the differences in moisture conditions of concrete, in the exposure period, in heating

rate and in concrete compositions.

In the literature, the behaviour of normal strength concrete under elevated temperature is well

documented. However, limited researches are available about the performance of other types of

concrete, in particular, when mineral admixtures are incorporated. Dias et al. studied the

mechanical properties of hardened cement paste exposed to high temperature up to 700 °C. A

decrease in compressive strength was observed at temperature around 100 °C. He attributed that to

the swelling of physically bound water layers. The original strength regained and maintained with

increasing the temperature to 300 °C. The regaining of strength could be due to the relief of the

build‐up pressure and the increased Van der Waals forces which result from the movements of

cement gel layers closer to each other. He suggested 300 °C as the critical temperature point with

respect to strength loss for unsealed Portland cement concrete [Dias, 1990]. Above this

temperature, rapid increase of porosity and micro‐cracking are occurring which leads to degradation

of mechanical properties. Another investigation has been carried out by Seleem et al. [Seleem,

2011]. He found that the compressive strength of concrete incorporating pozzolanic materials is

increased after elevated temperature exposure up to 200 °C. He attributed that to evaporation of

free water which results in an increase in friction between failure planes. In addition, at this level of

temperature, self‐autoclaving process is enhanced which accelerates the rate of cement hydration and

pozzolanic reaction.

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shocks. The results showed that the total porosity increased by about 12 % with heating up to 300

°C. Mechanical properties (elasticity and Poisson`s ratio) were reduced by about 17 %, which are in

agreement with the porosity results. Similar study was carried out by Piasta et al. [Piasta, 1984] on

the effect of elevated temperature on the microstructure of cement paste. He concluded that above

100 °C, water is evaporated and the generated steam affects the surrounding cement paste phases.

The temperature range of 100 ‐ 300 °C is suitable for formation of so‐called internal autoclaving. So,

additional hydration of unhydrated cement grains takes place. Up to 300 °C, the increase in total

porosity is relatively small due to additional hydration of unhydrated cement particles and increase

in the degree of carbonation. Similarly, Xu et al. [Xu, 2001] carried out a research to study the effect

of high temperature on porosity, durability and compressive strength of normal strength and high

strength concretes made of fly ash with replacement levels of 0, 25 and 55 %. The results indicated

that about 8 and 15 % increase in compressive strength has been gained for OPC and fly ash

concrete respectively after exposure to 250 °C. In addition, it was found that all concretes suffer

severe deterioration in permeability. He reported also that after exposed to 250 °C, the total

porosity is increased by about 1 ‐ 2 % only, and the pore diameter was coarsened. The micro‐

cracking was considered the main reason for deterioration when concrete exposed to high

temperature. Micro‐cracks is initiated around CH crystals and then around unhydrated cement

particles. Incorporation of fly ash reduces the thermal micro‐cracks due to the consumption of CH

and generation of additional C‐S‐H phases.

Li et al [Li, 1999] studied the microstructural characteristics of high performance concrete under

elevated temperature up to 200 °C intended for use in vault sealing in nuclear waste repositories.

Two types of concrete were used: one with oil well cement and the other with type K expansive

cement according to ASTM Standard C 192‐90a. It was found that concrete microstructure, matrix

morphology, mineral, and chemical composition showed no significant changes in the tested

temperature range up to 200 °C. Similar investigation was carried out by Khan et al. [Khan, 2010] to

study the effect of thermal cyclic loading with 200 °C on the properties of normal and fly ash

concrete. The results indicated that the compressive strength of plain and fly ash concrete is

increased with rising the heating cycles. The increase in compressive strength of fly ash concrete is

higher than that of normal concrete. It increased with increasing fly ash content up to 50 %.

However, with further increase of fly ash (i.e. 60 %), the increase in compressive strength with

heating is reduced and become lower than that with 20 % fly ash. Balendran et al. [Balendran, 2002]

presented an overview and discussion on the effect of high temperature on the strength and

durability of HPC incorporated pozzolanic materials. It is concluded that the addition SF reduces the

performance of HPC at high temperature. The risk of spalling and high loose of compressive strength


are associated with addition of SF. On the contrary, the addition of FA and MK has been found to

enhance the performance of HPC for both residual strength and durability.

2.4.3 Explosive thermal spalling

When exposed to high temperature, concrete suffers several degradation processes. Spalling is one

of the most important phenomena that may take place during the high temperature exposure. It

refers to sudden and violent separation of a surface layer of heated concrete. It is accompanied by a

loud bang and quick liberation of a large amount of energy and detaching of fragments of the

exposed concrete surfaces and can make severe deteriorations for the structure [Phan, 2002]. It

takes place if the heating rate is high, typically 20 °C/min [Lee, 2008]. It is prone to happen in HSC

and HPC even at a lower heating rate because of the dense microstructure. The explosive spalling is

a brittle failure observed first by Harmathy in normal concrete subjected to fire [Bažant, 1996]. Two

main mechanisms are commonly known to explain the spalling risk [Mindeguia, 2010]. The first is a

thermo‐mechanical interaction. Heating of concrete elements causes high temperature gradients in

the first area of the hot surface. If the heating rate is rapid, high compressive stresses are generated

closed to the heated surface. As self‐equilibrium, thin layer near the heated surface will be in

compression while the interior part is in tension. The compressive stress in the thin layer in the

vicinity of the surface causes buckling and deportation of some pieces of concrete surface. The

second mechanism is a thermo‐hygral process. With heating to high temperature water inside the

concrete begins to vaporize, and as a result the pore pressure is sharply increased in the hot zones

which create a pressure gradient with cold zones. Because of the pressure differences, the vapour

tries to migrate to low pressure zones (cold zones) and condensed there because of the low

temperature. With continuous heating, more water is condensed in the cold zone until reaching the

saturation state. At this point, the vapour is forced to migrate through the hot dry zone to escape

into the atmosphere. If the permeability of concrete is slightly low and the rate of heating is rather

high, the vaporized water cannot escape fast, because the rate of vaporization is higher than the

rate of release to atmosphere. Spalling will take place when the summation of pore pressure and

thermal stresses is higher than the tensile strength of concrete. So, the probability of spalling

occurrence depends mainly on heating rate, saturation level and the permeability of concrete

[Consolazio, 1998]. If the concrete is cracked, the vapour pressure will release, and the probability of

explosive spalling is decreased [Lee, 2008].

Bažant studied the explosive spalling and found that the pore pressure can work only to trigger the

cracking but cannot make the explosion [Bažant, 1979]. He mentioned that another supply of

energy should make this explosion; it is the potential energy of the thermal stresses. Nevertheless,

the role of pore pressure cannot be neglected because the experiments confirmed that the


explosive spalling occurred only in wet concrete which is saturated with water. Phan et al. [Phan,

2002; Phan, 2002] studied the explosive spalling in high strength concrete. Contrary to Bazant

explanation, it was found that internal pore pressure plays the primary cause, whereas the thermal

stresses play a secondary role in this failure. The w/c ratio has an important role in explosive

spalling; as it reduces, the risk of spalling increases. The addition of polypropylene fibers reduces the

pore pressure significantly and thereby reduced the tendency for spelling. It increases the

permeability during heating above 160 °C. It melts at a temperature of about 160 °C and provides

open channels in concrete to release the vapour and moisture [Khoury, 2000]. It is mentioned that

addition of 1.5 kg/m3 prevents the explosive spalling [Phan, 2002]. Noumowe and Debicki concluded

that addition of 2 % of polypropylene fibers by weight enhances the permeability of HPC and

reduces the risk of explosive spalling [Noumowe, 2002]. In the literature, many investigations have

been carried out in order to study the risk of explosive spalling of different types of concrete. The

following references provide further information about the explosive spalling of concrete when exposed

to high temperature [Bažant, 1979; Bažant, 1996; Chan, 1996; Consolazio, 1998; Kodur, 1999; Phan,

2001; Poon, 2001; Noumowe, 2002; Phan, 2002; Dehn, 2004; Hainz, 2004; Naus, 2006; Lee, 2008;

Mindeguia, 2010].

2.4.4 Effect of autoclaving on concrete properties

In some applications, concrete is subjected to both high temperature and saturated vapour

pressure. Nuclear power plant containment is an example where both hydraulic and thermal loads

attack concrete. In such applications, leak‐tightness of concrete should be ensured either by

installing a liner or by using very dense concrete in order to prevent water and vapour leakage. In

the French 1300 and 1400 MWe nuclear power plants, the reactor containment consists of two

concentric containments [Granger, 2001]. The outer one is designed to resist the external

environmental conditions; however the inner one is used to withstand the designed accidents (140

°C and 5 bars). In most containments the leak‐tightness is rely on a steel liner, but in the French

project, the leakage is controlled by using prestressed concrete wall with thickness of 120 cm. The

wall is designed to remain in compression during the accident. In the inner containment, HPC with

improved microstructure has been used in order to avoid the risk of thermal cracking, shrinkage and

to guarantee high impermeability. For similar application, Debicki et al. studied the leak‐tightness

integrity of containment wall made of HPC without liner under accidental conditions [Shekarchi,

2002; Shekarchi, 2003]. The test was carried out on HPC cylindrical specimens with 1.3 m thickness.

Two accidental conditions were applied; the first was 160 °C and 6.5 bars, and the second was 200

°C and 15 bars. The results indicated that after hydrothermal exposure, the permeability decreased

and no outgoing gas was measured through the specimen. Moreover, the maximum pressure was

found to be located near the heated surface and remains lower than the applied pressure and much


lower than the tensile strength of concrete. The porosity measurements showed that the volume of

fine pores increases and the volume of big pores are decreased with slight increase of the total

porosity of about 2.2 % due to hydrothermal exposure with 200 °C and 15 bars.

In the last few years, several energy storage tanks have been built to store solar energy from

summer to winter time with storing temperature up to 95 °C. Most of these tanks are constructed

with normal concrete and the leak‐tightness is ensured by installing a steel liner. The economic

studies assessed the costs of the liner as about one‐fifth to one‐fourth of the total cost of the

project. In Stuttgart University a dense high performance concrete (HPC) has been developed in

order to be used in seasonal energy storage tanks without internal lining. The developed HPC has

been used in Hannover hot water tank with capacity of 2750 m3 to store water up to 95 °C without

internal steel liner. Jooß et al studied the permeability and diffusivity of the high dense HPC at high

temperature (up to 80 °C). The results showed that the permeability increased by about 62 % with

increasing the temperature from 20 to 50 °C, and by about 55 % with increasing the temperature

from 50 to 80 °C. Diffusivity also was increased by about 10 ‐ 21 % with increasing the temperature

from 20 to 50 °C and by 8 – 21 % by increasing the temperature from 50 to 80 °C [Jooß, 2002].

The behaviour of concrete under hydrothermal conditions is complex because concrete is subjected

to repeated cycles of heating and cooling as well as repeated cycles of vaporization and

condensation (autoclaving cycles). These conditions are similar to well cementing and autoclave

curing of aerated concrete to a large extent. The chemistry of autoclaving is different from that at

normal conditions or at heat only. In the first period of hydrothermal exposure, the strength is

increased due to the additional hydration of cement and pozzolanic materials. The high temperature

and high moisture content of concrete are ideal for creation of additional hydration products and

these in turn lead to microstructure densification as well as porosity and permeability reduction

[England, 1995]. Later, considerable reduction or increase of compressive strength is occurred due

to formation of different C‐S‐H phases depending on the temperature and the chemical composition

of the bulk materials [Mindess, 2003].

Ghosh and Nasser [Ghosh, 1996] carried out a research project in order to study the effect of high

pressure and temperature up to 232 °C on strength and elasticity of fly ash and silica fume concrete.

The results indicated that compressive strength and modulus of elasticity were decreased with

hydrothermal treatment at temperature of 232 °C. On the other hand, Hilsdorf [Hilsdorf, 1986]

studied the hydrated cement system under hydrothermal conditions and noticed a decrease in

strength and increase of porosity in the system of neat OPC. He reported that the loss of strength

can be prevented by addition of fly ash or ground quartz which is contrary to the finding of Ghosh.

Another study has been carried out by Xi et al. [Xi, 1997] on the effect of autoclaving with


temperature of 125 and 175 °C on the properties of OPC and slag cement. The results revealed that

slag cement mix suffered strength loss after it reached the maximum strength. It lost more than 50

% of its original strength. In addition, the porosity was increased and the pore sizes became coarser.

However, the addition of silica fume enhanced the strength and reduced the porosity with

autoclaving. Moreover, the total porosity is reduced, the microstructure became denser and the

average pore radius became finer. These results have been interpreted as follow, the main

hydration product of the system of OPC‐slag is αC2SH and C‐S‐H (I). However, in the system

containing silica fume, the calcium ions is consumed and the pH of the pore solution is reduced. In

addition, the hydration reaction is totally changed and αC2SH is not more found. It was found also

that despite the hydrothermal treatment, the hydration of slag in the system OPC‐slag‐silica fume is

very slow due to the reduced pH value of the system.

2.4.5 Stability of C-S-H phases at hydrothermal conditions

Even though cement and concrete have been studied for several decades, questions still remain

about nature, morphology and interrelationship of hydration products and mechanisms of cement

hydration. Concrete is a heterogeneous material composed mainly of fine and coarse aggregate

bounded together by hydrated cement paste. Cement is the active constituents of concrete, and its

properties largely determine the performance of concrete. The primary constituents of OPC are C2S

and C3S, which make up about 70 ‐ 80 % of OPC [Ramachandran, 2001; Hewlett, 2003; Mindess,

2003; Aïtcin, 2004]. With addition of water at normal temperature, C2S and C3S start to hydrate

according to equation 2.6 and 2.7 [Nelson, 1990; Bažant, 1996; Barnes, 2002; Neville, 2004].

2C3S + 6H C3S2H3 + 3Ca(OH)2 (2.6)

2C2S + 4H C3S2H3 + Ca(OH)2 (2.7)

Under ambient conditions of temperature and pressure, the main product of OPC hydration (more

than 50 %) are the very low crystalline calcium silicate hydrates (C‐S‐H). It is an amorphous phase of

variable composition, and therefore, is usually written as C‐S‐H, which mean that no particular

chemical composition is implied [Taylor, 1997; Barnes, 2002; Mehta, 2006]. The exact structure of C‐

S‐H is not easily determined because it is too irregular and disordered [Neville, 2004]. However, it

can be roughly considered as a layered structure with large surface area, ranging from 100 to 700

m2/g [Mehta, 2006]. The formed C‐S‐H is a strong binding material responsible for strength of

cement based materials at temperature up to 110 °C due to Van der Waals force of attraction and

chemical bond [Noumowe, 2002; Mindess, 2003]. It has very low solubility in water under ambient

conditions, and its specific density varies between 2.3 ‐ 2.6 depending upon its chemical

composition, age, water/solid ratio, and temperature [Mindess, 2003]. The lime/silica ratio (C/S) is


one of the most important characteristics of C‐S‐H gel that governs its stability and performance, in

particular, at high temperature. For OPC, it ranges from 1.5 ‐ 2 depending on the measurement

method [Taylor, 1986; Bažant, 1996; Ramachandran, 2001; Mindess, 2003; Neville, 2004; Mehta,

2006; Lee, 2008]. In addition to the C‐S‐H gel, about 20 ‐ 25 % of the hydration products of OPC is

high crystalline hexagonal plates of calcium hydroxide (CH) with specific density of about 2.24

[Taylor, 1997; Ramachandran, 2001; Hewlett, 2003; Mindess, 2003; Mehta, 2006]. The use of

supplementary materials consumes the CH and reduces the C/S ratio of the formed C‐S‐H depending

upon the replacement level [Meller, 2005].

When exposed to hydrothermal conditions, the microstructure of concrete is subjected to strong

alteration. C‐S‐H gel which is an excellent binding material at ambient conditions becomes unstable

and can be transformed to different phases with lower compressive strength and higher

permeability at high temperature [Jupe, 2008; Meducin, 2008]. C‐S‐H systems are very complex with

more than 30 stable phases [Taylor, 1997; Shaw, 2000; Garbev, 2004]. When C‐S‐H gel of OPC is

subjected to autoclaving with temperature higher than 110 °C in the absence of reactive silica, it is

quickly converted to lime‐rich phases with high crystallinity [Hewlett, 2003; Mindess, 2003]. Phases

such as αC2SH, hillebrandite and ‐tricalcium silicate are the most stable phases in the system with

high lime content. These phases are accompanied by an increase in the density which causes

shrinkage of the solid products and therefore the porosity increases and the strength is markedly

decreased [Taylor, 1997; Glasser, 2003; Mindess, 2003; Luke, 2004]. The loss of strength due to

hydrothermal treatment is commonly known as strength retrogression [Nelson, 1990; Taylor, 1997;

Barnes, 2002; Hewlett, 2003; Meller, 2009]. The lime‐rich phases are detrimental for the mechanical

properties and durability of concrete. The addition of siliceous materials such as fine quartz can

significantly modify the system and change the structure and morphology of the formed phases.

Small amount of silica brings the C/S ratio of the bulk to about 2, which is the ideal for formation of

αC2SH [Taylor, 1997]. However, with large addition of silica, pozzolanic reaction is accelerated,

portlandite is consumed, C/S is considerably reduced and the formation of lime‐rich C‐S‐H phases is

prevented [Nelson, 1990; Glasser, 2003]. Instead, numerous silica‐rich phases such as gyrolite,

xonotlite and 1.1 nm tobermorite are formed. The optimum addition of quartz is the maximum

which can be taken up in the reaction [Taylor, 1997]. It ranges between 35 ‐ 40 % of the binder

depending upon the chemical composition of the bulk materials [Nelson, 1990; Barnes, 2002;

Hewlett, 2003; Mindess, 2003; Ramachandran, 2003; Meller, 2005]. These silica‐rich phases are

generally associated with high compressive strength and lower permeability. With addition of

sufficient amount of silica, 1.1 nm tobermorite is the dominant below 180 °C. However, with higher

temperature it converts to xonotlite with minimal deterioration [Taylor, 1997; Barnes, 2002]. It is

demonstrated that the highest strength is associated with formation of tobermorite and the lowest


with αC2SH formation [Ramachandran, 2001]. Xonotlite gives compressive strength lower than that

of tobermorite by about 20 – 25 %. Truscottite gives even lower strength than xonotlite but its

permeability is lower than xonotlite [Luke, 2004]. With very low C/S ratio, only gyrolite is formed at

200 °C [Meller, 2005; Meller, 2009]. The phase transformation under hydrothermal conditions can

be written as in the following equations:

C-S-H gel + CH C-S-H Phases (high C/S) ( 2.8)

C-S-H gel + CH + Silica C-S-H phases (low C/S) (2.9)

Tobermorite can be considered as one of the most important phases of cement hydration products

at normal and high temperatures. It can be classified into three types: 1.4 nm, 1.1 nm, and 0.9 nm.

1.4 nm tobermorite is formed below 80 °C, whereas 1.1 nm tobermorite is built in water at

temperature greater than 100 °C [Fujii, 1983; Shaw, 2000; Garbev, 2004]. It decomposes to 0.9 nm

tobermorite with heating at 300 °C [Shaw, 2000]. Several approaches have been carried out to

clarify the process and mechanism of tobermorite formation [Tsuji, 1991; Luke, 2004; Meller, 2005;

Meller, 2009; Kikuma, 2011; Matsui, 2011]. It is strongly affected by maximum temperature,

reactivity and amount of added silica, the addition of aluminium compounds and the addition of

alkali [Matsui, 2011]. The high reactive amorphous silica supports the formation of tobermorite by

enhancing the polymerization of C‐S‐H [Matsui, 2011]. The optimum addition of silica promotes the

formation of C‐S‐H with C/S ratio of 0.9 ‐ 1, which tends to crystalize to 1.1 nm tobermorite. At

higher temperature, tobermorite can be converted to xonotlite with volume reduction [Taylor,

1997; Jupe, 2008]. This volume reduction can be prevented by the addition of some aluminium,

which extends the stability of 1.1 tobermorite at higher temperature. The addition of aluminium

reduces the quartz solubility and thereby accelerates the formation of tobermorite and retards the

xonotlite formation [Taylor, 1997]. It is thought that aluminium ions may enter the lattice of the

tobermorite in expense of silicon ions and affect its properties and crystallinity [Kalousek, 1957;

Beaudoin, 1979; Jupe, 2008]. On the other hand, quartz reactivity has a significant role in

tobermorite formation. When reactive quartz is used as a source of silica, in the early stage

tobermorite is crystallized rapidly, but at late stage the rate of crystallization of tobermorite is

significantly reduced. However, when less reactive quartz was added, the formation of tobermorite

is continuing until reaching the equilibrium state. The final content of tobermorite is much more

than that in the case of reactive silica. However, in all cases at least about 10 % of the added quartz

does not react at all [Siauciunas, 2004].

The reaction of silica with lime is a pozzolanic reaction which is accelerated with autoclaving. So,

pozzolanic materials such as fly ash and silica fume could be used as sources of silica. By the use of


pozzolanic materials, portlandite is consumed during the pozzolanic reaction, and its absence is

advantageous for concrete from the viewpoint of aggressive attack of chemicals [Jupe, 2008]. In

addition, high pozzolanic materials help to prevent the formation of lime‐rich phases such as alpha

dicalcium silicate because of the high content of silica. Luke studied the influence of hydrothermal

conditions at 180 °C on various systems containing OPC, class C and F fly ash, silica fume and quartz

flour [Luke, 2004]. The results showed that with addition of 35 % of class C fly ash, αC2SH with C/S

ratio of about 1.92 is the predominant phase, and therefore, very low compressive strength is

obtained. However, replacing OPC with 35 % class F fly ash reduced the C/S ratio to 1.38 and at this

ratio αC2SH formation competes with the formation of silica‐rich phases such as poorly crystalline

tobermorite and hydrogrossular‐type phases. As a result, the compressive strength is relatively low;

however it is higher than that of 35% class C fly ash. In the case of silica fume addition (35 % of

cement), very low compressive strength of the products is observed. In this system, despite of the

high silica content in the system, the predominant phase is αC2SH. In addition, a number of different

metastable phases are formed, most of which are undefined. On the other hand, by addition of 35 %

quartz flour to cement, xonotlite was the major phases in the system and the compressive strength

was higher than fly ash and silica fume cases. With a combination of 30 % class F fly ash and 35 %

quartz flour, the bulk C/S ratio of the system was 0.64. At this ratio, the predominant phase is the

highly crystalline 1.1 nm tobermorite which is accompanied with high compressive strength.

However, combination of 35 % quartz and 30 % silica fume resulted in quite increase in compressive

strength which equals about 50 % of the cement‐class F fly ash‐quartz system.

Meller et al. carried out several researches to study the system of cement, silica, alumina and water

at different temperatures up to 350 °C [Nelson, 1990; Meller, 2005; Meller, 2007]. They developed a

diagram for the Cao‐Al2O‐SiO2‐H2O system (CASH) based on Taylor work (Figure 2.13). The results of

their work revealed that under hydrothermal condition at 200 °C, pastes made of neat Portland

cement exhibited portlandite and lime‐rich phases such as αC2SH and jaffeite which are deleterious

to compressive strength. With small addition of silica and alumina (< 25 %), the C/S ratio was still

high thereby hillebrandite and α C2SH are formed. However, at higher temperature these phases are

not found because they are replaced by reinhardbraunsite. With increasing silica addition to 35 %,

the C/S ratio is reduced to about 1 which promoted the formation of silica‐rich phases in expense of

lime‐rich ones. 1.1 nm tobermorite is formed at temperature higher than 110 °C, and transformed

to xonotlite or gyrolite at about 150 °C depending upon the quantity of the added silica. With

addition of 20 ‐ 50 % silica to the system at 200 °C, 1.1 tobermorite coexisted with xonotlite. The

addition of more quartz leads to formation of gyrolite and the excess quartz is remained with it.

Tobermorite decomposes normally at 150 °C to form xonotlite and gyrolite, however the addition of

aluminium stabilized the tobermorite at higher temperature up to 250 °C. When silica addition

Literatur

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Figure 2[Meller, 20

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improve

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36

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37

ength and

Materials and methods 39

3. Materials and methods

3.1 Introduction

To achieve the objectives of this investigation, the research has been divided into three parts:

1. Optimizing a dense high performance concrete mixture.

2. Study the effect of hydrothermal conditions on the properties of densely packed concrete.

3. Studying the influence of autoclaving on the properties of cement paste.

For the first two parts, twenty three concrete mixes have been prepared, cured, and tested. All

these mixes have approximately similar aggregate grading and content, whereas the differences are

only in the cementitious materials composition and water/binder ratios. The experimental work

includes the measurements of properties of fresh concrete. In addition, durability and mechanical

properties were measured at ambient conditions and after hydrothermal exposure (200 °C and 15.5

bars) for several autoclaving cycles, up to 50. To study the effect of hydrothermal treatment on C‐S‐

H stability and properties, 10 cement pastes have been prepared and tested. Details of the

compositions and tests of these pastes are presented in chapter 7.

3.2 Materials

3.2.1 Aggregates

Natural quartzite aggregates with maximum aggregate size of 16 mm has been used in this

investigation. The coarse and fine aggregate were dried until constant weight and then stored in the

room temperature before use. A mean specific density and water absorption of 2.61 and 0.5 % were

obtained for the used aggregates respectively. Three aggregate classes were used in the experiments:

0 ‐ 2, 2 ‐ 8, and 8 ‐ 16 mm. The sieve analysis of the used aggregate is presented in Figure 3.1.

Figure 3.1: Sieve analysis of the used aggregate

0

10

20

30

40

50

60

70

80

90

100

0.125 0.25 0.5 1 2 4 8 16 32

Cumulative passing %

Sieve size mm

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Aggregate 2‐8

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Figure 3

Table 3.

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CEM III/B

Table 3.

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Hackley, 2004

size distribu

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gO Na2O

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4]. In additio

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to the prod

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Se

3.17

3.12

3.01

2.96

e been used

32.5 R and C

e used (CEM

h slag conten

re shown in

experimenta

rent types o

on, the spec

materials

ucers)

O3

70

0

4

49

etting time (

Initial

105

140

235

285

40

d. Ordinary

CEM I 42.5

M III/A 32.5

nt of about

n Table 3.1

ally in the

of cements

ific density

(min)

Final

180

210

285

340


Figure 3.3: Particle size distribution of the used cements measured with laser granulometry

3.2.3 Fly ash

Fly ash is a waste product, remaining from the combustion of hard coal for energy production

purposes. Around 18 million tons of hard coal fly ash is produced in Europe yearly. Most of it is

consumed by the building materials sector, where it is used as a major ingredient for the cement

production or as additive material for the concrete production [Hunger, 2010]. In this research, Class

F fly ash according to DIN EN 450‐1 has been used. Three different sizes of fly ash have been used in

this investigation; normal fly ash, fine fly ash (M20) and fine fly ash (M10) with mean particle sizes of

17, 9, and 5 µm respectively. Table 3.3 shows the chemical composition of fly ash used in this

investigation according to the producers. However, physical properties of fly ash have been

determined experimentally. Table 3.4 shows the measured characteristics of different types of fly

ash. Figure 3.4 shows the particle size distribution of the used fly ash measured by laser granulometry.

In addition, SEM measurement of fine materials has been performed as can be seen in Figure 3.5.

Figure 3.4: Particle size distribution of the used fly ash

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

Cumulative passing %%

Size (µm)

CEM I 32.5 RCEM I 42.5 NCEM III A 32.5CEM III B 32.5

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

Comulative passing %

Size (um)

M10M20FA

Material

Table 3.

M10

M20

Fly ash

Table 3.

M10

M20 Fly ash * Without

Figure 3

ash (M2

3.2.4 S

Silica fu

element

extreme

grade 9

properti

the part

ls and metho

.3: Chemica

CaO

6

5

3.1

.4: Physical

Blai

superplasticize

3.5: Scannin

20), c) fine fl

Silica fume

me is very

tal silicon o

ely small par

71 accordin

es of the us

icle size dist

ods

al compositi

O SiO2

53

52

49.2

properties

ne Fineness

6400

6000 2877

r according to P

ng electron

ly ash (M10

e

fine non‐cr

r alloys con

rticle size, a

ng to DIN

ed silica fum

ribution of t

on of the us

Al2O3 Fe

25

25

27.6 7

of the used

s (cm2/g)

Puntke [Puntke,

microscopy

0), d) silica f

rystalline m

ntaining silic

nd large sur

EN 13263‐1

me are given

the used silic

sed fly ash (

e2O3 MgO

6 1

6 1

7.6 2.1

fly ash

Water dem

22

220

, 2002].

y (SEM) of f

fume

aterial prod

con. It is a

rface area. I

1 has been

n in Table 3.5

ca fume.

a

c

(according t

O Na2O K2

1.5 1

1.5 1

0.90 5

mand (%)*

2.1

220.8

fine materia

duced as a

morphous m

In this expe

n used. Che

5 and Table

to the produ

2O SO3

1.9 1.1

1.9 1.1

5.0 0.7

Specific d

als particles

by‐product

material wit

rimental wo

emical comp

3.6 respecti

ucer)

density (g/cm

2.49

2.45 2.29

: a) fly ash,

of the pro

th high SiO

ork, Elkem m

position an

ively. Figure

b

d

42

m3)

b) fine fly

duction of

O2 content,

micro silica

d physical

3.6 shows


Table 3.5: Chemical properties of silica fume (according to the producer)

CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 Cl

0.2 98.4 0.2 0.01 0.10 0.15 0.20 0.10 0.01

Table 3.6: Physical properties of silica fume

Fineness cm2/g* Water demand (%)** Density (g/cm3) Bulk weight (kg/m3)*

200000 51.8 2.2 500

* Densified according to Manufactures datasheet, ** Measured without superplasticizer according to Puntke [Puntke, 2002]

Figure 3.6: Particle size distribution of silica fume

3.2.5 Filler

In this research, the term filler mean the powder materials which applied in concrete to achieve high

packing density, it is almost non‐reactive at ambient temperature. In this investigation, two different

types of commercially available quartz filler which meet the requirements of DIN EN 12620 were

used: quartz powder and quartz sand. The mean particle size of quartz sand (QS) and quartz powder

(QP) are about 180 and 60 um respectively. Chemical composition of the used fillers can be found in

Table 3.7. However, physical properties and particle size distribution have been experimentally

determined as can be seen in Table 3.8 and Figure 3.7 respectively.

Table 3.7: Chemical composition of quartz sand and quartz powder (according to the producer)

CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O

Quartz sand 98.5 0.7 0.06

Quartz powder 0.013 99.61 0.11 0.012 0.004 0.006 0.024

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100


Particle Size (um)


Table 3.8: Physical properties of quartz sand and quartz powder

Blaine Fineness (cm2/g) Water demand (%)* Density g/cm3

Quartz sand 760 ‐ 2.67

Quartz powder 2683 24.4 2.69

*Without superplasticizer according to Puntke [Puntke, 2002]

Figure 3.7: Particle size distribution of quartz sand (QS) and quartz powder (QP) measured with

laser granulometry

3.2.6 Superplasticizer

In this investigation, to achieve the satisfied consistency class, superplasticizer (Muraplast FK 63.3) has

been used. The used superplasticizer is polycarboxylate‐based (PCE), as that type proved to be very

effective in densely packed systems with fine particles [Hunger, 2010], and it complies with DIN EN

934‐2. It has a density of about 1.06 g/cm3, and the recommended dosage is 2 ‐ 50 g per kg of cement.

3.3 Specimens preparation and curing

The concrete was mixed in a mixer with capacity of about 60 liters. In order to enhance the

distribution of all particles and make the mixture more homogenous, the sequence of the addition

of concrete constituents into the mixer has been carried out according to the following diagram

(Figure 3.8). The mixing procedures for all mixes were performed in the same manner. After mixing,

fresh concrete properties were measured. Then, the moulds were cast and compacted to remove

any entrapped air and to ensure full compaction according to DIN EN 12390‐2. A number of test

specimens, mainly 100 mm cubes and 150 x 300 mm cylinders were prepared from each mix for

different tests according to DIN EN 12390‐1. After 24 hours of adding the water to the mixture, the

test specimens were demoulded. The curing process took place in water basin at temperature of 20

± 1 °C until the day of testing.

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000


Particle Size (um)

QP

QS

Material

Addit

p

Additio

Additio

Figure 3

3.4 Te

3.4.1 F

3.4.1.1

In this in

It is basi

manner

of the di

Figure 3

ls and metho

tion of aggre

powder and

1/2 minu

on of part of

mixing for

on of silica fu

3.8: The seq

ests

Fresh conc

Consisten

nvestigation,

cally a meas

for 15 times

iameter from

3.9: Determin

ods

egates and q

quartz sand

te mixing

f mixing wat

1/2 minute

ume, M10 an

quence of ad

crete prope

ncy

, consistency

surement of

s in around

m 3 different

ning the con

quartz

d

ter and

nd M20

ddition of co

erties.

y was measu

f the spread

30 seconds.

t directions

crete consis

oncrete con

ured using a

of a fixed q

The mean d

as can be se

stency using

Ad

Add

stituents in

a flow table t

quantity of c

diameter is c

een in Figure

g flow table te

1/2 min

ddition of ce

1/2 min

dition of res

and sup

Mixing f

the mixer a

test accordi

oncrete afte

calculated fr

e 3.9.

est accordin

nute mixing

ement and f

nute mixing

sidual mixin

perplasticizer

for 2 minute

and the mixi

ng to DIN EN

er jolting in

rom the mea

ng to DIN EN

45

fly ash

g water

r

es

ng time

N 12350‐5.

a standard

asurement

N 12350-5


3.4.1.2 Air content

The air content of the fresh concrete mixes has been determined according to DIN EN 12350‐7. In

this test, a standard five liters steel cylinder is fully filled with fresh concrete and compacted. Then,

the cover is fixed and the pressure is applied on to the surface of concrete in the bowl, causing a

change in the volume. This change is attributed to the air in the concrete since other constituents in the

bowl are incompressible. The air content of concrete is then directly read from the scale on the cover.

3.4.1.3 Fresh density

The fresh concrete density has been measured as specified in DIN EN 12350‐6. A five liters bowel is

filled and compacted until no large bubbles of air appear on the surface. After compaction, the top

surface of concrete shall be struck off and finished smoothly. All excess concrete shall then be

removed and the filled bowel is weighed. The density of fresh concrete is the weight of net concrete

divided by the volume of the bowel.

3.4.2 Mechanical properties.

3.4.2.1 Compressive strength

The compressive strength of concrete was determined using 100 mm cubes at ages of 28 and 91

days according to DIN EN 12390‐3. A modern digital crushing machine (Toni Technik) was used. After

curing period, the specimens were dried at room temperature (20 ± 1 °C). Then, compressive strength

test was carried out. Three cubes of each mix were tested and the mean value was considered.

3.4.2.2 Splitting tensile strength

The splitting tensile strength test was applied on cylinders (150 mm x 300 mm) according to European

Standard DIN EN 12390‐6. The following formula was used to determine the splitting tensile strength.

s

t

ld

F

2

(3.1)

Where: Ft is the force in KN, L is the specimen length in mm and ds is the specimen diameter of in mm.

3.4.2.3 Modulus of elasticity

The modulus of elasticity of concrete has been determined on cylinders (150 mm x 300 mm)

according to European standard EN 1048‐5. Concrete samples were prepared and cured under

water until the testing day. At age of 91 days, the specimens were dried at room temperature (20 ±

1 °C). After that, the static modulus of elasticity was determined (Toni Technik) as can be seen in

Figure 3.10.

Material

Figure 3

3.4.2.4

The den

volume o

Where M

3.4.2.5

The rebo

concrete

compres

surface w

were fix

samples

influence

ls and metho

3.10: Determ

Density of

sity of harde

of about 1 li

Mc is the ma

Rebound

ound hamm

e. By refere

ssive strengt

which shoul

ed in the m

before and

e of hydroth

ods

mination of m

f hardened

ened concre

ter. The foll

cc V

M

ss of concre

Hammer

mer accordin

nce to the c

th. When co

ld be flat an

achine as ca

d after auto

hermal cond

modulus of

concrete

ete has been

owing formu

c

c

V

M

te sample (k

ng to DIN E

conversion c

onducting th

nd smooth. I

an be seen i

oclaving for

itions on co

elasticity of

n determined

ula has been

kg) and V is t

EN 12504‐2

chart, the re

he test, the h

t has been u

in Figure 3.1

5, 15, and

ncrete hard

f concrete a

d according

n implement

the volume

has been u

ebound valu

hammer sho

used perpen

11. The mea

50 cycles in

ness.

ccording to

to DIN EN 1

ted to calcul

(3

of concrete

used to mea

ue can be u

ould be held

ndicularly to

asurement w

n order to e

DIN 1048-5

2390‐7 on a

late the den

3.2)

specimen (m

asure the ha

used to dete

d at right ang

o the specim

was done for

exactly dete

47

5

a cube with

sity:

m3).

ardness of

ermine the

gles to the

mens which

r the same

ermine the

Material

Figure 3

hammer

3.4.3 D

3.4.3.1

Permeab

Numero

concrete

to DIN E

of 5 bars

of penet

the split

Figure 3

ls and metho

3.11: Determ

r) according

Durability o

Water per

bility is the

us test meth

e [Dinku, 199

EN 12390‐8 w

s to the surf

tration of th

surface of t

3.12: Determ

ods

mination of

g to DIN EN

of concrete

rmeability

ease with w

hods have b

96]. In this i

was used. T

face of harde

he waterfron

the tests spe

mination of w

hardness o

12504-2

e

which liquid

been develo

investigation

he basic pri

ened concre

nt is measur

ecimen (mois

water penet

of concrete u

ds or gases

ped and are

n, the water

nciple of thi

ete for 72 ho

red. The dep

st concrete

tration depth

using non-d

can travel t

e in use to e

r penetration

is test is tha

ours. The spe

pth of penet

being darke

h of concret

destructive t

through con

valuate the

n depth und

at water is a

ecimen is th

tration is fo

r as can be s

te according

test method

ncrete [Nevi

water perm

der pressure

pplied unde

en split and

und by obse

seen in Figur

g to DIN EN

48

d (rebound

ille, 2008].

meability of

e according

er pressure

d the depth

ervation of

re 3.13).

N 12390-8

Material

Figure 3

3.4.3.2

In this in

Because

dried in

tempera

and 20 m

specime

permeab

Where:

height o

dynamic

Figure 3

ls and metho

3.13: Determ

Air perme

nvestigation,

of the impo

n the oven

ature and sen

mm thicknes

n. The time

bility coeffici

K

Kair is air pe

f the concre

c viscosity of

3.14: Determ

ods

mination of t

ability

, the air perm

ortant effect

until cons

nt to air per

ss is fixed in

at which a u

ent can be o

tA

VK f

air .

rmeability c

ete sample (

f air in N.s/cm

mination of a

the water pe

meability of

t of moisture

stant weigh

meability de

n the device.

unit volume

obtained usin

pt

h

a .

..

coefficient (c

cm), A: cros

m2.

air permeab

enetration d

concrete ha

e content on

t. Then, th

evice. In this

. Air flow un

of the gas p

ng the Hagen

cm2), Vf: flow

s section are

bility of conc

depth (moist

as been mea

n air permea

he specimen

test, a conc

nder pressur

passes throu

n‐Poiseuille f

w volume (c

ea of the sa

crete accord

t concrete b

asured accor

ability, the sa

ns were co

crete specim

re is applied

gh the speci

formula [Din

cm3), ta: pass

mple (cm2),

ding to TGL

being darker

rding to TGL

amples have

ooled down

en of 50 mm

on one surf

imen is mea

nku, 1996]:

sing time (se

p: pressure

21094-12

49

r)

21094‐12.

e been first

n to room

m diameter

face of the

asured. The

(3.3)

econds), h:

N /cm2, η:

Material

3.4.3.3

A standa

water is

containe

pulls wa

determin

the incr

coefficie

Figure 3

15148

3.4.3.4

The test

chloride

or morta

applicati

determin

penetrat

diffusion

depth of

The follo

ls and metho

Absorption

ard method

included in

er of water s

ater up into

ned over va

rease in ma

ent of concre

3.15: Determ

Chloride d

t to determ

to be deter

ar specimen

ion, the spe

ned by mea

tion depth,

n coefficient

f chloride th

owing equat

ods

n

of evaluatin

n DIN EN ISO

such that th

o the specim

rious period

ass versus

ete can be ca

mination of

diffusion tes

mine the ch

rmined with

under the i

cimen is spl

ns of indicat

the applie

t was meas

rough the co

ion was use

ng the water

O 15148. Th

e immersion

men is the c

ds of time u

the square

alculated.

water abso

st

loride migra

in a short p

influence of

lit and the p

tor solutions

ed voltage,

sured accor

oncrete spe

d to determ

r uptake of a

e method in

n depth of t

capillary suc

p to 24 hou

root of tim

rption coeffi

ation coeffic

eriod of tim

f an electrica

penetration

s. The chlori

and other

ding to BAW

cimen was d

mine the diffu

a concrete sa

nvolves plac

the specime

ction. The in

rs. Then a s

me. From t

icient of con

cient enable

me. The pene

al field is exa

depth of the

ide diffusion

parameters

W Merkblat

determined

usion coeffic

ample which

cing a concre

n is 5 mm. T

ncrease of t

straight line

this relation

ncrete acco

es the resis

etration of c

amined. At t

e front of th

n coefficient

s. In this re

tt [BAW, 20

after 7 days

cient:

h partially im

ete specime

The primary

the specime

is fitted to t

nship, the a

ording to DIN

stance of co

chlorides into

the end of t

he free chlor

is calculate

esearch, th

012]. The p

s of beginnin

50

mmersed in

en within a

force that

en mass is

the plot of

absorption

N EN ISO

oncrete to

o concrete

the voltage

ride ions is

d from the

e chloride

enetration

ng the test.

Material

With

Where:

Dcl: Z: F: U: R: T: h: xd: t: erf ‐1: cd: co:

Figure 3[BAW, 20

3.4.4 C

3.4.4.1

The effe

water di

weight s

constant

ls and metho

2

migration cvalency, forFaraday conabsolute pogas constanabsolute mheight of thmean peneduration ofinverse errochloride cochloride co

3.16: Determ12]

Concrete m

Effective p

ective porosi

isplacement

should be m

t weight. Fr

ods

coefficient (mr chloride ionstant, F = 9otential diffent, R = 8.315ean temperhe test specitration deptf the test (s) or function ncentration ncentration

mination of c

microstructu

porosity

ity which is

t. The sampl

easured. Th

om the kno

12

m2/s) ns z = 1 9.649*104 J.erence (V)5 J* (K*mol)‐

ature of themen (m)th of the chl

at which thof the potas

chloride diff

ure

accessible b

es must be

hen, the satu

owledge of t

(Volt.mol)‐1

1

e solutions d

oride ions in

e colour chassium hydro

fusion coeffi

by water has

saturated u

urated samp

the weight i

uring the te

n each half o

anges, cd = 0oxide solutio

cient

s been meas

under water

ples are dried

in both dry

(3

(3

st (K)

of the test sp

0.07 (mol.l‐1)on (mol.l‐1)

sured using

at least for

d in oven at

(mdry) and s

3.4)

3.5)

pecimen (m)

)

the simple

24 hours an

t 105 °C with

saturated st

51

)

method of

nd the wet

h dry air to

tates (msat)

Material

and the

equation

3.4.4.2

The heliu

inside th

Concrete

concrete

pycnome

the open

Where ε

sample b

Figure 3

3.4.4.3

The prin

the pore

ls and metho

weight und

n:

o

Helium py

um pycnom

he pores is s

e specimen

e pieces we

etry (microm

n porosity ca

ε is the tota

by its volum

3.17: Helium

Mercury in

nciple of this

es of a dried

ods

der water (m

sat

sat

m

m

ycnometry

etry is a dev

substituted

is dried at

ere finely gr

metrics) to

an be calcula

(1

al porosity,

e), ρs is the s

m pycnometr

ntrusion po

s technique

d and evacu

msub), the ef

sub

dry

m

m

vice used to

by helium a

105 °C with

round (< 0.

measure dir

ated by the f

)s

b

ρb is the b

specific den

ry device us

rosimetry (

is based on

ated porous

ffective por

measure th

and the amo

h dry air to

063 mm). T

rectly the sp

following eq

ulk density

sity, (determ

sed to meas

(MIP)

n penetratin

s medium. T

osity (εo) ca

e specific de

ount of the i

constant w

Then the sa

pecific dens

quation:

(calculated

mined by usi

sure the spe

g a non‐rea

The dried sa

an be calcul

(3

ensity of por

inert gas is d

weight. Samp

amples wer

ity. By know

(3

by dividing

ng helium p

ecific densit

cting liquid

mple is plac

ated by the

3.6)

rous materia

detected [Kr

ples of abou

e sent to t

wing the bu

3.7)

g the dry m

pycnometer)

ty of concret

such as me

ced into a d

52

e following

als. The air

rus, 1997].

ut 5‐7 g of

the helium

ulk density,

mass of the

.

te

ercury, into

ilatometer

Material

and evac

(Porosim

used to

expresse

Where:

applied

measure

Figure 3concrete

3.4.4.4

It is a the

measure

phenom

provide

ls and metho

cuated. The

meter 2000 W

calculate th

ed as:

r(

r is the rad

pressure (b

ements proc

3.18: Mercue

Thermal g

ermal analys

ed as a func

mena, such a

informatio

ods

pressure is

WS ) where

he porosity.

mp

cos..2(

ius of the in

bar), and θ

cedures can

ry intrusion

gravimetric

sis method i

ction of inc

as phase tra

on about c

then raised

the pressur

The relation

)

ntruded por

is contact a

be found in

porosimetr

analysis (T

in which cha

creasing tem

ansitions inc

chemical ph

to atmosph

re is monito

nship betwe

re (nm), γ is

angle betwe

[Gluth, 2011

ry to measu

TGA)

anges in phy

mperature. T

cluding vapo

henomena

eric pressure

ored automa

een the pore

surface ten

een mercur

1].

ure the poro

ysical and ch

TGA can pro

orization an

including c

e before it is

atically. The

e size and th

(3.8)

nsion of me

ry and the

osity and po

emical prop

ovide inform

nd desorptio

chemisorpti

s fitted to th

Washburn e

he exerted p

rcury (N/m)

pore walls.

ore size dist

perties of ma

mation abou

on. Likewise

on dehydra

53

he machine

equation is

pressure is

), pm is the

Details of

tribution of

aterials are

ut physical

e, TGA can

ation and

Material

decomp

and the

measure

(TherMa

and afte

Figure 3

ls and metho

osition. As t

weight pe

ement proc

ax 700) is us

r autoclavin

3.19: Therm

ods

the tempera

ercentage of

edures can

sed to meas

ng for 50 cyc

ogravimetry

ature increa

f each resu

be found

ure the CH

les.

y to measur

ses, various

ulting mass

in [Ramac

content of d

re the weigh

component

change can

chandran, 2

different cem

ht loss of co

ts of the sam

n be measu

2001]. In th

ment pastes

oncrete sam

mple are de

ured. More

his investiga

s at normal

ple with hea

54

ecomposed

details of

ation, TGA

conditions

ating

Optimizing a high dense concrete mixture 55

4. Optimizing a high dense concrete mixture

4.1 Introduction

When a concrete wall subjected to hydrothermal cycles, water and vapour try to penetrate the wall.

The penetration rate depends mainly on the temperature and pressure of the water and vapour as

well as on the microstructure, durability and porosity of concrete. The aggregate, the matrix, and

the interface between them affect concrete porosity, microstructure and durability. The matrix

depends not only on water/cement ratio but also on the granulometry and reactivity of the

cementitious material. Therefore, in this research, concrete mixture optimization is primarily based

on three fundamental mechanisms:

1‐ Maximizing the packing density of solid particles.

2‐ Designing a dense cement matrix.

3‐ Densifying the interfacial transition zone.

Unlike normal concrete, it is no longer sufficient to base the mix composition on the principle of

compressive strength and w/c ratio relationship. For the sake of developing high dense concrete,

the above mechanisms has been optimized. In this chapter, detailed description of these

mechanisms and their influence on concrete performance and properties will be introduced.

Moreover, explanation of how these mechanisms can be optimized will be given. In addition, the

design and composition of 23 different mixes will be presented. The packing density of all concrete

mixes has been calculated using Rene LCPC software which based on the compressible packing

model (CPM). For these calculations, the packing density of all concrete constituents has been

measured and used as input for this model. At the end of this chapter, the measured properties of

fresh concrete including workability, air content and density will be presented and discussed.

4.2 Maximizing the packing density of solid particles

Excellent concrete can be made with a wide range of aggregate grading. Obviously, there are limits

of grading outside which it is not possible to make good concrete, but by choosing homogeneously

graded mixtures, it is possible to achieve the desired concrete mix within the desired borders. The

concrete mixture volume composed generally of about 65 – 80 % of aggregate and about 15 – 30 %

of water and binder [Rached, 2009]. The importance of aggregate gradation can be clarified by

considering concrete as a softly compacted cluster of aggregate particles bonded together with

cement paste [Mindess, 2003]. The properties of aggregate have crucial effects on the performance

of fresh and hardened concrete. The chemical composition of the aggregate is usually less important

than the physical characteristics, such as volume, shape, size, and grading. Therefore, the aggregate

should be chosen mainly on the base of its physical properties and to a lesser extent on its chemical


characteristics. The proper sieve distribution and packing of aggregate particles, together with other

particles in the mix, is the direct and systematic way to ensure the high density of hardened

concrete. Spherically shaped grains are considered as more advantageous compared to the angular

shaped aggregates. The porosity increases from 40 to 60 % when the angularity factor of aggregate

is doubled [Nehdi, 1998]. Round materials like gravel give a denser concrete than broken stone

[Fuller, 1907]. The maximum grain diameter should be accurately chosen to improve workability and

to reduce stress concentrations. Increasing the maximum aggregate size is considered as an

advantage for some concrete properties and a disadvantage for some others. Increasing the

maximum aggregate size reduces the paste requirement for the mix. Moreover, the smaller the

average size of aggregate, the lower the packing of aggregate [Powers, 1968; Mindess, 2003].

However, large aggregate sizes increase the internal stresses and this tends to reduce the strength.

Nevertheless, Larrard stated that with an aggregate size of 25 mm it is possible to produce high

performance concrete with compressive strength up to 130 MPa at 28 days [Larrard, 1999]. Results

of studies on the effect of the maximum aggregate size on the strength of HPC are mixed. Large size

aggregates can reduce the water demand, which leads to a considerable increase in concrete

strength. On the other hand, the use of small size can eliminate internal defects in aggregate

particles and results in a smaller cement paste/aggregate transition zone, which enhances the

concrete mechanical properties. However for HPC, the aggregate/cement paste bonding must be

strong enough to transfer significant stress to the aggregate. The failure going through the

aggregate particles are commonly observed on the surfaces of HPC specimens. Hence, the aggregate

can become the weak link, and may represent the limiting factor for the strength of the concrete.

On the other hand, concrete durability can also be improved with increasing the maximum aggregate

size because there will be less paste subjected to the chemical or physical attack [Mindess, 2003].

In this research, the concrete mixture proportioning is based on the granular optimization of solid

content by means of the Ideal Fuller curve (Figure 4.1). Compared to the well‐known Fuller parabola

for the highest density of aggregate, the Ideal curve has a higher content of fines and a lower coarse

aggregate content as can be observed in Figure 4.1. The packing density and the concrete mixture

proportioning will be calculated on the basis of the grading of all concrete constituents, not only the

aggregate grading. Brouwers stated that whilst the packing of the aggregate plays a major role, the

packing of all solid particles in the concrete mix should be the basis for the mix design [Brouwers,

2005]. In this investigation, in order to reduce the water demand and cement content, and at the same

time to hold the micro‐cracks as low as possible, the maximum grain size was taken as 16 mm. According

to the Ideal Fuller curve, the required aggregate volume (d > 125 µm) is 85.13 %, whereas the fine

materials content (d < 125 µm) is 14.87 % by volume.


Figure 4.1: The grading curve of aggregates and binder according to Ideal Fuller curve According to [Fuller, 1907]

Figure 4.2: Comparison of Ideal Fuller with the standard grading lines of aggregate According to DIN 1045-2

Figure 4.2 shows the grading of aggregate according to Ideal Fuller curve compared to standard

areas A, B, and C according to DIN 1045‐2. It is clear that, the curve has a little bit higher coarse

aggregate content than the grading line A (from 4 to 16 mm), however, at lower sizes, the curve has

higher fine material content (< 2 mm). These fines are needed to maintain good cohesion and to

prevent segregation. In the fresh state, high powder content is necessary for a stable fresh concrete

that does not segregate during installation and in which the coarse aggregate does not settle after

compaction [Nischer, 2007]. The better the particle size distribution of concrete mixture, the lesser

the required powder for a stable fresh concrete. The increase in powder will improve the stability of

fresh concrete only when it does not adversely alter the particle size distribution in the powder

9.53 12.0114.87

18.5423.29

29.1435.22

44.47

62.98

4.49 6.25 8.8412.5

17.6825

35.36

50

70.71

100

0

10

20

30

40

50

60

70

80

90

100

0.03125 0.0625 0.125 0.25 0.5 1 2 4 8 16

Passing % (volume)

Size (mm)

Ideal Fuller curve

Fuller parabola

88

74

62

49

34

18

7

76

56

42

32

20

85

60

36

2112

832

100

3030

30

56.5

34.77

23.916.769.9

4.3130

10

20

30

40

50

60

70

80

90

100

0.0625 0.125 0.25 0.5 1 2 4 8 16

Passing % (Volume)

Size (mm)

C

B

A

U16

Ideal Fuller


range. However, high amounts of particles smaller than 0.001 mm (e.g. micro silica) in concrete

frequently do not lead to an improvement in fresh concrete stability, because in this case, large

amount of water and/or superplasticizer is required.

In this investigation, to get a good size distribution, the skeleton of aggregate size fractions should

be viewed as a whole rather than two separate entities; coarse and fine aggregate. The combination

of well graded coarse aggregate and well graded fine aggregate considered separately does not

necessarily result in a well graded aggregate mix. Coarse and fine aggregate as they come from the

quarry do not normally have size distributions that fit the dense packing curve. So, the addition of

“correcting” aggregates could help to reduce excesses or deficiencies in some sizes [Quiroga, 2003].

Because of the bad grading of the natural aggregates, it is difficult to achieve the highest packing

density as can be seen in Figure 4.3. It is obvious on the curve that there are some gaps between the

concrete mixture and the Ideal Fuller curve. Sometimes the mixture is higher as in the range of 0.5 ‐ 2

mm, therefore in the mixes preparation, sieve analysis of aggregate was done and the required amount

of each size has been taken. On the other hand, the mixture has lower content of particles in the size

range 0.063 ‐ 0.25 mm, therefore, quartz sand and quartz powder has been added to close this gap.

Figure 4.3: The grading curve of non-optimized concrete mixture and the Ideal Fuller curve

4.3 Designing a dense cement matrix

4.3.1 Introduction

Concrete can be considered as a composite material whose microstructure is randomly arranged.

It consists mainly of aggregate and cement paste and in between the interfacial transition zone

0

10

20

30

40

50

60

70

80

90

100

0.03125 0.0625 0.125 0.25 0.5 1 2 4 8 16

Passing % (volume)

Size (mm)

Ideal Fuller curve

Mixture without optimization


(ITZ). Similarly, cement matrix can also be considered as a random composite material, made up

of unreacted cement, C‐S‐H, CH, capillary pores, and other chemical phases [Hilsdorf, 1995].

Properties of cement matrix, particularly porosity and microstructure have a great influence on

the durability and strength of concrete. All transport processes depend primarily on the properties

and structure of the hydrated cement paste [Neville, 2004], which are related mainly to its

porosity and pore size distribution. The latter relies on the packing density of the cement particles

and fine materials as well as on the filling ability of the hydration products. The reduction in pore

size results in a significant decrease in transport properties like water permeability and ionic

diffusion, which are often the rate limiting properties in chemical attack. In addition to the

durability, the strength of concrete is fundamentally a function of the volume of voids in it

[Neville, 2004]. The strength of the cement matrix is governed by the packing density and the

homogeneity of the cement particles and fine materials; an increase of 40 % in strength could be

obtained just by adjusting the particle size distribution of the cement [Nehdi, 1998]. The strength

and durability of concrete depend essentially on the dense packing of cement matrix as well as on

the hydration products characteristics. The effect of these two parameters on cement and

concrete properties will be explained in the following sections.

4.3.2 Dense packing of cement matrix

Understanding the filling mechanism of particles can help to design durable high performance

concrete mixture with high density. The packing density of cement powder alone is relatively low

because of its narrow particle size distribution which makes the inter‐particle voids bigger. Systems

with the same particle sizes have high porosity and low density. The use of various fine materials

with different particle size distributions such as quartz powder, fly ash and silica fume largely affects

the voids content within the cementitious materials mixture. The efficiency of these materials in

filling up the voids or in improving the packing of the system is dependent mainly on their fineness

and particle shape. In general, a broader range of particle size distribution would yield higher

packing density. This is because with a broader range of particle size distribution, the medium

particles would fill up the voids between the large particles, and the fine particles would fill up the

voids between the medium particles and the very fine particles would fill up the voids between the

fine particles and so on, leading to the removal of more voids by the successive filling effect. Particle

system with a larger range of particle sizes can pack together in more efficient way than one with a

smaller range and as a result less space for water is remained [Moosberg, 2004]. Furthermore, the

improved gradation of the binder particles provides a lubrication effect through decreasing

aggregate interlocking.


On the other hand, the addition of too much filler may have a reveres effect on the system due to

the reduction of the packing density. The reason for that lies in the high surface area of these fillers

which need more water to maintain the required workability. However, in order to prevent the

flocculation and reduce the amount of mixing water, superplasticizer which has dispersing

properties shall be used. Superplasticizer is more efficient in a dense particulate system with high

surface area, and has only limited effect in a porous low‐density system. A combination of an

appropriate superplasticizer dosage to ensure optimal dispersion of fine particles and an

appropriate filler content to fill up the voids between bigger particles, can dramatically enhance the

packing and densify the microstructure of HPC.

4.3.3 Hydration products characteristics

The structure of hardened cement paste is created after the transition from a fluid to a rigid phase

with the progress in hydration of cement components. It is a continuous process that starts after

mixing cement with water and lasts a long time with a decreasing rate. Hardened cement paste is

the product of the hydration process which fills the voids between aggregate grains and binds all

elements of the concrete microstructure together. During the hydration process, the size and the

continuity of the pores control the permeability of the hardening concrete. The hydration of

Portland cement produces generally about 50 to 70 per cent by volume of calcium silicate hydrates

(C‐S‐H) which are very good binding materials at normal temperature. In addition, calcium hydroxide

crystals (CH, also called portlandite) constitute about 20 to 25 per cent of the volume of solids in the

hydrated paste. In contrast to the C‐S‐H, calcium hydroxide is a compound with a definite

stoichiometry, Ca(OH)2. It tends to form as thin long crystals of hexagonal plates with size of tens of

µm across [Ramachandran, 1996]. The morphology usually varies from nondescript to stacks of

large plates, and is affected by the available space, temperature of hydration, and impurities

present in the system. Compared to C‐S‐H, the strength‐contributing potential of calcium hydroxide

is limited as a result of a considerably lower surface area. It is a very weak material that adds no

structural strength to concrete. The presence of large calcium hydroxide crystals reduces the

adhesion capacity, not only because of the lower surface area and correspondingly weak van der

Waals attraction forces, but also because they serve as preferred cleavage sites owing to their

tendency to form an oriented structure [Mehta, 2006]. Because of its increased crystallinity degree

and the high density, calcium hydroxide contains an extensive network of capillary pores. It is the

major factor responsible for the poor strength of the interfacial transition zone in concrete and the

initiation of micro‐cracks. Its presence contributes to increased porosity, efflorescence, decreased

aggregate/cement paste bond and decreased concrete durability [Lamond, 2006]. Calcium

hydroxide is a highly soluble material, thus, pastes that have a high content of calcium hydroxide are

likely to be more prone to leaching and efflorescence and to have a greater potential for


deterioration in severe environment [Mindess, 2003]. Leaching of calcium hydroxide provides a

point of entry for aggressive agents by increasing permeability and diffusivity, as well as weakening

the matrix. This leaching out creates several channels available for the ingress and penetration of

water and deleterious substances inside the concrete [Siddique, 2011]. Under severe continued

conditions, depletion of the calcium hydroxide destroys and breaks down the normal structure

of the hardened cement paste, resulting in softening of the mortar and poor strength of

concrete [Hewlett, 2003]. Regarding mechanical properties and durability, reducing the calcium

hydroxide content is considered as an advantageous because it has several detrimental effects

as mentioned above.

4.3.4 Optimizing a dense cement matrix

From the above discussion, it is clear that most of concrete problems come from cement paste.

Therefore, concrete durability can be improved significantly by using low amount of cement in the

mix. This can be achieved by applying packing density theory in the mix design. Using low cement

content helps to reduce the chemical contraction, and reduces the generation of hydration heat,

which could cause thermal cracking in the structural elements. More enhancements can be attained

by the use of cements containing blast furnace slag. These cements, in contrast to ordinary Portland

cement, show lower permeability, lower hydration heat, lower effective alkali content, and lower

steel corrosion. The beneficial effects of blast furnace slag arise from the significant decrease in CH

content. The slag retains the alkali and CH in its hydration products (i.e. C‐S‐H) which in turn lead to

formation of denser microstructure of hydrated cement paste. That mean more of the pore space

being filled with C‐S‐H than in Portland‐cement paste [Siddique, 2011]. Furthermore, the hydration

process of slag cement is very slow compared to ordinary Portland cement. Thus, the formed C‐S‐H

phases are strong and thick and the hydration heat is low, however, in the case of Portland cement,

the hydration heat is high, causing thermal cracking, and the formed C‐S‐H phases are long and thin.

Generally, the use of slag results in a hardened cement paste with denser microstructure and

smaller pore sizes than equivalent OPC paste, thus durability aspects; porosity, permeability and

ionic diffusivity, are notably reduced [Virgalitte, 1995].

The use of fine materials basically enhances the durability and strength of concrete. They affect the

concrete microstructure in different ways. On the physical level (filler effect), when the added

particles fill the voids between cement particles and thus improve the compactness of concrete

mixture. On the chemical level, the fine particles may provide nucleation sites for the growth of

hydration products. Consequently, the microstructure of concrete will be more homogenous and

denser. Further improvement could be gained if the filler itself is pozzolanic. Mineral admixtures

with pozzolanic activity such as fly ash and silica fume are used extensively for improving concrete


properties. The improvements in durability and mechanical properties result from the reduction in

calcium hydroxide content and changes in pore structure. The pozzolanic materials modify the

microstructure of hydrated cement matrix perfectly. It produces excess amount of the strength‐

forming hydrates, C‐S‐H. The increased content of C‐S‐H leads to more homogeneous and denser

microstructure. Moreover, in the pozzolanic reaction, the capillary water and the calcium hydroxide

are largely consumed, therefore, further reduction of capillary porosity occurred and hence more

densification of microstructure is achieved. In addition to fly ash and silica fume, quartz can be

considered an inert material, but finely ground quartz could also react if it has a very high surface

area. The solubility of quartz depends mainly of the temperature, particle size and the alkalinity of

the medium. It is reported that the dissolution rate of quartz is approximately 2 ‐ 3 times higher at

pH value of 12 compared to pH value of 7 [Moosberg, 2004].

In this investigation, in order to achieve dense and homogeneous cement matrix, it is suggested to

use CEM III/B 32.5 N‐LH/HS/NA with slag content of about 68 %. To evaluate the performance of this

cement compared to ordinary Portland cement, OPC with two different strength classes have been

used, CEM I 42.5 N, CEM I 32.5 R. Furthermore, to study the influence of slag content on the

behaviour and properties of concrete, slag cement CEM/III A 32.5 N‐LH/NA with lower slag content

(about 50 %) has been used. On the other hand, in order to enhance the packing density of cement

matrix, different fine materials have been implemented. Several pozzolanic materials with different

particle sizes, shapes, and reactivity including fly ash, fine fly ash (M10 and M20) and silica fume

have been implemented. Quartz sand and quartz powder were used as fillers to densify the matrix

and to close the gaps between cement and aggregates.

4.3.5 Optimization of water/binder ratio

The durability of concrete cannot be characterized with a uniform value, but the impermeability of

concrete against water and gases is always of the most crucial aspects. The transports of liquids and

gases, which can be harmful to concrete, occur exclusively through the capillary pore system of the

cement matrix. Accordingly, minimizing the volume of capillary pores is of vital importance for

concrete impermeability and durability. It is theoretically known that, capillary pores begin to form

at a water/cement ratio higher than 0.42 [Neville, 2004]. On the other hand, the added water should

be sufficient to achieve the required consistency class. Therefore, in this research, the water/binder

(w/b) ratio has been chosen in the range of 0.27 to 0.42. The effectiveness factor (k factor) is

assumed to be 0.4 and 1 for fly ash and silica fume respectively.


4.4 Densifying the interfacial transition zone

The interface is known as the region of direct contact between two materials or two phases [Brandt,

2009]. In general, interfacial transition zone is a region up to 50 µm wide around each aggregate,

containing higher porosity and larger pores than the bulk cement paste. According to Neville, the

microstructure of the interface zone consists of a thin layer of CH crystals, about 0.5 µm thick,

covers the surface of aggregate, behind which there is a layer of C‐S‐H with the same thickness

[Neville, 2004]. Moving further away from the aggregate, there is the main interfacial zone, with 50

µm thick. The properties of the interface are related mainly to the texture and roughness of the

particles surfaces, their purity, the ability of wetting of material with the other, volume fraction of

all components, type of aggregate, maximum and minimum size of aggregate, and the particle size

distribution of the constituents [Brandt, 2009]. The transition zone is the weakest link in concrete

under mechanical action, and the first micro‐crack starts in it before propagation from aggregate to

aggregate. It is also the favourite place for penetration of aggressive components. Therefore,

chemical and microstructural characteristics of the interfacial zone between paste and aggregate

have important influences on the durability and mechanical properties of concrete. The interface

zone occupies one‐third to one‐half of the total volume of hardened cement matrix in concrete and

its microstructure is totally different from that of the bulk hydrated cement paste [Neville, 2004]. It

has less unhydrated cement, lower density, high porosity. The pores are generally larger than those

formed in the bulk paste. It was also observed that, this region contains less C‐S‐H, large oriented

crystals of CH, and high concentration of ettringite [Mindess, 2003]. The morphology of the

transition zone in normal concrete includes large and continuous voids formed around the coarse

aggregate. The porosity of the transition zone is higher than that of the bulk cement matrix, it is

about 3 times higher than the bulk porosity. Within the interface zone, the porosity is higher near

the aggregate and decreases with increasing distance from the aggregate down to the bulk cement

porosity. The aggregate concentration plays a crucial role on the properties of the transition zone.

At low aggregate concentration, transport properties are dominated by the bulk cement matrix

properties. However, increasing the aggregate content tends to bind interfacial zone close together

and they become interconnected, which create continuous channel for gas and liquid movement. In

this case, the transition zone may act as short circuit for diffusion of ions.

The weakness and high porosity of this ITZ can be attributed to different reasons. One of these

reasons is the low packing density at this region. The particles cannot pack together in an efficient

way as in the case of free space. Since the aggregate is many times larger than cement particles,

aggregate surface appears locally flat to the surrounding particles. This inefficient packing causes

less cement and high porosity to be presented initially near the aggregate surface, and this

condition exists so even after hydration. The width of the interfacial transition zone depends then


mainly on median particle size of cement [Hilsdorf, 1995]. So, the use of coarse cement will increase

the thickness of the transition zone and also result in the presence of large pores in this region.

During mixing, dry cement particles are unable to pack efficiently around the aggregate surfaces.

Thus, less cement is present to hydrate and fill the original void in this zone. Therefore, the interface

zone has much higher porosity than the hydrated cement paste [Neville, 2004]. It is also observed

that, high concentration of large crystals of CH are found closed to the aggregate surfaces, where

local bleeding is considered to have been likely [Taylor, 1997]. Within the consolidation process,

large particles, depending on their size, shape, and surface texture prevent the homogeneous

distribution of water and lime in fresh concrete. Because of this localized wall effect, some water

tends to accumulate at the surface of coarse aggregate particles, as bleed water. Then, the local w/c

ratio in interfacial zone becomes higher than the w/c ratio in the bulk cement paste [Aïtcin, 2004]. It

contains products of cement hydration with larger crystals of CH but without any unhydrated

cement. The complete hydration of cement can be considered as an indication of the high w/c ratio

at the interface more than elsewhere, this supports the wall effect hypothesis [Neville, 2004]. In

addition to the poor packing density of particles on the aggregate surfaces, the one sided growth of

hydration products can be considered as one of the reasons for existence of this zone. In this zone,

the available porosity is filled with the hydration products, growing from one direction only, unlike

the bulk paste where the products grow inward from all directions. Although composed of the same

components and hydration products, the microstructure and properties of the transition zone are

different from the bulk cement paste. In fresh compacted concrete, water films cover the surfaces

of large aggregate particles. This is due to the high w/c ratio closed to the large aggregate than that

away from it. Therefore, ettringite and CH owing to form at the high w/c ratio. These crystalline

products in the vicinity of the coarse aggregate exist as relatively large crystals, which in turn form a

more porous microstructure than in the bulk cement paste [Mehta, 2006]. The aggregate grading

also plays an important role on the properties of the transition zone. During the compaction of fresh

concrete, segregation may occur in mixtures with poorly graded aggregate, accordingly, a thick

water film can be formed on the aggregate surfaces especially beneath the particles [Mehta, 2006].

Accordingly, a transition zone with high porosity is produced and consequently high concentration of

CH crystals aligns uniformly towards the aggregate surfaces.

The interfacial transition zone (ITZ) is generally weaker than either of the two main components of

concrete, namely, the aggregate and the bulk hydrate cement paste. Therefore, it has a greater

effect on the mechanical behaviour of concrete than it is reflected by its size [Mehta, 2006]. Because

of the high porosity of the ITZ, the strength is very weak and it represents the position where micro‐

cracks begin to appear [Neville, 2004]. It is also found that concrete toughness depends mainly on

the fracture toughness of aggregate‐cement paste interface and is less affected by the fracture


toughness of the hydrated cement paste [Hillemeier, 1977]. The differences between the

microstructures of transition zone and bulk cement matrix play an important role in determining the

mechanical properties of concrete [Aïtcin, 2004]. In ordinary concrete, the ITZ has less crack

resistance than either hydrated cement paste or the aggregate due to high concentration of large

crystals of calcium hydroxide which makes this zone as the weakest link in concrete. In addition, this

zone subjects to the greatest stress due to differences in elasticity and Poisson ratio between bulk

cement matrix and aggregate, thus the fracture occurs preferentially in this zone. Therefore, it has

the largest content of pores and micro‐cracks which affects the overall performance of concrete.

The effect of mineral admixtures on enhancing the microstructure of interfacial transition zone

depends substantially on their reactivity and particle size distribution, assuming an efficient

dispersion of fine particles [Hilsdorf, 1995]. The small sized pozzolanic mineral admixtures can

basically offset the wall effect whereas they have not any influence on the one‐sided growth

phenomena. Moreover, they offer an additional benefit by converting CH into larger volume of

pozzolanic C‐S‐H. The filler effect is assumed to be the result of the fine and spherical shape of fine

particles. These particles eliminate bleeding and can be packed efficiently at aggregate surfaces.

Thereby, they can prevent the formation of water filled voids around the aggregate. By substituting

cement with SF, slag, fine fly ash, a smaller transition zone with lower CH content can be formed.

The incorporation of fine pozzolanic materials modifies the microstructure of the transition zone to

be like that of the bulk cement matrix. By the addition of pozzolanic materials, the reaction with

calcium hydroxide starts very quickly and additional C‐S‐H is formed making the ITZ denser. It

increases also the internal cohesion of fresh concrete; reducing the bleed water and the

accumulation of bleed water beneath the coarse aggregate particles. As a result, CH concentration

at the paste aggregate interface is significantly reduced. In addition, the thickness of ITZ is reduced

as the amount of fine pozzolans increases.

In this research, in order to produce high performance concrete with dense interfacial zone, several

supplementary materials have been used and tested. Fly ash and fine fly ash (M10 and M20) have

been used as pozzolanic materials to strengthen the transition zone and to reduce its porosity. In

addition, silica fume was also used for the same target and to compare its influence with that of fly ash

and fine fly ash. As mentioned before, fine materials can enhance the packing at the aggregate

surfaces, but it does not have any role to modify the one‐sided growth of the hydration products.

Therefore, to reduce this effect, a certain sequence of adding the concrete constituents into the mixer

has been used as can be seen in figure 3.8. It is suggested that, after adding the aggregate, it is useful

to add some water (about 30 % of the mixing water) to moisten the aggregate surfaces and then adding

the fine materials. By this sequence, the fine materials stuck well to the aggregate surfaces and reduce


the porosity of ITZ and as a result the concentration of calcium hydroxide in this area will be

considerably reduced.

4.5 Mix design and mixes composition

Based on the above discussions, several mixes have been designed and tested. The following steps

have been applied in concrete mix design.

1. Determination of the maximum aggregate size considering its effect on water and binder

content as well as on the hardened concrete properties (in this research 16 mm).

2. Calculating the solid materials fractions using the Ideal Fuller curve (Figure 4.1) which applied for

both aggregate and binder (for 16 mm, aggregate content is 85.13 % and binder content is 14.87

% by volume).

3. Choosing the aggregate grading that fit the curve in order to achieve the maximum packing

density.

4. Selecting the cement type to control chemistry of the hydration products (CEM III/B was chosen,

and for comparison, CEM I and CEM III/A were also applied).

5. Optimization of the binder proportioning (< 0.125 mm) by using various supplementary

materials, type and content, in such a way that maximum packing can be attained

(granulometric viewpoint), meanwhile, dense and durable cement matrix microstructure as well

as thin and dense interfacial transition zone can be achieved (chemical viewpoint).

6. Determination of the required fillers addition (quartz sand and quartz powder) to fill the gaps

between the mixture and the targeted curve and to make the mixture fit the curve as exact as

possible.

7. Calculating the water content from rheological, physical and chemical points of view (w/b

0.42, in order to reduce the capillary porosity as low as possible).

8. Adapting the superplasticizer dosage, that is compatible with the used cement, to get the

required consistency, and to enhance the packing density of fine materials.

In order to achieve the target of this part, produce high dense concrete and compare the effect of

different cementitious materials on the properties of concrete, 23 mixes have been prepared. Table

4.1 shows the composition of all concrete mixes.


Table 4.1: Composition of concrete mixes

Mix

Cementitious materials composition (wt.%)

SP (wt.%)

Aggregatekg/m3

QP kg/m3

QS kg/m3

w/b ratio

Cement FA M20 M10 SF

Type %

1 CEM III/B 100 0.7 1854 46 84 0.42

2 CEM III/B 70 30 1 1854 46 84 0.42

3 CEM III/B 70 30 1 1854 46 84 0.42

4 CEM III/B 70 30 1 1854 46 84 0.42

5 CEM III/B 70 15 15 1 1854 46 84 0.42

6 CEM III/B 70 20 5 5 0.9 1854 46 84 0.42

7 CEM III/B 90 10 0.7 1854 46 84 0.42

8 CEM III/B 65 25 10 1.1 1854 46 84 0.42

9 CEM III/B 65 12.5 12.5 10 1 1854 46 84 0.42

10 CEM III/B 65 15 5 5 10 1 1854 46 84 0.42

11 CEM I 42.5 N 100 0.87 1854 46 84 0.42

12 CEM I 42.5 N 70 20 5 5 1 1854 46 84 0.42

13 CEM I 42.5 N 70 15 15 1.8 1854 46 84 0.42

14 CEM I 42.5 N 65 12.5 12.5 10 2.2 1854 46 84 0.42

15 CEM I 42.5 N 65 15 5 5 10 1.9 1854 46 84 0.42

16 CEM I 42.5 N 90 10 1.2 1854 46 84 0.42

17 CEM III/A 65 12.5 12.5 10 1.7 1854 46 84 0.42

18 CEM I 32.5 R 100 0.87 1854 46 84 0.42

19 CEM I 32.5 R 65 12.5 12.5 10 1.4 1854 46 84 0.42

20 CEM III/B 67 12.5 12.5 8 1.8 1887 45 86 0.36

21 CEM III/B 67 12.5 12.5 8 4 1947 48 88 0.27

22 CEM III/B 70 15 15 3 1885 47 86 0.36

23 CEM III/B 70 15 15 4 1947 48 88 0.27

FA: fly ash M20: fine fly ash M10: fine fly ash SF: silica fume SP: superplasticizer QS: quartz sand QP: quartz powder W/b: water/binder ratio


4.6 Measuring the packing density

In order to ensure the high density of concrete, the compressible packing model (CPM) developed

by Larrard has been used to predict the packing density of the concrete mixes in the dry state

[Larrard, 1999]. This model is based on the concept of virtual packing density and compaction index.

The virtual packing density is the maximum possible density which occupies the minimum space if

the particles have been placed one by one (ideal case). However, in practice it is impossible to

achieve this density, usually the particles are randomly placed depending of the applied compaction

energy. For example, the virtual packing density of a system of monodisperse spheres equals 0.74,

while the actual packing density that can be measured in a random mix is close to 0.60/0.64.

Therefore, the compaction index K is introduced. If K tends to infinity, the real packing density αj

tends to the virtual packing density. The compaction index (K) depends mainly on the type of

compacting and is independent on the properties of the tested material (table 4.2). Two interaction

effects have been considered in this calculation: the wall effect exerted by coarser grains and the

loosening effect exerted by the finer particles. The effect of particle shape and texture is indirectly

included via measuring the actual packing density experimentally. Rene LCPC, commercially

available software based on the compressible packing model, has been used to determine the

maximum density or the porosity of the dry mixture. This software can calculate the compaction

index for any mixture at a given porosity, or calculate the porosity of any mixture at a given

compaction index. According to Larrard, after measuring the packing density of many samples, the

error provided by this model is generally less than 1 % [Larrard, 1999]. The general equation of the

compressible packing model which represents the virtual packing density of a mixture of n number

of size classes with category i being the dominant is expressed as follow [Fennis‐Huijben, 2010]:

jj

iij

n

ijjj

iiji

i

j

iti

rarb ]1[)]1

1(1[11

1

1

(4.1)

Where:

ti = calculated virtual packing density of a mixture when size class i is dominant.

i = virtual packing of size class i.

rj = volume fraction of size class j.

n = number of size classes in a mixture.

j = the virtual packing density of size class j.

For a single size particle class i with actual packing density αj, which can be experimentally

determined (section 4.7), j can be calculated by the following equation:

)1

1/(kjj

(4.2)


The loosening effect aij and the wall effect bij can be calculated from the following equations:

02.1)1(1i

jij d

da

(4.3)

50.1)1(1dj

db i

ij (4.4)

Where

di is the diameter of dominant size class i.

dj is the diameter of particle class j.

tit

iinii

ni

rKK

/1/1

/11

(4.5)

The packing density of the mixture αt is determined indirectly from equation 4.5. Based on the

above equations, this model has been used to calculate the packing density and porosity of

polydisperse mix, from the knowledge of three parameters: packing density of monosized classes,

particle size distribution of the all components and compaction index (K). The particle size

distribution of fine and coarse aggregate has been determined using sieve analysis. However, the

particle size distribution of the fines has been measured using laser granulometry as discussed in

chapter 3. In addition, to calculate the packing density of the mixture it is required to determine the

actual packing density of each concrete component individually (αj).

Table 4.2: Compaction index (K) for different compaction methods according to [Larrard, 1999]

Implementation Loose Striking

with a rod

vibration Wet

packing

Vibration with

Comp. 10 kPa

Virtual

K 4.1 4.5 4.75 6.7 9 infinity

4.7 Measuring the actual packing density αj

The maximum packing density of dry particles can be determined according to EN 1097‐3 for loose

bulk density (which incorporates the shape and surface texture effects). External loads such as

consolidation and vibration can be also implemented in this method to determine the packing

density at a certain compaction level [Fennis‐Huijben, 2010]. This method is acceptable for coarse

and fine aggregates, where the gravitation forces and shear forces between particles are dominant.

However, for powders (< 125 µm), the inter‐particle forces become increasingly important. These

forces can cause agglomeration of particles, and lowering the packing density. Therefore, it is

important to measure the packing density under the same conditions as when the particles would

be used in concrete, with water. On the other hand, the addition of superplasticizer alters the


microstructure in two main ways [Larrard, 1999]. First, it deflocculates the fine particles and

enhances the packing density of the system. Also, it can lubricate the solid surface, thus reduce the

friction forces between particles. In this investigation, two methods were used to measure the

packing density of fine materials: thick paste method developed by Larrard as well as Puntke

method for measuring the water demand of fine materials. Details of these methods can be found in

[Larrard, 1999; Puntke, 2002]. After determining the water demand, the following equation is used

to calculate the maximum packing density αj for each material:

p

wj

M

M

1000

1000

(4.6)

Where, Mw is the mass of water (kg), Mp is the mass of the powder (kg) and ρ is the powder density

in kg/m3.

4.8 Results of packing density

4.8.1 Packing density of fine materials.

The measured packing densities of all materials (αj) were used as input for Rene LCPC software to

calculate the packing density of all mixtures. Figure 4.4 shows the results of the maximum packing

density measurements for fine materials according to Larrard (thick paste method). The wet packing

densities of all materials were measured twice; once without superplasticizer and once with

superplasticizer. The packing densities of all materials depend basically on shape, surface texture,

fineness and particle size distribution. Certainly, the addition of superplasticizer increases the

packing density for all materials as can be seen in Figure 4.4. This is attributed to the role of

superplasticizer in dissolving flocks and dispersing the particles, which results in a looser but more

homogenous particle packing. Due to the high inter‐particle forces, the particles cannot fill the

spaces of their own classes without excess of water or superplasticizer. Through its action on the

surface, superplasticizer can partially liberate the water that adsorbed as a surface layer, but does

not decrease the amount of free water. Thus, the dispersion influence of superplasticizer is more

manifest in dense particulate systems with high surface area, while in a porous low‐density system it

has a limited efficiency [Moosberg, 2004]. Compared to the results of the wet measurement, the dry

packing density is lower for all fine materials. The reason for that lies mainly in the influence of

inter‐particle forces which become higher as the particle sizes reduce. These forces can create

agglomeration of particles and as a result the packing density is reduced. This effect is more visible

in silica fume which consists of particles in the size range of 0.1 to 0.5 µm. In the dry state, silica

fume has the lowest packing density which is 20 %. In spite of its spherical shape, the packing

density of silica fume is not as high as that of fly ash. This can be explained probably by the

Optimizi

differenc

particle

because

nearly th

differenc

(M20) ha

the inter

superpla

the fine

prove fo

and silica

Figure 4

4.8.2 P

The pac

The inpu

packing

the max

packing

packing

attribute

10

20

30

40

50

60

70

80

Packing den

sity %

ing a high de

ces in the p

forces. On

of its partic

he same pro

ce is the pa

as lower pac

r‐particle for

asticizer, bec

particles fro

or that are th

a fume (SF);

4.4: Dry and

Packing de

king density

ut data for

density resu

imum packi

densities. T

density. Mi

ed to the ba

0

0

0

0

0

0

0

0

CEM I32.5 R

ense concret

articles size

the other

cle size and s

operties as th

article size. M

cking densit

rces are very

cause it cann

om lying clos

he results of

in which all

d wet packin

ensity of dry

y of all mixe

this softwa

ults of differe

ng density w

The addition

ix 2 with 30

all bearing e

CEM I42.5 N

CE3

Wet P.D

te mixture

s. It is thoug

hand, norm

spherical sha

hose of norm

Moreover, f

ty. Indeed, it

y strong. The

not totally d

se together

f the packing

l particles ha

ng densities

ry concrete

es was meas

re is in mas

ent mixes m

was 87.3 %.

n of normal

0 % fly ash

effect exerte

EM III A32.5 N

CEM 32.

D Wet P.D

ght that the

mal fly ash h

ape. The sam

mal fly ash b

fine fly ash

t is thought

ese forces ca

disperse all fi

in the dry s

g densities o

ave a spheric

(P.D) of the

e mixtures.

sured using

ss ratio for

made with bla

However, m

fly ash app

has the hig

ed by fly ash

IIIB.5

FA

D with superp

e lower the

has the high

me trend can

but with quit

(M10) with

that there i

ause a poor

ine particles

tate as well

of fly ash, fin

cal shape.

e used fine

Rene LCPC

all substanc

ast furnace s

mixes 2, 3, 4,

pears to be

ghest packin

h particles w

M 10

plasticizer

particle size

hest packin

n be observe

te lower pac

higher fine

s a particle

packing den

s. The inter‐p

as in water

ne fly ash (M

materials (s

software as

ces classes.

slag cement

, 5, and 6 w

more effec

ng density o

which reduc

M 20 S

Dry P.D

e, the bigger

g density, a

ed for fine f

cking density

eness than f

size limit be

nsity, even w

particle forc

‐fines suspe

M20), fine fly

size < 125 µ

s previously

Figure 4.5

t (CEM III B).

ith fly ash h

ctive in enha

of 88.4 %. T

ces the part

SF QP

71

r the inter‐

and this is

ly ash with

y. The only

ine fly ash

elow which

when using

es prevent

ension. The

ash (M10)

µm)

discussed.

shows the

. For mix 1,

ave higher

ancing the

his can be

icles inter‐

Optimizi

locking [

10 % silic

clumping

used [Ta

forces b

[Larrard,

even wit

more ef

variation

Figure 4

Figure 4

86.0

86.5

87.0

87.5

88.0

88.5

89.0

Packing den

sity %

8

8

8

8

8

8

8

Packing density %

ing a high de

[Hüsken, 201

ca fume rep

g of fine ma

aylor, 1997]

ecome impo

, 1999]. Sim

th fly ash ad

ffective than

n in the surfa

4.5: Packing

4.6: Packing

Mix 1

86.0

86.5

87.0

87.5

88.0

88.5

89.0

Mix 1

III

I

ense concret

12]. Howeve

placement ha

terials whic

. Silica fume

ortant and o

milarly, mixe

dition. Com

n fine fly as

ace area and

g density of

g density of

Mix 2 M

1 Mix 12

III‐FA III‐

I‐FA

te mixture

er, the additi

ad the lowes

h is favoured

e particles h

overcome th

es 7, 8, 9 an

paring the r

sh in enhan

d particles si

concrete m

concrete m

Mix 3 Mix

Mix 13

‐M20 III‐M1

I‐M20/10 I‐M

ion of silica f

st packing de

d to occur in

have sizes lo

he natural te

nd 10 made

esults of mix

cing the pa

izes.

ixes made w

ixes made w

x 4 Mix 5

Mix 14 Mix

10 III‐M20/10

M20/10‐ SF I‐FA‐M

fume reduce

ensity which

n pastes wit

ower than 1

endency of

with silica

xes 2, 3, 4 an

cking densit

with CEM II

with differen

Mix 6

x 15 Mix 1

III‐FA‐M20/10

M20/10‐SF I‐SF

ed the packi

h is 86.4 %. T

h SF even w

1 µm and at

spherical pa

fume have

nd 5 showed

ty. This can

I/B

nt cement

Mix 7 M

16 Mix 17

III‐SF III‐F

IIIM20/10‐SF

ing density.

This may be

when superpl

this level t

articles to pa

low packing

d that norma

n be attribut

Mix 8 Mix

Mix 18

FA‐SF III‐M20/10

I I‐

72

Mix 7 with

due to the

lasticizer is

he surface

ack closely

g densities

al fly ash is

ted to the

x 9 Mix 10

Mix 19

0‐SF III‐FA‐M20/

‐M20/10‐SF

/10‐SF


Figure 4.6 shows the results of packing density of the second series of mixes (11‐19) made with

different cements. For mixes 11‐16 made with the CEM I 42.5, the addition of normal fly ash

improves the packing density, while the mixture containing silica fume (mix 12) has lower packing

density. As mentioned for the first series, normal fly ash is more effective in packing than fine fly

ash. In this group, the poor compactability of silica fume affects the packing density of concrete

mixes. Cement type also has an important effect on the packing density of concrete mixture. Mix 11

made with CEM I has a higher packing density than mix 1 with CEM III/B. Both mixes have the same

aggregate and cement volume content. The differences are the particle sizes and surface areas of the

cements. So, it can be said that, at a given aggregate content and grading, particle size distribution and

particle shape of the binders govern the packing density of concrete mixtures.

4.8.3 Results of fresh concrete properties

4.8.3.1 Air content

The air content of concrete has an important influence on determining the concrete mechanical

properties and durability. In non‐freezing environment, the presence of air voids in concrete has

harmful effects on its properties. In case of high dense concrete, high resistance to freezing and

thawing may be achieved automatically (no need for air voids) because of the low amount of

freezable water inside the concrete and because of the high resistance to water penetration

[Hilsdorf, 1995]. So, for high performance concrete it is important to keep the air content as low as

possible, below 2 ‐ 3 % with regard to strength and permeability [Larrard, 1999]. After hardening,

the air bubbles form a large volume of coarse voids inside concrete microstructure. Like other voids,

these voids affect the pore system and make it continuous and vulnerable to penetration of water

and aggressive materials. Furthermore, the presence of these voids deteriorates the mechanical

properties of concrete. Every increase of 1 % of air content reduces the compressive strength by

about 5 % [Neville, 2004]. Modulus of elasticity is also decreased by the same degree as strength

with increasing the air content [Ramachandran, 1996]. The existence of air inside concrete is

basically a result of an incomplete consolidation. In addition, the air content is affected by the mixer

speed, the compaction and the amount of concrete being mixed [Mindess, 2003]. Moreover, mix

proportions, sand content, fine materials content, aggregate shape and texture, and grading has a

crucial effect on the air content. Cement content and fineness also have a notable influence on air

content; lean mixes have higher air than rich ones.

In this investigation, the air content test has been performed according to DIN EN 12350‐7. Results

of air content of all mixes can be found in Table 4.3. The experimental results showed that the

addition of fly ash has an important role in reducing the air content [Ramachandran, 1996].

However, the addition of silica fume to slag cement results in an increase in air content (mix 7). This


may be due to the high surface area of silica fume and slag cement which need more water or

superplasticizer to completely disperse. On the other hand, mixes with fine fly ash exhibited higher

air content than mixes with normal fly ash at a given cement type. This can be also attributed to the

quite large size of normal fly ash particles compared to that of fine fly ash, which in turn enhances

the compactability and packing of particles. For most mixes, the results of air content are in good

agreement with the results of packing density measured with Rene LCPC. Generally, it can be said

that all mixes have air contents lower than 3 % and the differences between air contents for most

mixes are small. The main reason for that could be the high packing density of all mixes due to

applying the Ideal Fuller curve.

4.8.3.2 Workability

Concrete workability can be considered as an indicator for the behaviour or performance of

concrete in fresh state. The ACI defined the workability as “that property of freshly mixed concrete

or mortar which determine the ease and homogeneity with which it can be mixed, placed,

consolidated, and finished” [Neville, 2004]. Several factors affect the concrete workability but the

water content is the most important [Neville, 2008]. In addition, other parameters such as maximum

grain size, aggregate grading, texture and shape, cement fineness and content, and admixtures have

important influences on concrete workability. In this investigation, the flow table test according to

DIN EN 12350‐5 has been used to measure the workability. The experimental results of all concrete

mixes are presented in Table 4.3. The measured flow diameters of all mixes are in the desired range

(F3 ‐ F4) according to DIN EN 206‐1. Regarding superplasticizer content, mix 1 with CEM III/B needed

around 2.2 kg/m3 of superpalsticizer to achieve flow diameter of 53 cm, whereas, mix 11 with the

same proportions but with CEM I needed 2.72 kg/m3 to get 49 cm. This increase in admixture

dosage (about 23 %) may be attributed to the presence of slag which improves the workability and

makes the mix more flowable and cohesive. Slag particles are smooth and therefore absorb little

water during mixing and thereby leave more water for enhancing the workability [Neville, 2004]. This

effect is repeated in all mixes with slag cement when they are compared to mixes with Portland

cement.

Due to its spherical shape and its high compactability, normal fly ash mixes have high flowability

even with low superplasticizer dosage and water content. In spite of taking the effective factor as 1

and 0.4 for silica fume and fly ash, mixes with silica fume required more superplasticizer to achieve

the required consistency class. This is because the very large surface area of silica fume particles

needs a lot of water to be wetted and therewith increase the water demand and superplasticizer

amount. Nehdi explained the effect of silica fume on the workability on two opposite effects [Nehdi,

1998]. First, it works as filler because of its spherical shape and very small particles. The fine


particles fit into the void spaces between the relatively coarser cement grains and liberate the water

which fill this voids. Now this water participates to fluidize the concrete. On the other hand, as

mentioned above, silica fume particles tend to adsorb water because of their high fineness which

increases the water demand. Besides this explanation, another effect can be added that is the low

packing ability of silica fume particles which also increases the water demand. The experimental

results indicate that mixes with higher packing densities exhibit better workability. This is because of

the efficient packing of particles with different sizes. The successive filling of different classes make

the particles to displace the water molecules, thus the captured water between flocculated cement

particles can be released and participate in fluidizing the mix.

4.8.3.3 Density of fresh concrete

The density of fresh concrete has been determined by weighing a compacted known‐volume of

concrete according to European standards DIN EN 12350‐6. As the water is the lightest component

in concrete, the density is controlled basically by the water content. In addition, the density and

contents of other components have also important influences on fresh concrete density. It is clear

from the results that mixes 20 ‐ 23 with lower water content have the highest densities. On the

other hand, because of the higher bulk density of Portland cement (3.17 t/m3) compared to slag

cement (2.96 t/m3), mixes composed of OPC have higher density. However, mixes with silica fume

have the lowest density. This may be explained by the poor compactability of silica fume as

discussed before. In general, from the experimental results of concrete density, all mixes have a fresh

density higher than 2.4 t/m3. This can be explained by the high packing density of all solid materials

as a consequence of applying the Ideal Fuller curve. In addition, according to this curve, a high

content of aggregate is required and a lower amount of cement paste is needed to achieve the

maximum packing density. Thereby, the resulting mixes have high densities because of the relatively

high density of aggregate (about 2.61 t/m3) and the comparatively low density of the paste (about 2

t/m3). Furthermore, the reduction of the paste volume reduces the water demand which is one of

the major factors determining the concrete density. Finally, it can be said that the results of the

fresh concrete density agree well with the results of the packing density measured with Rene LCPC.


Table 4.3: Results of fresh concrete properties

Mix SP content (Kg/m3) Flow diameter (cm) Air content % Fresh density (t/m3)

1 2.2 53 1.5 2.43

2 2.19 52 1.3 2.45

3 3 48 1.6 2.45

4 3 51 1.4 2.46

5 3 51 1.3 2.46

6 2.6 54 1.3 2.44

7 2.54 48 2.3 2.42

8 3.1 50 2 2.44

9 2.8 50 2.2 2.45

10 3 50 2 2.44

11 2.72 49 1.4 2.46

12 2.19 50 1.6 2.48

13 3.94 50 1.9 2.47

14 4.47 48 1.3 2.46

15 3.86 49 1.3 2.46

16 3.36 49 1 2.43

17 3.52 54 1.45 2.46

18 2.68 52 1.5 2.45

19 3.01 46 1 2.46

20 3.83 49 1.2 2.47

21 8.78 44 1.5 2.50

22 6.75 49 1.5 2.47

23 8.83 48 1.4 2.49

Results and discussion of the optimized concrete properties 77

5. Results and discussion of the optimized concrete properties

5.1 Introduction

In this chapter, mechanical properties, porosity and durability of the optimized concrete mixes are

presented and discussed. Experimental results of mechanical properties including compressive

strength, splitting tensile strength and modulus of elasticity are given. The results indicated that

most concrete mixes exhibited compressive strength higher than 60 MPa which reflected the

important role of enhancing the packing density. Moreover, the modulus of elasticity was higher

than 40 GPa for all mixes which demonstrates the high efficiency of concrete mixture optimization

in strengthening the interface zone between cement paste and aggregate and reducing the

probability of micro‐cracking. A modulus of elasticity of about 50 GPa has been attained with low

w/b. In addition, the concrete porosity was measured using three different methods; water porosity,

helium pycnometry and mercury intrusion porosimetry (MIP). Furthermore, the MIP method has

been used to study the pore size distribution and to calculate the capillary porosity which is

responsible for the transport properties of concrete. The porosity of concrete made with blended

cement is lower and the pores are finer than those made with ordinary Portland cement. The results

of porosity pointed out that concrete made with fine fly ash showed very low porosity. However, at

low w/b ratio, combination of fine fly ash with silica fume resulted in the lowest porosity, about 3 %,

which is comparable to that of UHPC. Moreover, the capillary porosity was very low (about 2 %)

which is advantageous with regard to concrete durability.

The durability of concrete has been evaluated via measuring the transport properties of concrete

which related directly to its resistance to aggressive attack. Transport of contaminants into concrete

can take place through three mechanisms; permeation, diffusion and absorption. Therefore, the

resistance of concrete against these mechanisms were measured and assessed. The experimental

results of water penetration depth indicated that most of concrete mixes have a very low

penetration depth. On the other hand, the use of fine pozzolanic materials resulted in concrete with

more dense and homogeneous microstructure due to both filler and pozzolanic effects. As a result,

the resistance to chloride attack is significantly increased. Additionally, the transition zone between

aggregate and cement matrix which is the locus of coarse pores became denser due to the

consumption of calcium hydroxide crystals by the pozzolanic reaction. The capillary suction results

also showed very low absorption coefficient especially for mixes with pozzolanic materials and low

w/b ratio. Compared to literature, the optimized concrete showed desirable performance

concerning durability and permeability. Because of the much data from the experimental

investigation, some results are presented and discussed in this chapter, while other data and tests

results are given in the appendices.

Results a

5.2 M

5.2.1 C

The com

ages of 2

concrete

is the w/

MPa at

mixes. In

especial

ages is h

becomes

days, the

was 62 M

Figure 5

The add

dependin

cement

results o

addition,

has com

ash, conc

ages of 2

2

4

6

8

10

12

Compressive stren

gth (MPa)

and discussio

Mechanical

Compressi

mpressive str

28 and 91 d

e mixes. It is

/b ratio. Mix

28 days and

n addition,

ly at early a

higher than

s to be nea

e compressi

MPa. Howev

5.1: Compre

ition of poz

ng on their

enhances th

f mixes 9 an

, the use of s

pressive stre

crete made w

28 and 91 da

0

20

40

60

80

00

20

Mix 1

Fc 2

Fc 9

III

on of the op

l propertie

ive strength

rength of th

days accordin

s clear on th

xes 21 and 2

d 99 and 94

the cement

ges (28 days

the strengt

rly the sam

ve strength

er, at 91 day

essive stren

zzolanic mate

type, conten

he strength o

d 19 which h

silica fume r

ength of 78 M

with silica fu

ays. The stren

Mix 2

28

91

III‐FA

ptimized con

es

h

he concrete

ng to DIN EN

e figure tha

23 with w/b

4 MPa at 91

t type has a

s). It is well

h of slag ce

e. The resu

of mix 1 wi

ys both mixe

gth of differ

erials has an

nt and prop

of concrete

have compre

esulted in an

MPa which is

ume or fine f

ngth develop

Mix 5 MIII‐M20/10

ncrete prope

samples ha

N 12390‐3. F

t, the main

ratio of 0.27

1 days respe

also an imp

known that

ment, while

lts of mixes

th slag ceme

s exhibited r

rent concret

n important

perties. Comb

more than

essive streng

n improveme

s higher than

fly ash or a c

pment of mix

Mix 7 Mix

III‐SF III‐M20

erties

ve been de

Figure 5.1 sh

factor that c

7 exhibited c

ectively, wh

ortant role

the develop

e at later ag

s 1 and 18 c

ent was 52

roughly simil

e at 28 and

t influence o

bination of p

with slag ce

gth of 71 and

ent in compr

n mix 1 by ab

combination

x 2 with norm

x 9 Mix 18

0/10‐SF I

termined us

hows the te

controls the

compressive

hich are the

on the dev

pment of str

ges the final

confirmed t

MPa, while

lar strengths

91 days

on concrete

pozzolanic m

ement as ca

d 82 MPa at 9

ressive stren

bout 15 %. C

of both sho

mal fly ash is

8 Mix 19

I‐M20/10‐SF

sing 100 mm

st results fo

compressiv

e strength of

highest am

velopment o

rength of OP

compressiv

his assumpt

for mix 18 w

s (70 ± 2 MPa

compressive

materials wit

n be noticed

91 days resp

ngth as expec

Compared to

wed higher

s very slowly

Mix 21

III‐M20/10‐SF

78

m cubes at

or different

ve strength

f 82 and 80

mong other

of strength

PC at early

ve strength

tion. At 28

with OPC it

a).

e strength;

th Portland

d from the

pectively. In

cted. Mix 7

o normal fly

strength at

y compared

Mix 23

III‐M20/10

Results a

to other

Figure 5.

compres

only at t

exhibited

5.2.2 S

In spite

valuable

The expe

w/b rati

results o

exhibited

tensile s

0.42 and

Figure 5

Accordin

strength

Where ft

shows th

obvious t

Splitting tenslie stren

gth M

Pa

and discussio

mixes with

1. At low w/

ssive strength

he same w/b

d similar com

Splitting ten

of the defin

e especially r

erimental re

io, packing

of tensile str

d higher te

trength of c

d reached 6

5.2: Splitting

ng to Nevill

h is governed

t is the splitti

he experimen

that, the rela

0

1

2

3

4

5

6

7

Mix 1

III

on of the op

different po

/b ratio (0.27

h of mix 21

b ratio (0.27

mpressive stre

nsile streng

nitive impor

regarding th

esults showe

density, cem

rength mea

ensile streng

concrete. The

MPa for mix

g tensile stre

e [Neville, 2

d by the follo

ft = 0.3 fc

ing tensile st

ntally deduce

ationship bet

Mix 2

III‐FA II

ptimized con

ozzolanic ma

), silica fume

with silica fu

). However,

ength as mix

gth

rtance of co

he cracks dev

ed that like c

ment type,

sured at ag

gth. Howev

e tensile stre

xes with w/b

ength of diff

2004], the

owing formu

c 2/3

trength and f

ed relationsh

tween the m

Mix 5 M

I‐M20/10 I

ncrete prope

aterials, part

e looked to b

ume and fine

at w/b ratio

x 5 with fine f

mpressive s

velopment a

compressive

and supple

e of 91 day

er, the add

ength range

b ratio of 0.2

ferent concr

relationship

ula:

fc is the com

hip between

measured resu

Mix 7 Mix

II‐SF III‐M20/

erties

icularly at ea

be more effec

e fly ash is h

o of 0.42, mix

fly ash only.

strength, kno

and failure m

e strength, te

mentary ma

ys. It is clear

dition of po

es from 3 to

27.

rete at age o

p between t

mpressive stre

tensile stren

ults agree wi

9 Mix 18

10‐SF I

arly ages as

ctive in enha

higher than m

x 9 with fine

owing the t

modes of co

ensile streng

aterials. Figu

r that mixes

ozzolanic ma

5 MPa for m

of 91 days

tensile stren

(5.1)

ength of con

ngth and com

ith theoretica

Mix 19

I‐M20/10‐SF

can be clea

ncing the str

mix 23 with f

e fly ash and

ensile stren

oncrete [Nev

gth depends

ure 5.2 illus

s with lower

aterials enh

mixes with w

ngth and co

ncrete (MPa)

mpressive str

al ones to lar

Mix 21 M

III‐M20/10‐SF II

79

arly seen in

rength. The

fine fly ash

silica fume

ngth is also

ville, 2004].

s mainly on

strates the

r w/b ratio

hances the

w/b ratio of

ompressive

. Figure 5.3

rength. It is

rge extent.

Mix 23

II‐M20/10


Figure 5.3: Relationship between compressive strength and splitting tensile strength

5.2.3 Modulus of elasticity

From structural point of view, measuring the elastic modulus of concrete is very important in order

to design concrete elements to resist structural loads, moments and deflection. Although concrete is

non‐elastic material, determination of the elastic modulus is important to predict the stresses

generated by strains due to environmental effects. The elasticity of any material depends mainly on

the fractions and properties of its components. Aggregate content and properties, cement matrix

properties and transition zone features are the main factors influencing concrete elasticity. In this

research, the modulus of elasticity has been determined according to DIN 1048‐5 at age of 91 days.

The experimental results of elasticity modulus are found in Figure 5.4. It is obvious that, all mixes

have modulus of elasticity of more than 40000 MPa. Mix 21 with w/b ratio of 0.27 has the highest

elasticity, about 51000 MPa. Mix 1 and mix 18 showed roughly the same elasticity modulus (about

43000 MPa), that mean slag cement and OPC have the same effect on concrete elasticity as

mentioned by [Virgalitte, 1995].

Modulus of elasticity of concrete is related directly to its compressive strength as can be seen in

Figure 5.5. Both are affected by the porosity and density of concrete although not to the same

degree. At low w/b ratio, with the addition of fine materials, the porosity of concrete was decreased

while the strength and elasticity are increased. Moreover, the transition zone between cement

paste and aggregate becomes denser which results in more improvement in strength and elasticity.

According to Mehta et al. [Mehta, 2006], the relationship between compressive strength and

modulus of elasticity is governed by the following formula for normal weight concrete:

0

2

4

6

8

10

50 60 70 80 90 100 110

Splitting tensile stren

gth (MPa)

Compressive strength (MPa)

Experimental

Theoretical

Results a

Where fc

Figure 5

modulus

compres

Figure 5

Figure 5

10

20

30

40

50

60

Modulus of elasticity (MPa)

and discussio

cm and Ec are

5.5 shows t

s of elastici

ssive strengt

5.4: Modulus

5.5: Relations

0

0000

0000

0000

0000

0000

0000

Mix 1

10

20

30

40

50

60


III

on of the op

Ec =2.15 *

e the averag

he relations

ty. It is cle

h and modu

s of elasticit

ship betwee

1 Mix 2

0

0000

0000

0000

0000

0000

0000

60

III‐FA

ptimized con

104 (fcm/10)1

ge compressi

ship betwee

ear that the

lus of elastic

ty of differen

en compress

Mix 5

70

III‐M20/10

ncrete prope

1/3

ive strength

en theoretic

e relationsh

city agreed w

nt concrete

sive strength

Mix 7 M

80

Compressiv

III‐SF III‐M2

erties

and modulu

cal and mea

hip between

well with the

at age of 91

h and elastic

Mix 9 Mix 1

90

ve strength (M

E

T

20/10‐SF I

(5

us of elasticit

asured com

n the exper

theoretical

1 days

modulus of

18 Mix 19

100

MPa)

Experimental

Theoretical

I‐M20/10‐SF

5.2)

ty in MPa re

pressive str

rimentally d

relationship

f concrete at

Mix 21

110

III‐M20/10‐SF

81

espectively.

rength and

determined

.

t 91 days

Mix 23

III‐M20/10

Results a

5.3 Po

5.3.1 P

Figure 5

21 and 2

Howeve

fine fly a

to mix 1

about 9.

about 9

porosity

Figure 5

5.3.2 W

In this m

realistic.

Figure 5

that the

pycnome

the dive

overall r

pycnome

0

2

4

6

8

10

12

Total porosity %

and discussio

orosity

Porosity me

.6 presents

23 with w/b

r, mix 18 wi

ash and 10 %

18 with OPC

.8 %. More r

%. Similarly

as can be se

5.6: Total po

Water poro

method, wat

. In this test

5.7 illustrate

porosities m

etry. This m

erse in the s

results. The

etry method

Mix 1

III

on of the op

easured wi

the experim

b ratio of 0

ith OPC only

% silica fume

C, the use of

reduction in

y, the incorp

een from the

orosity of co

osity (effect

er was used

method, co

s the exper

measured wi

may be due t

size of speci

porosity res

d.

Mix 2 M

III‐FA III‐M

ptimized con

ith helium p

mental result

.27 have ve

y has the hig

e (mix 19), th

f slag ceme

n the porosit

poration of

e results of m

oncrete mea

tive water p

d to calculat

ncrete speci

imental resu

ith water sat

to the differ

mens. Gene

sults of all co

Mix 5 Mix

M20/10 III

ncrete prope

pycnometry

ts of total po

ery low poro

ghest porosi

he total poro

nt (mix 1) le

ty has been

silica fume

mix 9.

asured with

porosity).

e the poros

imens were

ults of the t

turation are

rences in th

erally, these

oncrete mix

x 7 Mix 9

I‐SF III‐M20/

erties

ry

orosity of co

osities which

ty which is

osity is signif

eaded to a

achieved by

together w

helium pycn

ity of concre

cured unde

total porosit

quite highe

e methodol

differences

es have the

9 Mix 18

10‐SF I

oncrete mixe

h are 3.6 an

11 %. By rep

ficantly redu

quite decre

y the additio

with fine fly

nometry at 9

ete, which m

r water unti

ty at age of

er than that

ogies of the

s are small a

same trend

Mix 19

I‐M20/10‐SF

es. As expec

nd 4.2 % re

placing OPC

uced to 7 %.

ase in the p

on of fly ash

ash reduced

91 days

make the re

l the day of

f 91 days. It

measured w

e two tests

and did not

d as that in t

Mix 21

III‐M20/10‐SF

82

cted, mixes

spectively.

with 25 %

Compared

porosity to

(mix 2) to

d the total

sults more

testing.

is noticed

with helium

along with

affect the

the helium

Mix 23

III‐M20/10

Results a

Figure 5

5.3.3 P

Figure 5

at age o

concrete

on concr

tradition

w/b rati

[Teichma

and mic

capillary

framewo

sheet of

Figure 5

and 23 w

On the o

0

2

4

6

8

10

12

14

Total porosity %

and discussio

5.7: Total por

Porosity an

.8 presents

of 91 days. T

e. In additio

rete porosity

nal concrete

o (21 and 2

ann, 2004].

crostructure

y porosity is

ork. Therefo

f MIP measu

.9 shows the

with w/b rat

other hand m

Mix 1

III

on of the op

rosity of con

nd pore siz

the experim

The results c

n, concrete

y. Compared

e which lies

23) exhibite

Undoubtedl

of concret

of most con

ore, the cap

rements. It

e experimen

tio of 0.27 h

mix 18 with w

Mix 2 M

III‐FA III‐M

ptimized con

ncrete measu

ze distributi

mental resul

confirmed t

component

d to literatu

between 15

d porosity i

ly, measurin

te. Howeve

ncern. Most

illary porosi

is assumed

ntal results o

have very low

w/b of 0.42

Mix 5 M

M20/10 III‐

ncrete prope

ured with eff

ion measur

ts of the tot

hat w/b rat

ts proportio

ure, most of

5 to 20 % [T

n the range

ng the poros

er, regarding

transport m

ity of concre

to be in the

of capillary p

w capillary p

has the high

ix 7 Mix

‐SF III‐M20/

erties

fective water

red with M

tal porosity

io is the ma

ons and prop

these mixes

Teichmann,

e of UHPC w

ity gives a g

g durability

mechanisms

ete mixes ha

range of 30

porosity of d

porosity whi

hest capillary

9 Mix 18

10‐SF I

r method at

IP

of concrete

ain factor co

perties have

s have lowe

2004]. More

which is in t

ood indicati

y and deter

take place

as been calc

0 nm to 10 µ

different con

ich is 1.9 an

y porosity w

8 Mix 19

I‐M20/10‐SF

91 days

e specimens

ontrols the p

e important

r porosity th

eover, mixe

the range of

ion of the po

rioration of

via the capi

culated from

µm [Teichma

ncrete mixes

nd 2.96 % re

which is 5.6 %

Mix 21

F III‐M20/10‐SF

83

measured

porosity of

influences

han that of

s with low

f 4 to 6 %

ore system

concrete,

llary pores

m the data

ann, 2007].

s. Mixes 21

spectively.

%.

Mix 23

III‐M20/10

Results a

Figure 5

concrete

Figure 5

the size

5.4 D

5.4.1 P

5.4.1.1

The wate

Figure 5

revealed

concrete

0

2

4

6

8

10

12

Total porosity %

0

1

2

3

4

5

6

Capillary porosity %

and discussio

5.8: Total po

e mixes at a

5.9: Capillar

range of 30

urability

Permeabili

Water pen

er penetrati

5.10 illustra

d that all m

e can be con

Mix 1

Mix 1

III

III

on of the op

orosity of c

age of 91 da

ry porosity

0 nm to 10 µ

ity

netration de

on depth ha

tes the exp

mixes exhibit

nsidered imp

Mix 2

Mix 2

III‐FA II

IIIII‐FA

ptimized con

concrete me

ay

of concrete

µm

epth

as been mea

perimental

ted low per

permeable i

Mix 5 M

Mix 5 M

II‐M20/10 II

II‐M20/10 III

ncrete prope

easured with

e measured

asured accor

results of w

rmeability (

if the penet

Mix 7 mix

Mix 7 mix

I‐SF III‐M20/

‐SF III‐M20

erties

h mercury i

using the m

rding to DIN

water pene

depth < 20

ration depth

x 9 Mix 1

x 9 Mix 18

/10‐SF I

0/10‐SF I

intrusion po

mercury int

EN 12390‐8

etration dep

0 mm). Acco

h is lower th

8 Mix 19

8 Mix 19

I‐M20/10‐SF

I‐M20/10‐SF

orosimetry o

rusion poro

8 at the age o

pth tests. T

ording to N

han 50 mm,

Mix 21

Mix 21

III‐M20/10‐SF

III‐M20/10‐SF

84

of different

osimetry in

of 91 days.

The results

Neville, the

and if the

Mix 23

Mix 23

III‐M20/10

III‐M20/10

Results a

penetrat

aggressiv

permeab

penetrat

other ha

volume

penetrat

depth of

and whi

show th

penetrat

Figure 5

5.4.1.2

The tran

several p

type and

the conc

permeab

penetrat

properti

air perm

four tim

1

1

2

2

3

Water pen

etration dep

th (mm)

and discussio

tion is lowe

ve conditio

bility of con

tion depth w

and, the cem

and connec

tion depth o

f 18 mm. At

ch have pen

hat slag cem

tion depth o

5.10: Water

Air perm

nsport of ai

parameters.

d pozzolanic

crete age, cu

bility measu

tion depth,

es. It is obv

meability (3.4

es due to re

0

5

10

15

20

25

30

Mix 1

III

on of the op

er than 30

ns [Neville,

ncrete is th

which is abo

ment type an

ctivity whic

of 8 mm wh

the same ti

netration de

ment concre

of mix 1 was

penetration

meability

r via concre

The air per

materials co

uring, entrai

urements o

the results

ious that mi

4 x 10‐17 m2)

educing w/b

Mix 2

III‐FA

ptimized con

mm, then

2004]. The

he w/b ratio

ut 3 mm. Ho

nd mineral a

h control t

hich is lowe

me, mix 5 h

epths of 11 a

ete showed

15 mm, whe

n depth of th

ete occurs t

meability of

ontent and p

ned air cont

of concrete

reflected th

ix 21 prepar

compared t

b ratio from

Mix 5

III‐M20/10

ncrete prope

the concre

e results sta

o. Mix 21 w

owever, mix

admixture pr

he water p

er than mix

as lower de

and 13 mm

d higher res

ereas mix 18

he concrete

through the

f concrete d

properties. F

tent and tem

specimens

he major rol

red with w/b

to other mix

0.42 to 0.27

Mix 7 M

III‐SF III‐M2

erties

te can be

ated also th

with w/b r

x 23 has a pe

roperties ha

penetrability

2 with norm

pth than mi

respectively

sistance to

8 exhibited p

at age of 9

e connected

epends on w

Furthermore

mperature. F

at age of

le of w/b ra

b ratio of 0.

xes with w/b

7. However,

ix 9 Mix 1

0/10‐SF I

regarded as

hat the mai

atio of 0.27

enetration d

ave crucial in

y. Mix 5 wi

mal fly ash

xes 7 and 9

y. The result

penetration

penetration

1 days

pore syste

w/b ratio, ag

e, the air pe

Figure 5.11 s

91 days.

atio on dete

27 and silica

b ratio of 0.4

, mix 23 mad

18 Mix 19

I‐M20/10‐SF

s impermea

in factor go

7 showed t

depth of 5 m

nfluences on

th fine fly

and with p

made with

ts of mix 1 a

n than OPC

depth of 18

em which de

ggregate typ

rmeability d

hows the re

Similar to

ermining the

a fume has

42. It is redu

de with the

Mix 21

III‐M20/10‐SF

85

able under

overns the

the lowest

mm. On the

n the pores

ash has a

enetration

silica fume

and mix 18

C mix. The

8 mm.

epends on

pe, cement

depends on

esults of air

the water

e transport

the lowest

uced about

same w/b

Mix 23

III‐M20/10

Results a

ratio but

other ha

concrete

10‐17 m2

replacem

shown f

relatively

Figure 5

5.4.2 A

The capi

concrete

absorpti

increase

of mass

Figure 5

concrete

Where:

(from th

of calcu

0

2

4

6

8

10

12

14

16

Air permeability x 10‐17 (m

2)

and discussio

t without sil

and, the inco

e made with2) is around

ment of OPC

rom the res

y twice the a

5.11: Air per

Absorption

illarity of co

e. In this inv

on coefficie

e in the mass

was applied

.12 present

e mixes. The

absorpK

mtf is the v

he fitted line

lated absor

Mix 1

III

on of the op

lica fume ha

orporation o

h normal fly a

d two folds

C with silica

sult of mix 1

air permeab

rmeability of

n (capillary

ncrete has a

vestigation,

ent according

s of the spe

d at certain p

s the exper

e following e

ption

m

value of incr

on the figu

rption coeff

Mix 2

IIIII‐FA

ptimized con

as a little bit

of fine fly as

ash. The res

higher than

fume and

9. The air p

bility of mix 1

f different co

suction)

a very impor

the capillar

g to DIN EN

ecimen due t

periods of tim

imental mea

quation was

f

otf

t

mm

reased mass

re), and tf is

ficient of di

Mix 5 M

III‐M20/10

ncrete prope

higher perm

sh improved

istance of m

n mix 2 wit

fine fly ash

ermeability

19 (7.2 x 10‐

oncrete mea

rtant influen

ry suction te

ISO 15148.

to the rise o

me; at the b

asurements

s used to cal

s at the end

s the testing

ifferent con

Mix 7 Mix

III‐M20I‐SF

erties

meability (3.

the imperm

mix 5 with fin

th normal f

significantly

of mix 18 w17 m2).

asured at 9

nce on the in

est has bee

. The main i

of water by

beginning, 20

of the mas

lculate the w

of the test

period (hou

ncrete mixes

9 Mix 18

0/10‐SF I

.9 x 10 ‐17 m2

meability of

ne fly ash to

fly ash (9.7

y reduced t

was about 14

1 days

ngress of agg

en used to m

dea of this t

capillary for

0 minutes, 1

s increase w

water absorp

(5

(kg), mo is

urs). Figure 5

s. The rate

8 Mix 19

I‐M20/10‐SF

2) than mix

concrete co

air permeab

x 10‐17 m2

he air perm

4.3 x 10‐17 m

gressive mat

measure the

test is to me

rce. The mea

, 2, 4, 8, and

with time fo

ption coeffic

5.3)

the m at 0

5.13 shows

of water a

Mix 21

III‐M20/10‐SF

86

21. On the

mpared to

bility (4.8 x

). Partially

meability as

m2 which is

terials into

e concrete

easure the

asurement

d 24 hours.

or different

cient:

0 time (kg)

the results

absorption

Mix 23

III‐M20/10

Results a

depends

cementit

of water

hr0.5. Ho

0.32 kg/

concrete

mixes, t

OPC, mix

reductio

Figure 5

Figure 5

Increase in

mass per unit area (kg/m

2)

Water absorption coefficien

t (kg/m

2 hr0

.5)

and discussio

s on w/b ra

toius mater

r. Mix 21 m

wever, for m

/m2 hr0.5. It i

e mixes with

he absorptio

x 19 with re

n of absorpti

5.12: The inc

5.13: Absorp

0

0.5

1

1.5

2

0

Increase in


)

0

0.1

0.2

0.3

0.4

0.5

Mix 1

III

on of the op

atio, age, c

ials compos

ade with w/

mix 18 with w

s about 4 fo

h slag cemen

on coefficie

placing part

ion coefficien

crease in co

ption coeffic

1

Mix 1 (IIMix 5 (IIMix 9 (IIMix 19 (IMix 23 (I

1 Mix 2

III‐FA

ptimized con

uring, comp

ition and pr

/b ratio of 0

w/b ratio of

olds higher t

nt have lowe

nt ranges b

of cement w

nt to 0.1 kg/m

oncrete mas

cient of conc

1

Square ro

II)II-M20/10)II-M20/10-SF)I-M20/10-SF)III-M20/10)

Mix 5

III‐M20/10

ncrete prope

paction and

roperties hav

0.27 has the

f 0.42 and w

than that of

er absorptio

between 0.1

with 25 % of

m2 hr0.5.

ss with time

crete at age

2

oot of time (ho

Mix 2 Mix 7 Mix 1Mix 2

Mix 7 M

III‐SF III‐M2

erties

d aggregate

ve also an im

e lowest abs

with OPC, the

mix 21. It is

on than that

to 0.2 kg/m

f fine fly ash

e due to cap

e of 91 days

3

ours)

(III-FA)(III-SF)8 (I)1 (III-M20/10)

Mix 9 Mix

20/10‐SF I

properties

mportant eff

sorption coe

e water abso

s obvious fro

of mix 18 w

m2 hr0.5. Com

h and 10 %

pillary suctio

4

18 Mix 19

I‐M20/10‐S

[Hilsdorf, 1

fect on the a

efficient of 0

orption coeff

om the resu

with OPC. Fo

mpared to m

silica fume e

on

Mix 21

SF III‐M20/10‐S

87

1995]. The

absorption

0.08 kg/m2

ficient was

ults that all

or all these

mix 18 with

exhibited a

5

Mix 23

III‐M20/10F

Results a

5.4.3 C

Diffusion

Differen

investiga

2012]. T

Figure 5

chloride

chloride

m2/sec,

OPC wit

alone, it

ash exhi

alone (6

m2/sec).

is twice

chloride

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Figure 5

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Chloride di

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5.15 and Fig

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Mix 23

F III‐M20/10


elasticity and late strength are markedly improved. Secondly, it enhances the packing density which

results in better compactness, thus the strength also increases [Peng, 2009]. Thirdly, fly ash addition

reduces the water demand which contribute to enhance the strength development. In addition, it

enhances the concrete microstructure through reducing the distances between the particles, which

leads to improving the mechanical properties of concrete. Moreover, the reduction of CH content

due to pozzolanic reaction makes the transition zone denser and stronger. Thus, the mechanical

properties, especially, elastic modulus are significantly improved. These effects are more noticeable

with using fine fly ash and silica fume. Fine fly ash is more reactive than normal fly ash, therefore,

mechanical properties are more enhanced [Maibaum, 2004; Chindaprasirt, 2005; Bentz, 2011].

Furthermore, the addition of fine materials reduces the micro bleeding and prevents the formation

of coarse pores in the transition zone and as a result the mechanical properties are significantly

improved [Erdem, 2012]. On the other hand, the strength of concrete is controlled by the strength

of the weakest zone, the interface between cement paste and aggregate [Popovics, 2011]. So,

strengthening the transition zone by adding fine materials leads to considerable increase in

mechanical properties. The results of modulus of elasticity, compressive strength and tensile

strength confirm the aforementioned explanation. The mechanical properties of mixes made with

fine materials (silica fume, fine fly ash or both) are superior to those with cement only or cement

with coarse fly ash. Because of its bigger size, the pozzolanic reaction of coarse fly is very slow which

reduces the strength development rate. On the other hand, the use of a low water content along

with the addition of fine particles prevent the internal bleeding beneath the aggregate particles that

after drying make the ITZ weaker and locus of micro‐cracks. As a result, the ITZ and microstructure

become denser and stronger which engender better mechanical properties. The compressive

strength of mix 21 reached 99 MPa while the modulus of elasticity was more than 50 GPa. This

enhancement in mechanical properties can be attributed directly to high density of the system.

In fact, it should be pointed out here that at different w/b ratio the behaviour of fine materials is

somewhat different. At w/b ratio of 0.42, the comparison of mix 5 and 9 showed that compressive

strength, tensile strength and modulus of elasticity of mix 5 with fine fly ash only was a little bit

higher than that of mix 9 with both fine fly ash and silica fume. While, at low w/b ratio (0.27), the

situation is totally reversed, the addition of both silica fume and fine fly ash enhanced the

mechanical properties more than the case of fine fly ash alone (mixes 21 and 23). The reason for

that may be the inefficient dispersion of silica fume in the first case because of the high content of

fine materials compared to the added superplasticizer dosage. However, in the other case (mix

21), higher dosage of superplasticizer was added which probably ensured better dispersion of fine

materials in the system. Therefore, the mechanical properties were significantly improved at the

same w/b ratio level. On the other hand, although the cement content is low, 312 kg/m3, (can be


said lean mixes), the mechanical properties of mixes with fine fly ash are as high as that of high

strength concrete which need more cement, more silica fume and more superplasticizer content

per cubic meter. This can be reasoned by the higher packing density of the system, because the

strength depends fundamentally on the porosity and pore size distribution. The voids between

each particle classes are filled with the particles from the smallest class. Especially at the micro

scale, the gap between cement particles with average diameter of about 30 µm and the silica

fume with average diameter of 0.5 µm are filled with fine fly ash with diameter in the range of 1‐

10 µm, thus the packing density is notably enhanced. The improved packing density increases the

strength indirectly via improvement of workability, cohesion and compaction at fresh state as well

as reduction of porosity and increase of the matrix density at the hardened state. Furthermore,

the high filling ability of fine fly ash spheres enhances the transition zone strength which is the

locus of weakness and micro‐cracks. As a consequence, the bond between aggregate and cement

paste increases which results in the considerable improvement of mechanical properties as the

experimental results revealed.

5.5.2 Porosity

Pore structure of concrete has been studied for a long time as it strongly affects the concrete

durability and strength. Because of its major influence on concrete properties, three different

methods have been used to measure the porosity using different media: helium gas, mercury and

water. The results of the measured porosities with different methods were in the same trend. Indeed,

it should be pointed out here that the porosity measured with MIP was a little bit lower than the

porosities yielded via the other two methods. This is perhaps due to the pores shape and geometry,

which prevent the mercury molecules to penetrate and reach all voids, known as bottle neck

phenomena [Diamond, 2000]. Furthermore, the porosity measured using water is normally higher

because of the drying process during the specimens’ preparation which in addition to evaporating the

water in the voids, probably some of the inter‐layer water of C‐S‐H can be also evaporated. In this

part, only the experimental results of concrete porosity measured with MIP will be discussed.

The experimental results of concrete porosity measured with MIP indicated that the maximum

porosity was 10 % for mix 18 with OPC and w/b ratio of 0.42 and without supplementary materials,

which is comparable to conventional concrete. In order to evaluate the experimental results of

porosity in this investigation, a small comparison with traditional, high performance and ultra‐high

performance concrete will be made. It is reported that the total porosity of normal concrete, HPC,

and UHPC are in the range of 15, 8.3 and 6 % by volume respectively. It is reported also that the

capillary porosity contents (0.03 – 10 µm) are 8.3, 5.2 and 1.5 % by volume respectively [Teichmann,


2004]. Results of mix 18 which can be considered as an optimized normal concrete showed that the

total and capillary porosities were 10 % and 5.6 % respectively, which are comparable to normal

concrete. These results significantly manifest the influence of enhancing the packing density on

concrete porosity. On the other hand, by maximizing the packing density of aggregate, the volume

of cement paste (cementitious materials and water) required to fill the voids between the aggregate

is reduced to about 23 %. However, for traditional concrete it is larger than about 30 %. So, if both

cement pastes have the same porosity, then the total porosity of the densely packed concrete will

be lower by about 25 %.

The use of supplementary materials appeared to reduce the porosity of concrete depending on their

type and content at w/b ratio of 0.42. The use of blended cement (slag cement), with higher fineness

than OPC, reduces the total and capillary porosities to about 9 % and 4.5 % respectively. More

reduction of porosities was attained by the addition of normal fly ash. The total and capillary porosities

of mix 2 with normal fly ash were reduced to about 7.8 and 3.9 % respectively. Similarly, mix 5 with

fine fly ash showed lower porosity; about 7 and 3.5 % for total and capillary porosities respectively,

which are comparable to the aforementioned porosities of HPC according to [Teichmann, 2004]. This

can probably be attributed to the efficient packing of fine fly ash particles which have small size (< 10

µm), spherical shape and smooth texture [Sinsiri, 2010]. The addition of silica fume to slag cement

reduces the porosity but not by the same way of fine fly ash. This may be due to the inefficient

dispersion and the possibility of agglomeration of the very fine particles of silica fume. On the other

hand, mix 19 made with OPC, fine fly ash and silica fume exhibited about 6.3 and 4.5 % for total and

capillary porosities respectively which are much lower than mix 18 without pozzolanic materials. At

low w/b ratio, the situation is clear and the porosity is significantly reduced. For mix 23 with w/b ratio

of 0.27 and with fine fly ash, the total and capillary porosities were 4.9 and 2.9 % respectively. More

reduction was gained at the same w/b ratio by the addition of silica fume to the system. Mix 21

exhibited around 1.9 % capillary porosity and about 3 % total porosity. These values are in the range of

UHPC which need more fines, cement, silica fume and fillers and more superplasticizer and special

mixing tools. The comparison of the measured porosity of mixes 21 and 23 with mixes 9 and 5

respectively emphasis the aforementioned assumption, that silica fume is more efficient in systems

with low w/b ratios than those with high w/b ratio.

To explain the previous results of total and capillary porosity of different mixes, it is basically worth

to repeat that all of these mixes prepared roughly with similar volume fractions of aggregate and

cementitious materials. The concrete mixture can be considered as a unit volume model that has to

be filled with the maximum well graded solid materials and at the same time to keep the produced

concrete as workable as possible. So, the aggregate content and grading has been optimised


according to Ideal Fuller curve which exhibited the lowest voids content. These voids should be also

filled with dense cement paste. In addition to the cement paste, the transition zone between

cement paste and aggregate should be also as dense as possible. Cement particles with sizes of

about 30 µm cannot achieve this target alone. Therefore, another material with smaller size which

can fit in the voids between cement particles should be implemented. Fine fly ash is the ideal in

these conditions because it has a smaller size, a spherical shape, a smooth texture and pozzolanic

reactivity [Droll, 2004]. Going deeper, similarly, the voids between fine fly ash should be filled with

smaller size particles. Silica fume with average particle size of about 0.1 ‐ 0.5 µm could be the best

material that can fill the voids between fine fly ash. For more optimization, two main problems face

this model; the first one is how the efficient dispersion of these fine materials can be ensured. The

second one is how much the optimum content of each material is required to achieve the highest

packing density, taking into account all parameters such as the water demand, inter‐particle forces,

etc…

The role of fine fly ash in reducing the porosity and enhancing the pore size distribution can be

interpreted basically from several aspects. Firstly, the enhancement of the packing density as

mentioned above, which is responsible for reducing the porosity of the mixture [Droll, 2004].

Secondly, fine fly ash enhances the packing density at the aggregate surfaces, or by other words,

reduces the transition zone thickness. Thirdly, it reacts with CH in the transition zone, therefore, the

transition zone become denser and the pores become finer [Chindaprasirt, 2005]. Fourthly, the

pozzolanic reaction with CH produces pozzolanic C‐S‐H which granted more gel for densifying and

strengthening the matrix and the transition zone. Compared to normal C‐S‐H gel from OPC

hydration, pozzolanic C‐S‐H has lower porosity; the porosity of C‐S‐H gel is about 28 %, whereas

pozzolanic one has about 19 % [Bentz, 2000]. Additionally, the pozzolanic reaction takes place within

the capillary pores, and the hydration products blocks and reduces the size of capillary pores and

prevents its connectivity. Furthermore, the pozzolanic reaction takes some water of the free water

that is found in the system, which indirectly helps to reduce the porosity. Fifthly, it enhances the

cohesion of fresh concrete and reduces the amount of bleed water beneath the aggregate, and as a

result the transition zone becomes denser. Sixthly, because of its spherical shape and smooth

texture, it reduces the water demand and makes the microstructure more homogenous and denser.

Finally, because of its low hydration rate compared to cement, the hydration heat is low which

result in smaller thermal stresses at early ages. On the other hand, the only difference between fine

and normal fly ash is the particle size. Because of its relatively big size (most particles > 10 µm),

normal fly ash cannot fit between cement particles with the same particle size [Droll, 2004] which

results in poor packing density in the size level of 10 µm. Moreover, the transition zone is more

porous and the pores are coarser in the case of normal fly ash compared to fine fly ash. However the


use of silica fume alone with cement (binary system) result in low packing density also because of

the high inter‐particle forces and the possibility of agglomeration. Silica fume should be uniformly

and homogeneously dispersed in order to achieve its pozzolanic and filler effects [Marchuk, 2004].

Agglomeration of the particles occurs with very fine substances which provoke poor packing density.

Therefore, the use of adequate cementitious materials with optimum composition, with the use of

appropriate superplasticizer that is sufficient to efficiently disperse the fine particles will lead to concrete

with dense microstructure and with very low porosity.

5.5.3 Durability

Durability is of major concern in the nowadays concrete mix design. Practically, the interest has

gradually shifted from the compressive strength towards other properties of the materials, such as

high modulus of elasticity, high density and durability. Concrete durability cannot be measured with a

uniform value, but the resistance of concrete to the ingress of water and gases is always of the most

crucial aspects. According to ACI committee [Virgalitte], durability of concrete is defined as its ability

to resist weathering action, chemical attack, abrasion or any other process of deterioration. The

transport of liquids and gases, which can be harmful to any concrete structure, occur exclusively through

the capillary pore system of the hardened cement matrix. In this investigation in order to assess the

durability of concrete; permeability, diffusion, and capillary suction tests have been implemented.

The transport of liquids and gases take place through the continuous pore system inside concrete.

So, enhancing the packing density reduces the penetration of contaminates into concrete. The

increased volume of aggregate close the transport passes and makes it longer. The experimental

results of penetration and air permeability tests clarify the important role of the increased packing

density on transport mechanisms. All concrete mixes exhibited water penetration depth lower than

20 mm which can be classified as impermeable concrete under aggressive conditions (< 30 mm)

according to Neville [Neville, 2004]. Because of its higher fineness, the use of slag cement reduces

the permeability more than OPC concrete. This result agrees with [Virgalitte, 1995; Güneyisi, 2008].

This is attributed to the dense microstructure and the lower content of CH crystals in the transition

zone in the case of slag cement concrete.

The addition of fine fly ash and silica fume notably reduces the concrete permeability and diffusivity

[Thomas, 2012]. The main effect of these fine materials is the enhancement of transition zone

properties and making it denser. This zone is known as the locus of micro‐cracks which influence not

only the mechanical properties but also the permeability and durability [Mehta, 2004]. Due to its

high pozzolanity, fine fly ash and silica fume increase the homogeneity of the microstructure by

replacing CH crystals with excess of C‐S‐H gel. Therefore, the probability of micro‐cracking is reduced

and the transition zone becomes thinner. Moreover, the size and content of capillary pores, which


are responsible for transport as well as the CH‐crystal concentration, are reduced with progress of

the pozzolanic reaction. Furthermore, the produced C‐S‐H gel blocked the pores and reduced its size

and interrupted its connectivity. On the other hand, due to the high fineness and small particle size,

fine materials fill the space between cement particles and therefore refine the microstructure and

densify the matrix (filler effect). In addition, it reduces the wall effect around the aggregate surfaces,

thus allowing better packing of cement particles at the interface between cement paste and

aggregate. This filler effect makes the pores finer. The influence is clear with using different classes

of fine materials. The filler effect is greater the wider the differences between fine materials and

cement [Reschke, 2000]. These factors together lead to very positive influences on concrete

properties, particularly result in significant reduction in the permeability and considerable

enhancement in durability of concrete.

The incorporation of fine fly ash or both fine fly ash and silica fume with low w/b ratio result in an

effective reduction in the transport of contaminants into concrete. The results of absorption and

chloride diffusion confirm this influence. The water absorption coefficient of mix 23 was about 0.09

kg/m2 h0.5. However, a combination of fine fly ash and silica fume along with low water content

leaded to a much lower water absorption coefficient, which equals 0.07 kg/m2 h0.5. Similar results

were obtained for chloride penetration. The chloride penetration into concrete is considered to be

the most important factor that can be used to evaluate the concrete durability. The chloride

penetration of concrete without fine materials was about 42 mm. However, the use of blended

cement reduced the penetration to around 15 mm (mix 1). The use of fine fly ash at the same w/b

ratio of 0.42 reduced the penetration to about 7 mm. Mix 21 with w/b ratio of 0.27 showed the

lowest chloride penetration (about 3 mm) which is in the range of UHPC [Teichmann, 2004]. The

experimental results of mixes 5, 9, 21 and 23 confirm that silica fume is more effective in reducing

the transport of gases and fluids into concrete at low w/b ratio more than at high w/b ratio. Similar

conclusion has been emphasized by Bentz who reported that silica fume is more efficient for

reducing diffusivity in low w/c ratio concretes [Bentz, 2000]. The high resistance of concrete to

chloride penetration can be interpreted by several reasons; high packing density, optimization of

cementitious materials and low w/b ratio. The role of high packing density in reducing the chloride

diffusion coefficient can be clarified by comparing the experimental results from this investigation

with the results reported by Brandenburger [Brandenburger, 2006]. He studied the effect of fine fly

ash on chloride diffusion of concrete and found that fine fly ash concrete exhibited a chloride

diffusion coefficient of 1.5 x 10‐12 m2/sec. However the optimized mixes in this investigation showed

higher resistance to chloride diffusion than these values by using the same materials. For example,

the chloride diffusion of mix 5 made with the same w/b ratio and with the same fine fly ash content

is 3.8 x 10‐13 m2/sec which is 4 times lower than Brandenburger concrete with fine fly ash. This small


comparison accurately elucidated the important effect of packing density on concrete durability. On

the other hand, the high resistance of concrete with supplementary materials to chloride ingress is

attributed to many parameters as discussed in the above sections. In addition to the

aforementioned parameters, the C/S ratio of the produced pozzolanic C‐S‐H has an important role.

The C‐S‐H from the pozzolanic reaction has a C/S ratio of about 1 which is much lower than that of

C‐S‐H gel from OPC hydration. It is stated that the lower the C/S ratio, the lower the chloride binding

[Droll, 2004]. It was found also that C‐S‐H with low C/S ratio has lower porosity than C‐S‐H with high

C/S ratio [Bentz, 2000]. Therefore, the pozzolanic C‐S‐H has important role in improving the

durability and reducing the porosity of concrete.

5.6 Porosity & mechanical properties relationship

In order to understand the interdependence between the measured properties of concrete in the

present investigation, the relationships between different properties have been studied. In general

the strength of most materials is increased when decreasing the porosity. An optimum addition of

fine materials helps to reduce the porosity and to improve the mechanical properties at the same

time. However, too much addition of fine materials increases the shrinkage which causes micro‐

cracking. The microstructure which is the main factor controlling the mechanical and chemical

properties depends mainly on the particle size distribution of the starting raw materials [Lange,

1997]. In addition, the properties of interface zone, porosity and micro‐cracking fundamentally

affect the mechanical properties. Figure 5.16 illustrates the mechanical properties and total porosity

relationship. It is clear that the relationship between porosity and mechanical properties is positively

correlated to a certain extent. Contrary to durability, all pores (open and close pores) affect the

mechanical properties of concrete [Beaudoin, 1979]. Strictly speaking, mechanical properties of

concrete are affected by the volume of all voids in concrete: entrapped air, capillary pores, gel

pores, and entrained air if present. Generally, from fracture mechanics point of view, at a given

porosity, smaller pores lead to a higher strength of the cement paste. Additionally, decreasing the

porosity and making the microstructure denser and more homogeneous results in beneficial effects

with respect to strength, resistance to crack propagation and stiffness. Besides, reducing the

porosity especially in transition zone makes the transition zone denser and enhances the bond

between cement paste and aggregate which has a direct influence on concrete elasticity and

strength. The concentration of CH crystals, which is a very weak material that weakens the

concrete, is reduced by minimizing the porosity in the TZ, thus mechanical properties of concrete

is more enhanced.


Figure 5.16: Relationship between total porosity and modulus of elasticity and compressive

strength of different concrete mixes at age of 91 days

5.7 Permeability & capillary porosity relationship

The entrance of aggressive substances into concrete takes place via the pore system within concrete

microstructure and transition zone. The porosity of any material can be divided into two categories;

close and open. The open porosity can be also subdivided to two classes. The first type is the pores

that start from the surface and reach to the interior of the body and then stop. These pores do not

participate in transport properties. The second type is continuous pores from one side to the other

side permitting the passage of liquids and gases [Därr, 1973]. It is proved that the permeability of

concrete is well correlated to the average pore size and porosity measured with MIP [Hamami,

2012]. Figure 5.17 shows the relationship between the measured water penetration and air

permeability with capillary porosity. It is obvious from the figure that a clear relationship existed

between permeability and capillary porosity of concrete. The addition of fine pozzolanic material

which refine the pores and make it denser have a positive effect on permeability. On the other

hand, the experimental results showed that most concrete mixes have very low capillary porosity. It

is stated that at lower porosities (< 20 %), the capillary pores became discontinuous and the

transport is controlled by the properties of nano‐pores in C‐S‐H microstructure [Bentz, 2000].

10000

20000

30000

40000

50000

60000

2 4 6 8 10 12


Total porosity %

0

20

40

60

80

100

120

2 4 6 8 10 12

Compressive stren

gth (MPa)

Total porosity %


Figure 5.17: Relationship between capillary porosity measured with MIP in the range of 30 nm to

10 µm and the air permeability and water penetration depth at age of 91 days

5.8 Absorption & capillary porosity relationship

The absorption coefficient which represents the capillary suction effect has been determined using

the water absorption test. The dependence of absorption on capillary porosity is presented in

Figure 5.18. Certainly, as the capillary porosity increases, the capillary suction also increases.

Nevertheless, experimental results of both measurements do not exactly fit the line. This might be

due to the differences of the measuring media; water and mercury. Additionally, beside the size, the

shape and geometry of the pores play important roles in determining the transport of fluids and

gases through concrete. Furthermore, the measuring period of the water absorption affects the

results because it is continued for 24 hours.

Figure 5.18: Water absorption coefficient and capillary porosity relationship at age of 91 days

0

2

4

6

8

10

12

14

16

1 2 3 4 5 6

Air permeability 10‐17 (m

2)


0

5

10

15

20

25

1 2 3 4 5 6

Water pen

etration depth (mm)


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 2 3 4 5 6

Water absorption coefficient

(kg/m

2hr0

.5)



5.9 Chloride diffusion & capillary porosity relationship.

Chloride ions diffusion is most effective when the pores are totally saturated, but it occurs also in

partially saturated ones [Neville, 2004]. Figure 5.19 presents the relationship between chloride

diffusion coefficient and the capillary porosity. It is clear that the relationship between capillary

porosity and chloride diffusion is positively correlated; as the capillary porosity increases the

diffusion coefficient increases. Similar relationship between capillary porosity and diffusion has been

documented by Bentz [Bentz, 2000]. On the other hand, the addition of fine pozzolanic materials

modifies the pore structure of the cement paste. The pozzolanic reaction takes place within the

capillary pores and the products may fill and block some of these pores and make them

discontinuous. Such pore modification and blocking are more pronounced in the case of very fine

materials. As a result of this refinement, the chloride penetration is considerably reduced.

Figure 5.19: Chloride diffusion coefficient & capillary porosity relationship at 91 days

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6

Chloride diffusion coefficien

t x 10‐13 (m

2/sec)


Effect of hydrothermal conditions on the properties of densely packed concrete 101

6. Effect of hydrothermal conditions on the properties of

densely packed concrete

6.1 Introduction Although the behavior of normal concrete at high temperature and fire is intensively studied in the

literature, there is a lack of information about the influence of elevated temperature on the

properties of high performance concrete containing mineral admixtures. Most investigations

concentrated only on the residual mechanical properties and a little information is available on

concrete durability which controls the service life of different structures. In addition, the available

information about the effect of hydrothermal treatment on properties of already cured concrete is

very rare. When the concrete is subjected to elevated temperature, changes in durability and

strength take place [Xu, 2001]. However, the mechanism causing these changes is complex as a

result of large number of physical and chemical processes in concrete microstructure. The literature

review about the behavior of concrete at high temperature give widely varying results which make it

difficult to draw definite conclusions. For example, the residual strength after exposing to 200 °C

could vary from a decrease of 50 per cent to an increase of 50 percent [Bažant, 1996]. The reasons

for that include the differences in moisture content, in exposing time, in concrete composition, and

in aggregate properties [Neville, 2004].

As one of the main targets of this investigation is to develop hot water tank to store solar thermal

energy at 200 °C and pressure of 15.5 bars, then the optimized concrete should be tested under

these hydrothermal conditions. Two aspects should be considered in this concern; the first is the

structural design including thermal stresses, geometry of the tanks and walls as well as the

structural behavior of concrete under hydrothermal cyclic loads (fatigue). The second aspect is the

behavior of concrete itself under hydrothermal conditions which is the main focus of this

investigation. The used concrete should be water‐tight at high temperature, durable and sustainable

for long working period. The principal goal of this chapter is to evaluate the effect of hydrothermal

conditions on properties of the densely packed concrete. Contrary to the case of the pure heat

exposure, where the concrete specimens are subjected to pressure gradient which may lead to

spalling, in the case of hydrothermal exposure (temperature and pressure), the pressure gradient is

not so aggressive like the first case. Additionally, in the case of heat only, the concrete suffers

dryness because it loses its pore water due to heating [Bažant and and Prasannan, 1986], whereas,

in the studied case (hydrothermal exposure), the pores are completely saturated with water. In this

investigation, in order to study the properties and microstructure of the optimized HPC under

different thermal conditions, a series of experimental tests have been performed.


This chapter presents the experimental results of the optimized concrete mixes after hydrothermal

exposure to a temperature of 200 °C and a pressure of 15.5 bars for several cycles up to 50.

Mechanical properties including compressive, tensile strength and hardness of concrete using

rebound number are introduced. Furthermore, porosity measurements using MIP will be also given.

In addition, the durability indicators including measurement of different transport phenomena;

diffusion, permeation and capillary suction are presented. At the end of this chapter, detailed

analysis and discussions of the experimental results are included.

6.2 Methods

In real situation of the hot water concrete tank, because the tank is totally closed, increasing the

temperature above 100 °C will generate water and steam pressure on the concrete walls. So, the

concrete will subject to temperature and pressure from one side while the other side is kept at

ambient conditions. However, due to the lack of information about the behaviour of the developed

densely packed concrete under hydrothermal conditions, it is favoured to study first the properties

of concrete after hydrothermal exposure from all sides. Therefore, in this experimental work, the

autoclave has been used to load the concrete specimens with temperature of 200 °C and pressure of

15.5 bars as seen in Figure 6.1. The height of the autoclave is about 1120 mm and the outer

diameter is about 550 mm with a pressure vessel from inside with volume of approximately 7.8

liters. The autoclave contains two heating units; bigger and smaller with capacity of 1600 and 400

watt respectively. A pressure gauge that enables setting the desired pressure value and pressure

valve are located on the enclosure. Additionally, a cooling fan is included in order to cool down the

pressure vessel if required. The pressure vessel has a diameter of about 170 mm and height of about

350 mm which makes it approximately suitable for 1 standard cylinder (300 x 150 mm) as a

maximum. The hydrothermal loading was applied for several autoclaving cycles, each one needs of 3

hours heating at a constant heating rate of 1 °C/min, and 6 hours cooling at constant rate of 0.5

°C/min for cooling down to the room temperature (20 ± 1 °C) as can be seen in Figure 6.1. Because

of the limited size of the used autoclave, either three cubes with edge length of 100 mm of each mix

or one cylinder (150 x 300 mm) were subjected to several hydrothermal cycles; 5, 10, 15 or 50 times.

Thereafter, the specimens were cooled down to ambient conditions and then tested at normal

temperature to study the variation in their properties due to autoclaving.

Effect of

Figure 6

6.3 M

6.3.1 C

The com

the plan

different

of some

showed

(strength

reduced

decrease

of norm

to reduc

was abo

compare

compres

quite di

reduced

both fine

only, mi

resistanc

slag cem

bit decre

f hydrotherm

6.1: High pre

Mechanical

Compressi

mpressive str

nned curing

t autoclavin

concrete m

that OPC a

h retrogress

to 61 % of

e of strength

al fly ash ap

ce. Neverthe

out 136 % of

ed to other

ssive strengt

fferent perf

to approxim

e fly ash and

ix 19 with p

ce to strengt

ment, fine fly

ease of abo

mal conditio

essure auto

l propertie

ive strength

rength has b

period at 2

g cycles wit

ixes, while t

and slag ce

sion). The st

f the origina

h to about 7

ppeared to i

eless, the fin

f its original

r mixes. M

th was abou

formance. T

mately its or

d silica fume

partial repla

th retrogres

y ash and sili

out 2 % can

ns on the pr

oclave and a

es results

h

been measu

20 ± 1 °C u

h 200 °C an

the results o

ment concr

trength of O

al strength.

8 % of its or

ncrease the

nal compress

l value, whic

ix 5 with f

ut 130 % of

The strength

riginal value

e is about 1

acement of

ssion after ex

ica fume ma

be noticed

roperties of

a schematic

red for all m

under water

d 15.5 bars.

of other mixe

rete exhibite

OPC concret

Similarly, s

riginal value

e strength un

sive strengt

ch represen

fine fly ash

the origina

h is increas

after 50 cyc

26 % of its

OPC with f

xposure to 5

anifested an

due to incr

0

50

100

150

200

250

0Temperature °C

densely pac

c diagram fo

mixes using

r, the concr

. Figure 6.2

es can be fou

ed decrease

te (mix 18)

lag cement

due to auto

ntil 15 cycle

h of mix 2 w

ts the maxi

showed s

l strength. M

sed with au

cles. Howeve

original valu

fine fly ash

50 cycles. At

increase of

reasing the

5

ked concret

or hydrother

100 mm edg

rete specim

presents the

und in the a

e of strengt

after autoc

concrete (m

oclaving for 5

es, after whi

with normal

mum increa

imilar trend

Mix 7 with s

utoclaving fo

er, the final

ue. Compare

and silica f

t low w/b ra

strength wi

autoclaving

10

Time (hours)

e

mal loading

ge length cu

ens were e

e experimen

ppendices. T

th due to a

laving for 5

mix 1) also

50 cycles. Th

ch the stren

fly ash afte

ase due to a

d, however,

silica fume

or 5 cycles

strength of

ed to mix 18

fume exhibit

atio (0.27), m

ith autoclavi

g cycles from

0 1

103

g period

ubes. After

exposed to

ntal results

The results

autoclaving

0 cycles is

suffered a

he addition

ngth began

r 50 cycles

autoclaving

, the final

revealed a

then it is

mix 9 with

8 with OPC

ted higher

mix 21 with

ing. A little

m 15 to 50

15


cycles. Nevertheless, the final compressive strength is about 112 % of its original value (without

autoclaving). On the other hand, mix 23 with w/b ratio of 0.27, with slag cement and fine fly ash

showed a negligible strength retrogression at all levels of autoclaving. The final compressive

strength was about 117 MPa which represents about 129 % of its original value.

6.3.2 Rebound number

The aim of this test is to measure the effect of autoclaving with various cycles on the hardness and

strength of concrete, on the same cube. After the designed curing period, the samples were

surface dried and then tested to determine its hardness in the original state without autoclaving.

Then, the specimens were put in the autoclave for different cycles. After autoclaving, the concrete

specimens were surface dried and tested again. In order to apply the rebound number test, the

specimens were fixed in compressive strength machine under a certain load with a certain loading

rate as can be seen in Figure 3.11. All samples are tested by the same way in order to get fair

comparison of the results. The experimental results of rebound number test on concrete

specimens are presented in Figure 6.3. It is not difficult to observe that the results of rebound

number are in good agreement with the results of compressive strength. The strength decrease

can be observed significantly in mixes 1 and 18 with slag cement and OPC concrete respectively. It

is clear that w/b ratio has a major effect on strength of concrete before and after autoclaving.

Furthermore, the results indicated that the addition of pozzolanic materials considerably

enhances the strength of concrete after hydrothermal exposure for different cycles; depending on

its type and content. It can be also indicated that mixes with fly ash continuously gain strength

with increasing the autoclaving cycles. However, mixes with silica fume gained high strength

quickly and then the strength is constant or slowly increases with autoclaving. The results of mix

18 showed that the strength is aggressively decreases with autoclaving for 50 cycles which is

typically the case in the compressive strength test results.


Figure 6.2: Compressive strength of concrete mixes at normal conditions and after autoclaving.

40

50

60

70

0 10 20 30 40 50Compressive strength

(Mpa)

Number of autoclaving cycles

Mix 18 (III)

50

60

70

80

90

100

110


(Mpa)


Mix 2 (III‐FA)

50

60

70

80

90

100


(Mpa)


Mix 5 (III‐M20/10)

50

60

70

80

90

100

0 10 20 30 40 50

Compressive strength

(Mpa)


Mix 7 (III‐SF)

50

60

70

80

90

100


(Mpa)


Mix 9 (III‐M20/10‐SF)

40

50

60

70

80

90

0 10 20 30 40 50


(Mpa)


Mix 18 (I)

60

70

80

90

100

110


(Mpa)


Mix 19 (I‐M20/10‐SF)

80

90

100

110

120

0 10 20 30 40 50


(Mpa)



80

90

100

110

120

0 10 20 30 40 50


(Mpa)


Mix 23 (III‐M20/10)


Figure 6.3: Results of rebound hammer for concrete at normal conditions and after autoclaving

for several cycles

30

35

40

45

50

0 10 20 30 40 50

Rebound number


Mix 1 (III)

35

40

45

50

55

0 10 20 30 40 50

Rebound number


Mix 2 (III‐FA)

30

35

40

45

50

55

0 10 20 30 40 50

Rebound number


Mix 5 (III‐M20/10)

30

35

40

45

50

55

0 10 20 30 40 50

Rebound number


Mix 7 (III‐SF)

30

40

50

0 10 20 30 40 50

Rebound number



35

40

45

50

0 10 20 30 40 50

Rebound number


Mix 18 (I)

35

40

45

50

55

0 10 20 30 40 50

Rebound number


Mix 19 (I‐M20/10)

40

45

50

55

0 10 20 30 40 50

Rebound number



40

45

50

55

0 10 20 30 40 50

Rebound number


Mix 23 (III‐M20/10)

Effect of

6.3.3 S

Cylindric

the desig

the test

week. Th

autoclav

tested.

conditio

at w/b ra

mix 19 i

and 5 ex

hand, m

addition

with 10

autoclav

fine fly a

same lev

Figure 6

6.4 Po

The mai

concrete

in the c

0

1

2

3

4

5

6

7

Splitting tensile stren

gth (MPa)

f hydrotherm

Splitting ten

cal specimen

gned curing

was perfor

he autoclav

ving, the sam

Figure 6.4 p

ns and after

atio of 0.42,

n which the

xhibited a sl

mix 1 reveale

of silica fu

0 % silica f

ving. At low

ash and silic

vel without a

6.4: Splitting

orosity me

in target of

e samples af

concrete mi

Mix 1

Normal

15 cycle

III

mal conditio

nsile streng

ns with dime

g period. Bec

med after 1

ing process

mples were

presents the

r hydrotherm

, most concr

e tensile stre

light decrea

ed the highe

me has a n

ume replac

w/b ratio, a

ca fume. How

any changes

g tensile stre

easured w

this test is

fter hydroth

crostructure

Mix 2 M

es

III‐FA III‐

ns on the pr

gth

ensions of 1

cause of the

15 cycles of

was perfor

allowed to c

e results of

mal exposur

rete mixes s

ength increa

se in tensile

est strength

egligible eff

cement exh

a little bit de

wever, only

s due to auto

ength at nor

with MIP

s to determ

ermal expos

e due to au

Mix 5 Mi

‐M20/10 III‐

roperties of

150 x 300 m

e limited vol

autoclaving

med one we

cool down t

f splitting te

e for 15 cycl

uffer a decr

ased by abou

e strength; a

reduction d

fect on prev

ibited abou

ecrease of st

y mix 23 wit

oclaving.

rmal conditio

ine the por

sure which c

utoclaving. A

ix 7 Mix

‐SF III‐M20/1

densely pac

m were sub

ume of the

g only, wher

eek before

to the room

ensile streng

les at age of

ease in tens

ut 13 % com

about 7 and

due to autoc

venting the

ut 40 % re

trength can

h fine fly as

ons and afte

re size distr

can help to

After curing

9 Mix 18

10‐SF I

ked concret

bjected to au

autoclave (1

re each cylin

the test, at

temperatur

gth of conc

f 91 days. Th

sile strength

mpared to it

d 10 % respe

claving, whi

tensile stre

duction of

be observed

sh held the t

er autoclavi

ibution and

better unde

g, the samp

8 Mix 19

I‐M20/10‐SF

e

utoclaving c

17 x 35 cm,

nder needs

age of 83 d

re (20 ± 1°C

crete mixes

he results ind

. The only ex

s original va

ectively. On

ch is about

ngth reduct

the strengt

d for mix 21

tensile stren

ing for 15 cy

the total p

erstanding th

ples were e

Mix 21

III‐M20/10‐SF

107

cycles after

7.8 liters),

about one

days. After

C) and then

at normal

dicate that

xception is

alue. Mix 2

n the other

41 %. The

tion. Mix 7

th due to

1 with both

ngth at the

ycles

porosity of

he changes

exposed to

Mix 23

III‐M20/10

Effect of

autoclav

the test

Figure 6

hydrothe

increase

OPC wit

porosity

pozzolan

result of

5.34 % d

very sma

porosity

conditio

Figure 6

figure re

autoclav

reduced

with poz

50 cycle

remaine

for 50 cy

reduced

Figure 6

autoclav

f hydrotherm

ving for 50 cy

ing device.

6.5 illustrat

ermal expo

ed from 10.3

th fine fly a

is still high

nic materials

f mix 5. The

due to autoc

all reduction

with autoc

n was 4.9 %

6.6 shows th

evealed that

ving for 50 c

from 3.5 to

zzolans exhi

s. At low w

d approxim

ycles. On th

from 2.96 %

6.5: Total

ving for 50 c

0

2

4

6

8

10

12

14

16

Total porosity %

mal conditio

ycles. After

Detailed ex

tes the tot

sure for 50

3 to 13.3 %

sh and silic

her than th

s seemed to

porosity of

claving for 50

n in porosity

claving for 5

, and it was

he experime

t mix 18 exh

cycles, 5.6 %

o 1.74 % aft

bited a decr

w/b ratio, the

ately withou

e other han

% at normal

porosity of

cycles

Mix 5

III‐M20/10

ns on the pr

cooling dow

planation o

tal porosity

0 cycles. Th

after autoc

a fume (mix

he original o

o have bette

mix 5 made

0 cycles. At

y to 2.98 % a

50 cycles wa

reduced to

ental results

hibited the h

% and 9.26 %

er hydrothe

rease in the

e capillary p

ut change, 1

nd, the capil

conditions t

f concrete

Mix 18

I

roperties of

wn to room t

n the meas

of concre

he results in

laving for 50

x 19), the in

one (from 6

er performan

e with slag ce

low w/b rat

after autocla

as clear for

2.65 % after

s of capillar

highest capi

% respectiv

ermal treatm

capillary po

porosity of m

1.9 % at amb

lary porosity

to 1.44 % aft

measured

Mix 1

I‐M20/1

densely pac

temperature

uring proce

ete mixes a

ndicated th

0 cycles. Ho

ncrease in p

6.2 to 7.9 %

nce concern

ement and f

io, mix 21 w

aving for 50

mix 23, wh

r hydrotherm

ry porosity

illary porosit

ely. Howeve

ment. Mix 19

orosity from

mix 21 with

bient condit

y of mix 23

ter hydrothe

with MIP

19 Mix

10‐SF III‐M20

ked concret

e, the sample

dures is fou

at normal

at the tota

owever, by p

porosity is r

%). The use

ing porosity

fine fly ash w

with bulk por

cycles. How

here the tot

mal treatme

of different

ty at norma

er, the capil

9 with partia

4.5 to 2.4 %

both fine f

tions and 1.8

with fine fly

ermal treatm

at normal

x 21 M

N

5

0/10‐SF III‐

e

es were tran

und in sectio

conditions

al porosity

partial repla

reduced, bu

of slag cem

y as can be s

was reduced

rosity of 3 %

wever, the re

tal porosity

nt for 50 cyc

t concrete m

al conditions

lary porosit

al replaceme

% after auto

fly ash and s

87 % after a

y ash was si

ment for 50 c

conditions

Mix 23

Normal

0 cycles

‐M20/10

108

nsferred to

on 3.4.4.3.

and after

of mix 18

acement of

ut the final

ment with

seen in the

d from 7 to

% showed a

eduction in

at normal

cles.

mixes. The

s and after

ty of mix 5

ent of OPC

oclaving for

silica fume

autoclaving

ignificantly

cycles.

and after

Effect of

Figure 6

6.5 D

When th

occur, pa

order to

conditio

properti

The mea

conditio

6.5.1 P

6.5.1.1

The wat

days. On

After au

were cut

applied f

device, t

measure

results o

autoclav

increase

value fo

f hydrotherm

6.6: Capillar

urability

he concrete

articularly d

o find out w

ns or not,

es of concre

asurements

ns and comp

Permeabili

Water pen

er penetrati

ne week bef

utoclaving, th

t and fixed i

for 72 ± 2 ho

they are sub

ed, the wett

of water pen

ving for 15

ed for all mix

r mix 1. It w

0

1

2

3

4

5

6

7

8

9

10


mal conditio

ry porosity o

e is exposed

durability, wh

whether the

a series of

ete which giv

include pe

pare them w

ity

netration de

ion depth te

fore the test

he samples

n the testing

ours. Therea

bjected to sp

ted zone ap

netration de

cycles. The

xes, but cert

was increase

Mix 5

III‐M20/10

ns on the pr

of concrete a

d to hydrot

hich can red

ere was deg

f experimen

ve indicatio

rmeability,

with those at

epth.

est according

t, the cylind

were coole

g device as c

after, during

plitting by th

ppeared dar

pth of concr

test result

tainly not in

ed from 15 t

Mix 18

I

roperties of

at normal co

hermal con

duce the rem

gradation of

ntal tests h

ns about the

absorption

t normal con

g to DIN EN

rical sample

d down to t

can be seen

g 10 minutes

he machine.

rker [Neville

rete at norm

ts indicated

the same m

to 50 mm d

Mix 1

I‐M20/10

densely pac

onditions an

ditions, sign

maining serv

f the concre

has been pe

e durability

and diffusio

nditions.

12390‐8 ha

es were exp

the room te

in Figure 3.

s of removin

The depth

e, 2004]. Fig

mal condition

that the p

manner. It w

ue to autoc

19 Mi

0‐SF III‐M2

ked concret

nd after auto

nificant cha

vice life of th

ete durabilit

erformed. I

of concrete

on after exp

as been imp

osed to aut

emperature.

13. A pressu

ng the speci

of water pe

gure 6.7 sho

ns (without

penetration

as about thr

laving for 15

x 21

N

5

20/10‐SF

e

oclaving for

nges in its

he structure

ty after hyd

n this sect

have been

posure to a

lemented at

oclaving for

. Then, the

ure head of

mens from t

enetration w

ows the exp

autoclaving

depth is dr

ree folds of

5 cycles. Ho

Mix23

Normal

50 cycles

III‐M20/10

109

50 cycles

properties

rapidly. In

drothermal

tion, some

measured.

autoclaving

t age of 91

r 15 cycles.

specimens

5 bars was

the testing

was directly

perimental

) and after

ramatically

its original

owever, for

Effect of

mix 18 w

times its

imperme

several t

depths.

and 5 m

Figure 6

15 cycle

6.5.1.2

The co

section 3

diamete

20 mm.

thereaft

105 °C u

conditio

carried o

concrete

Contrary

imperme

permeab

0

10

20

30

40

50

60

70

80

90

Water pen

etration dep

th (mm)

f hydrotherm

with OPC o

s original v

eability of co

times of its o

Compared t

m to 24 and

6.7: Water p

es

Air perme

ncrete air

3.4.3.2. Afte

r of 50 mm

Then, the

er they were

until a consta

ns to avoid

out as expla

e in three dif

y to the res

eability of m

bility due to

Mix 1

III

mal conditio

nly, the pen

value (18 m

oncrete afte

original valu

to its origina

d 22 mm for

penetration

ability

permeabili

er curing for

were taken

samples we

e cooled dow

ant mass. Th

d any moist

ained in the

fferent case

ults of wate

most concre

o hydrotherm

Mix 2

III‐FA II

ns on the pr

netration de

m). The ad

er hydrother

ue. At low w

al values, th

mixes 21 an

depth of co

ity was m

r the planne

n. Afterward

ere exposed

wn to the ro

hen, the sam

ure uptake.

e section 3.4

s; normal, w

er penetratio

ete mixes w

mal cycling.

Mix 5 M

I‐M20/10 II

roperties of

epth reache

ddition of fl

mal exposur

w/b ratio, mix

he penetrati

nd 23 respec

oncrete at n

measured ac

ed period, w

s, the samp

to autoclav

oom temper

mples were

. Then, the

4.3.2. Figure

with autoclav

on depth, th

with autoclav

It has the s

Mix 7 Mix

II‐SF III‐M20/

densely pac

ed about 82

y ash show

re. Neverthe

xes 21 and 2

ion depth af

ctively.

normal cond

ccording to

with the help

les were cut

ving for the

rature. The s

cooled natu

measurem

e 6.8 shows

ving for 15 a

he results in

ving. Only m

ame trend a

x 9 Mix 18

/10‐SF I

ked concret

2 mm which

wed a quite

eless, the pe

23 showed q

fter autocla

ditions and a

o the proc

p of drilling m

t to slices w

e designed n

saturated dis

urally to 20 ±

ents and ca

the results

and with aut

ndicated en

mix 18 show

as that of w

8 Mix 19

I‐M20/10‐SF

e

h represents

improveme

enetration de

quite high p

ving increas

after autocl

cedure des

machine sam

with thicknes

number of

scs were ove

± 1 °C in mo

alculations h

of air perm

toclaving for

hancement

wed an incre

water penetr

Mix 21

F III‐M20/10‐SF

110

s around 5

ent in the

epth is still

enetration

sed from 3

aving with

scribed in

mples with

ss of about

cycles and

en dried at

oisture free

have been

meability of

r 50 cycles.

in the gas

ease in air

ration test.

Mix 23

Normal

15 cycles

III‐M20/10

Effect of

For othe

2 with fl

initial va

fine fly a

w/b ratio

permeab

1.4 and

Figure 6

15 and 5

6.5.2 A

In this i

Concrete

days). O

for 15 c

constant

Thereaft

to allow

increase

The abso

of concr

coefficie

increase

coefficie

0

5

10

15

20

25

Air permeability x 10‐17(m

2)

f hydrotherm

er mixes, larg

y ash, after

alue. On the

ash resulted

o, a significa

bility at norm

1.53 x 10‐17

6.8: Air perm

50 cycles

Absorption

nvestigation

e cubes wer

ne week be

ycles. After

t weight. T

ter, the circu

flow of wat

e with time o

orption coef

rete samples

nt of differe

d for most

nt before an

Mix 1

III

mal conditio

ge reduction

autoclaving

other hand

d in a reduct

ant decrease

mal conditio

m2 respectiv

meability co

n (Capillary

n, the capill

re cast, dem

fore the tes

cooling to

hen, the sa

umferences

ter in one d

of concrete m

fficient has b

s. Figure 6. 1

ent mixes aft

mixes with

nd after aut

Mix 2

IIIII‐FA

ns on the pr

n in permea

g for 50 cycle

d, replacing

tion in air p

e in air perm

ons exhibited

vely.

oefficient of

y suction)

ary suction

moulded and

st, some cub

room temp

amples wer

of the speci

irection only

mixes after a

been calculat

10 presents

ter hydrothe

h autoclavin

toclaving for

Mix 5 M

II‐M20/10 I

roperties of

bility can be

es, the air p

part of ordin

ermeability

meability can

d much redu

concrete at

has been d

cured unde

bes were cut

erature, the

re cooled d

mens were

y. Figure 6.9

autoclaving f

ted accordin

the effect o

ermal exposu

g. Mix 18 w

r 15 cycles, i

Mix 7 Mix

II‐SF III‐M20/

densely pac

e noticed. Th

permeability

nary Portlan

to 42 % of

n be observe

uction with a

t normal con

determined

er water at 2

t, polished a

e specimens

own natura

coated with

9 shows the

for 15 cycles

ng to equatio

of hydrother

ure. The res

with OPC s

it was increa

x 9 Mix 18

/10‐SF I

ked concret

he maximum

was reduce

nd cement w

its original v

ed. Mixes 21

autoclaving

nditions and

according t

21 °C until t

and then su

s were oven

ally at mois

transparen

experiment

s.

on 5.3 based

rmal treatm

ults indicate

howed the

ased by abo

8 Mix 19

I‐M20/10‐SF

e

m reduction

ed to about

with 10 % SF

value (mix 1

1 and 23 wit

for 50 cycle

d after autoc

to DIN EN I

the day of th

bjected to a

n dried at 10

sture free c

t paraffin wa

tal results o

d on the ma

ent on the

e that the ab

maximum

out 56 % of

Mix 21

N

1

5

III‐M20/10‐SFF

111

was in mix

27 % of its

F and 25 %

19). At low

th very low

es to about

claving for

SO 15148.

he test (91

autoclaving

05 °C until

conditions.

ax in order

f the mass

ss increase

absorption

bsorption is

absorption

its original

Mix 23

Normal

15 cycles

50 cycles

III‐M20/10F

Effect of

value. Th

in mix 1

plays a p

quite inc

cement a

15 cycles

with fine

Due to h

absorpti

Figure 6

Figure 6

for 15 cy

Increase in m

ass per unit area (Kg/m

2)

0.

0.

0.

0.

0.

0.

Absorption coefficien

t (kg/m

2hr‐0.5)

f hydrotherm

he use of ble

due to auto

positive role

crease in th

and pozzola

s resulted in

e fly ash is th

hydrotherma

on coefficien

6.9: Mass in

6. 10: Water

ycles

00.20.40.60.81

1.21.41.61.82

2.22.42.6

0

0

.1

.2

.3

.4

.5

.6

Mix 1

III

mal conditio

ended cemen

oclaving was

in determin

he absorptio

nic materials

an increase

he only mix s

al treatment

nt of about 9

crease of d

r absorption

1

Mix 1 (III)

Mix 5 (III‐M2

Mix 9 (III‐M2

Mix 19 (I‐M2

Mix 23 (III‐M

Mix 2

III‐FA

ns on the pr

nt reduced t

s 34 % of its

ning the cap

n coefficien

s (mixes 5, 7

e of absorpti

showing a de

t for 200 °C

92 % of its or

ifferent con

n coefficient

Squar

20/10)

20/10‐SF)

20/10‐SF)

M20/10)

Mix 5

III‐M20/10

roperties of

the increase

s original va

pillary suctio

t can be re

7 and 9). At l

on coefficien

ecrease in its

and pressur

riginal value.

crete mixes

t of concrete

2re root of time

Mix 2 (III

Mix 7 (III

Mix 18 (I

Mix 21 (I

Mix 7 M

III‐SF III‐M2

densely pac

in absorptio

lue. Howeve

n of concret

ecognized fo

low w/b rati

nt by about

s absorption

re of 15.5 ba

.

s after autoc

e at normal

3e in hours

‐FA)

‐SF)

)

II‐M20/10‐SF)

Mix 9 Mix

20/10‐SF I

ked concret

on due to aut

er, the use o

te after hyd

or mixes wit

o, the hydro

7 % for mix

coefficient t

ars for 15 cy

claving for 1

conditions

4

18 Mix 19

I‐M20/10‐S

e

toclaving. Th

of pozzolanic

rothermal e

h combinati

othermal tre

21. In contr

than the orig

ycles, mix 23

5 cycles

and after a

9 Mix 21

SF III‐M20/10‐S

112

he increase

c materials

xposure. A

ion of slag

atment for

ast, mix 23

ginal value.

3 exhibited

utoclaving

5

Mix 23

Normal

15 cycles

III‐M20/10SF

Effect of

6.5.3 C

The chlo

section 3

from the

cycles. A

been im

experim

hydrothe

maximu

penetrat

is higher

increase

Howeve

penetrat

Furtherm

exposure

cycles (a

current,

concrete

conditio

mix 18 is

m2/s aft

than tha

Figure 6

for 50 cy

f hydrotherm

Chloride di

oride diffus

3.4.3.4. Cyli

e core of sta

After natural

mplemented

ental result

ermal expos

m penetrat

ted water re

r than the p

e in diffusion

r, for mixes

tion depth

more, mix 2

e. The pene

about 7 mm

the chloride

e mixes. Fig

ns and after

s higher tha

er autoclavi

at of mix 18 a

6.11: Chlori

ycles

0

10

20

30

40

50

60

Chloride pen

etration depth

(mm)

mal conditio

iffusion

ion of conc

ndrical spec

andard concr

lly cooled do

to measure

ts of chlor

sure. The r

ion depth (

eached the o

presented va

n due to au

s with low w

of mix 21

3 exhibited

etration dep

m). From the

e diffusion c

gure 6.12 p

r autoclavin

an 3.7 x 10‐1

ing for 50 cy

after autocla

de penetrat

Mix 5

III‐M20/10

ns on the pr

crete has be

cimens with

rete cylinde

own to the

e the chlorid

ride penetra

esults revea

(> 50 mm).

other side o

alue. On the

utoclaving fo

w/b ratio, h

increased

a good resi

pth remained

e knowledg

coefficient ca

presents th

g for 50 cyc2 m2/s. How

ycles. That m

aving with 5

tion depth o

Mix 18I

roperties of

een measur

diameter o

rs. Then, the

room tempe

de penetrati

ation depth

aled that af

Hence the

f the sample

e other hand

or 50 cycles

high resistan

from 3 to

istance to c

d at the sam

e of penetr

an be derive

e results o

cles. The res

wever, the d

means the r

0 cycles.

of concrete

8 MixI‐M20

densely pac

red accordi

of 100 mm a

e samples w

erature, the

on depth of

h of concre

fter exposu

thickness o

e. So, the re

d, mix 5 wit

s. Similar tre

nce to chlor

4 mm du

hloride pen

me value be

ration depth

ed according

f chloride

sults reveale

iffusion coef

resistance o

at normal

x 190/10‐SF III‐

ked concret

ng to the m

and thicknes

were exposed

e rapid chlor

f concrete. F

ete sample

re to 50 cy

of the samp

eal value of

th slag ceme

end can be

ide migratio

ue to autoc

etration eve

efore and af

h, testing pe

g to the Fick

diffusion of

ed that the d

fficient of m

f mix 21 is a

conditions

Mix 21‐M20/10‐SF

e

methods de

ss of 50 mm

d to autoclav

ride migratio

Figure 6.11

s after 50

ycles, mix 1

ples was 50

penetration

ent showed

observed f

on was obse

claving for

en after hyd

fter autoclav

eriod and th

k’s laws of di

f concrete

diffusion coe

mix 21 was 2

about 15 tim

and after a

Mix 23

Normal

50 cycle

III‐M20/10

113

escribed in

m were cut

ving for 50

on test has

shows the

cycles of

8 had the

0 mm; the

n of mix 18

a little bit

or mix 19.

erved. The

50 cycles.

drothermal

ving for 50

he applied

iffusion for

at normal

efficient of

2.47 x 10‐13

mes higher

utoclaving

l

es

Effect of

Figure 6

for 50 cy

6.6 D

6.6.1 M

The expe

each co

strength

confirme

calcium

strength

pressure

chemica

C/S ratio

formed

strength

properti

after aut

which w

quartz p

The incr

particles

strength

Chloride diffusion coefficien

t

f hydrotherm

6.12: Chlorid

ycles

iscussion

Mechanica

erimental te

ncrete mix

h of mix 18,

ed the sam

silicate hy

h. When ex

e, C‐S‐H is

al and mecha

o of the mat

which has c

h [Bezerra, 2

ies at high t

toclaving fo

was perhaps

powder may

rease of stre

s with incre

h retrogress

0

5

10

15

20

25

30

35

40x 10 ‐1

3 (m

2/sec)

mal conditio

de diffusion

al propertie

est results o

has a distin

made with

me behavior

ydrate (C‐S‐

xposed to

converted

anical prope

trix. For mix

crystalline st

2011]. In spit

emperature

or 50 cycles.

not sufficie

y be limited

ength after

easing the t

ion is obser

Mix 5

III‐M20/10

ns on the pr

n coefficient

s.

of compress

nct pattern

OPC, can be

. Hydration

‐H) which i

high tempe

into severa

erties of con

x 18, becaus

tructure, hig

te of the ad

e, the compr

This could

nt to modify

to the first

5 cycles ma

emperature

rved. Bažan

Mix 18

I

roperties of

of concrete

ive strength

of strength

e significant

of Portlan

is the main

erature, pa

al other pha

ncrete. The

se of the hig

gh specific m

dition of qu

ressive stren

be due to t

y the C/S ra

stage of aut

ay be also d

e. However,

nt has notice

8 Mix

I‐M20

densely pac

e at normal

h after hydro

h loss or ga

tly observed

d cement p

n compone

articularly c

ases, which

produced p

gh initial C/S

mass, high p

uartz powde

ngth of mix

he low cont

tio of the w

toclaving, w

due to acce

with increa

ed similar b

x 19

0/10‐SF II

ked concret

conditions

othermal ex

in. A severe

d. The result

produced ca

nt respons

combined w

h markedly

hases depe

S ratio, lime

permeability

er, which ma

18 is reduce

tent of the a

whole matrix

where the str

lerating the

asing the au

behavior, at

Mix 21

I‐M20/10‐SF

e

and after a

xposure indi

e loss of th

ts of reboun

alcium hydr

ible of the

with saturat

change the

nd mainly o

e‐rich phase

y and low co

ay have the

ed from 72

added quart

x. The influe

rength was

e hydration

utoclaving c

t saturated

Mix 23

Norm

50 cy

III‐M20/10

114

utoclaving

cated that

he residual

nd number

roxide and

e concrete

ted vapor

e physical,

on the bulk

s could be

ompressive

pozzolanic

to 44 MPa

tz powder,

ence of the

increased.

of cement

cycles, the

steam. He

mal

ycles


found a decrease in compressive strength to about 50 % of the original strength [Bažant, 1996].

The residual strength depends mainly on the maximum temperature, bulk C/S ratio, exposure

period and concrete type. Ghosh and Nasser found a significant strength loss after exposure to

high temperature and pressure [Ghosh, 1996]. They attributed that to a gradual deterioration of

the binding matrix with the rise in temperature. At high temperature, chemical transformation of

the gel happened, the dense C‐S‐H gel is subjected to changes and form a weak matrix which

responsible for the loss of strength at high C/S ratio.

Partial replacement of OPC with fine fly ash and silica fume seemed to prevent the strength

retrogression to a certain degree. The results of mix 19 confirm this claim. The residual strength

after 50 cycles of autoclaving was increased from 82 to 95 MPa. The reason for that may be due to

the hydrothermal conditions which accelerate the pozzolanic reaction between quartz powder,

fine fly ash and silica fume with calcium hydroxide. As a result of this reaction, excess of C‐S‐H

phases were formed which is responsible for the strength of concrete. Furthermore, the produced

C‐S‐H has low C/S ratio because of the high silica content of the bulk materials. This is because

when exposed to hydrothermal treatment, silica‐rich phases are transformed to other phases with

higher strength. Poon et al. argued the strength increase to the formation of tobermorite (two or

three times stronger than normal C‐S‐H gel) which is formed by the reaction between fly ash and

lime at high temperature [Poon, 2001]. The results of mix 19 are in good agreement with Khan

who studied the effect of fly ash on the residual compressive strength after exposed to 200 °C. He

found that the residual compressive strength increases with raising the percentage of fly ash

[Khan, 2010]. Another reason could also explain the increased compressive strength. The

interfacial transition zone in concrete usually plays a major role in controlling the mechanical

properties. This is because the high concentration of calcium hydroxide and the high porosity of

this zone. By addition of pozzolanic materials and due to the hydrothermal conditions the

pozzolanic reaction is markedly accelerated in this zone. Thus, denser phases are formed on the

aggregate surfaces which enhance the bond strength between cement paste and aggregate. The

rebound number results for mix 19 showed approximately the same behavior of the compressive

strength results. After hydrothermal exposure, the rebound number increased to about 112 % of

its original value. Similar results were achieved by Lehmann [Lehmann, 2009]. He reported that

compared to ambient conditions, the autoclaved samples showed better mechanical properties.

This was attributed to the high degree of hydration and to the more homogeneous and denser

microstructure.

The use of slag cement revealed low resistance to strength retrogression, as can be shown from

the results of mix 1. However, some of the previous works showed that the use of slag cement


enhances the concrete resistance to fire and to elevated temperature [Khoury, 2000; Poon, 2001].

This contradictory of the results with literature may be due to several reasons. Most of the

investigations tested slag concrete after exposure to elevated temperature only without pressure.

However, the presence of moisture has a significant influence on the compressive strength. It is

believed that water in concrete soften the cement gel, or weaken the surface forces between gel

particles, thus reducing the strength [Cheng, 2004]. Moreover, most of these investigations tested

the slag concrete after exposure to one cycle only. In addition, the concrete composition is totally

different from that tested in literature which certainly affects the results. In this investigation, the

strength of mix 1 with slag cement dramatically reduced with increasing the autoclaving cycles.

The final residual strength of mix 1 after hydrothermal treatment for 50 cycles was about 78 % of

the original value without autoclaving. Similar observation has been found from the results of

rebound number. The final rebound number for mix 1 was reduced to about 87 % of its original

value. Similar conclusion was found by Xi [Xi, 1997]. He noticed a strength retrogression of about

50 % of the original value for autoclaved cement paste made with blended cement in which the

slag content was about 67 %.

Combination of slag cement with fine fly ash (mix 5) exhibited improvement in compressive

strength after hydrothermal exposure. This is in accordance with the results concluded by other

researchers [Bažant, 1996; Xu, 2001]. The explanation was the increased Van der Waal’s forces as

a result of the cement gel layers moving closer to each other. It can be also said that the addition

of 30 % fine fly ash as well as quartz powder along with the slag cement is sufficient to ensure low

C/S ratio of the matrix. Consequently, silica‐rich phases are formed which is associated with high

strength. Additionally, the autoclaving process encourage the dissolution of quartz grains which in

turn produces a better cohesion between fillers and cement paste [Lehmann, 2009].

Consequently, a significant increase in the compressive strength takes place. On the other hand,

the use of silica fume along with slag cement (mix 7) prevents the strength retrogression, but no

significant increase in the final strength has been observed. Despite the strength increase of silica

fume concrete after 5 autoclaving cycles, a gradual decrease in strength with further autoclaving

for 10, 15 and 50 cycles took place. The final compressive strength was about 103 % of its original

value. The reduction in strength of concrete is probably attributable to the high C/S ratio of the

bulk, owing to the low amount of the added silica, about 10 % of the cementitious materials.

Khoury attributed the relatively low performance of silica after temperature exposure to the

probability of micro‐cracks development [Khoury, 2000].


At low w/b ratio, interesting features can be observed. Even after exposing to 50 autoclaving cycles,

the compressive strength of mix 23 was continuously increased with increasing the autoclaving

cycles. The results of compressive strength and rebound number assured this conclusion. The

residual compressive strength of mix 23 was 117 MPa which represented 129 % of its original value.

The possible reason is the greater percentage of unhydrated fly ash and cement particles due to the

low w/b ratio. At high temperature and in the presence of moisture, the hydration of unhydrated

cement grains is increased. In addition, the pozzolanic reaction of fine fly ash with the generated

lime from the hydration of cement is accelerated. Thus, more C‐S‐H with low C/S ratio is formed

which enhances the final compressive strength. Additionally, the microstructure became denser due

to the replacement of CH with C‐S‐H. In addition, due to the low C/S ratio of the bulk, the

hydrothermal treatment resulted in silica‐rich calcium silicate hydrate phases which normally

associated with high strength. Moreover, because of the low w/b ratio, the porosity is very low.

Nevertheless, the additional pozzolanic C‐S‐H fills the micro‐cracks, if any, and blocks the small

pores, which results in more homogeneous microstructure. Due to all these reasons, the

compressive strength of mix 23 showed a significant increase with autoclaving. Compared to mix 23,

mix 21 made with silica fume and fine fly ash revealed similar performance after hydrothermal

exposure. These results agree with Lawson who reported that reducing the w/b ratio enhanced the

residual compressive strength after exposure to 200 °C, particularly in combination with silica fume

[Lawson, 2000]. It is interesting to note from the results of the compressive strength that the

maximum increase for mix 21 was after autoclaving for 15 cycles, while for mix 23 it was directly

after 5 cycles. This could probably explained by the high reactivity of silica fume compared to fine fly

ash. In the case of silica fume, it consumed the produced calcium hydroxide from the hydration of

cement before autoclaving or at the first stages of it. On the other hand, during the first 10 cycles of

autoclaving, the slag cement hydration began to accelerate and more calcium hydroxide was

produced, which consumed by pozzolanic materials in the latter stages of autoclaving. However, for

mix 23, excess of calcium hydroxide could be found in the matrix, and with autoclaving the

pozzolanic reaction was accelerated, which directly resulted in an increase in compressive strength

in the early autoclaving stages.

6.6.2 Porosity

At high temperature, it is difficult to measure the porosity of concrete because it needs special

techniques, but it can be measured easily after cooling down. So, the change in porosity and pore

size distribution due to hydrothermal conditions can only be determined if they have irreversible

changes on the concrete microstructure. Measurement of porosity gives a clear description of the

pore structure and its changes due to hydrothermal treatment. In this investigation, the mercury


intrusion porosimetry has been used to measure the porosity of concrete before and after

autoclaving for 50 cycles.

The results showed a noticeable increase in the porosity of mix 18 made with OPC after autoclaving

for 50 cycles as expected. The transformation of lime‐rich C‐S‐H phases at high pressure and

temperature resulted in weak and porous phases such as α‐C2SH, which leads to a large increase in

the porosity. In addition, the pozzolanic reaction between quartz powder and calcium hydroxide

which favored to happen at these hydrothermal conditions may resulted in phases with high C/S

ratio. This is because of the low amount of the added quartz powder which is not sufficient to

prevent the formation of the weak and porous C‐S‐H phases. The optimum content to prevent the

formation of these phases is about 40 % [Mindess, 2003; Neville, 2004], while the added content

was only about 15 %. However, partial replacement of OPC with pozzolanic materials (mix 19)

reduced the porosity after autoclaving for 50 cycles. This can be attributed to the probability of

reducing the C/S ratio of the bulk system by the addition of silica‐rich materials as well as the

accelerated pozzolanic reaction which consumes the calcium hydroxide crystals. Nevertheless, the

residual porosity of mix 19 after autoclaving for 50 cycles is still higher than the original one and

increased from 6.2 % to about 7.9 %. On the other hand, mix 5 with 30 % fine fly ash and slag

cement with slag content of about 68 % showed an interesting behavior. After hydrothermal

treatment for 50 cycles, the total porosity reduced from 7 % to 5.3 %. This can be explained as

follow; the hydration of slag cement is very slow and produces low amount of calcium hydroxide

which is one of the main sources for calcium ions in the pore structure. With hydrothermal

treatment, the hydration process of slag is significantly accelerated and resulted in generation of

much more C‐S‐H which densifies the microstructure and reduces the porosity volume and sizes. In

addition, the system contains high amount of silica‐rich materials because there are many sources

for silica; from slag, from quartz, and from fly ash. This high silica concentration in the system

ensured the formation of C‐S‐H phases with low C/S ratio either from pozzolanic reaction of silica

with calcium hydroxide or from the transformation of the existed amorphous phases to crystalline

phases due to hydrothermal treatment. The formed phases have low porosity because of their low

density. Similar trend can be clearly recognized for mix 23 with low w/b ratio. Mix 23 with the same

composition as mix 5 behaved in the same way. The total porosity reduced from 4.9 % to 2.65 %

with autoclaving for 50 cycles. However, mix 21 with both silica fume and fine fly ash showed

approximately no change in the total porosity due to autoclaving. The causes of these phenomena

are not easily explainable. In the next chapter, the effect of hydrothermal treatment on the

properties of cement paste and the transformation of different C‐S‐H phases will be explained in

details. From the above discussion and results it can be noticed that compared to pure heat

exposure, hydrothermal treatment leads to significant decrease or increase in concrete porosity


depending mainly on the chemical composition of the bulk system. However, when the concrete is

subjected to high temperature only, a significant increase in porosity occurs. The increase in

porosity may be due to the release of adsorbed water and the coarsening of pore structure. In the

case of pure heat, a strong temperature gradient could be generated. This temperature difference

encourages the moisture escape from the substance during heating which causes an additional

moisture gradient. If the heating rate is rather high, the generated moisture differences may lead to

additional stresses and micro‐cracking or explosive spalling as in the case of fire. This influence is

vanished with autoclaving process which maintains stable moisture content inside the specimen,

thus reduces the micro‐cracking and stresses due to combined moisture and temperature gradient

[Müller, 2008].

The experimental results of capillary porosity (30 nm ‐ 10 µm) reflected some interesting features.

The OPC concrete showed a large increase in the capillary porosity due to hydrothermal treatment

for 50 cycles. The main reason for that as previously discussed is the formation of lime‐rich C‐S‐H

phases. In contrast, notable reduction in the capillary porosity can be clearly observed from the

results of mix 19. In spite of the increase of the total porosity of mix 19 after autoclaving for 50

cycles, the capillary porosity is significantly reduced from 4.5 to 2.4 %. The addition of fine fly ash

and silica fume is responsible for this reduction. The hydrothermal treatment accelerated the

pozzolanic reaction of silica fume and fine fly ash with calcium hydroxide. And as a consequence of

this reaction the calcium hydroxide is replaced by additional C‐S‐H which densifies the

microstructure and modifies the pore size distribution. This result is in agreement with the results of

Shekarchi [Shekarchi, 2002], who found the volume of fine pores tend to increase with autoclaving.

Mix 5 with slag cement and fine fly ash showed approximately similar behavior. The capillary

porosity is reduced from 3.5 % to 1.7 % due to autoclaving for 50 cycles. The main factor responsible

for the porosity reduction is the pozzolanic reaction which resulted in pore size refinement, matrix

densification, paste aggregate interface refinement and the consumption of calcium hydroxide. With

addition of pozzolanic materials, large amount of calcium hydroxide was transformed into C‐S‐H;

while the remaining CH tends to form smaller crystals compared to those in the OPC concrete. The

pozzolanic reaction normally occurs in the capillary voids and the generated C‐S‐H fills the capillary

pores or reduces its size. On the other hand, fine fly ash revealed better behavior than silica fume at

low w/b ratio after hydrothermal exposure. Although the capillary porosity of mix 21 with both silica

fume and fine fly ash nearly remained the same after hydrothermal treatment (about 1.9 %), mix 23

showed remarkable reduction of capillary porosity to about the half (from 2.96 to 1.44 %). This may

be because of the developments of micro‐cracks in the case of silica fume. In both cases, the

capillary porosity was very low (< 2 %). This may be attributed to the low w/b ratio, where the

capillary porosity is low and a considerable part of cement grain remains unhydrated. At high


temperature and pressure, the hydration of cement particles as well as the pozzolanic reaction were

accelerated, which significantly resulted in more reduction in capillary porosity.

Additional comments should be made concerning the pore size distribution. Autoclaving process

generated an obvious change in concrete microstructure and pore size distribution. Figure 6.13

presents the pore size distribution of some concrete mixes at normal conditions and after

autoclaving for 50 cycles. It can be observed that concrete pore structure varied very much with

autoclaving for different concrete mixes. For mix 5, the volume of pores with size lower than 1 µm

was reduced significantly due to hydrothermal conditions. However, mix 18 showed an obvious

enlargement in all pore sizes. This is due to the transformation of C‐S‐H gel into crystalline products

with high density and smaller solid volume which leads to an increase in the porosity and shrinkage.

The presence of silica‐rich materials such as fly ash and silica fume promote the formation of silica‐

rich C‐S‐H phases with smaller change in the density. Additionally, as a result of the accelerated

pozzolanic reaction, the cracks and small pores were filled with the produced C‐S‐H, as can be seen

from the pore size distribution of mix 19. An apparent decrease in the volume of pores in the range

of 10 to 100 nm of mix 19 after hydrothermal treatment for 50 cycles occurred. For mix 21, although

the changes of total pore volume remain rather small, a significant change of the cumulative

distribution in comparison with normal conditions can be observed. Mix 21 showed an increase in

the volume of pores in the size range of about 50 nm to 0.8 µm (capillary range), which

fundamentally affects the transport properties through concrete. However a little bit decrease in

the size smaller than 60 nm took place. Mix 23 with low w/b ratio showed a quite increase in the

volume of pores with size larger than about 140 nm. However a considerable decrease of the pores

with sizes lower than about 140 nm can be significantly observed. The reason for that as explained

above may be due to the late hydration of non‐hydrated cement particles as well as the acceleration

of pozzolanic reaction which resulted in pore refinement and denser microstructure. These results

are in good agreement with those obtained by [Poon, 2001]. He reported that a significant decrease

in porosity and average pore diameter was observed by addition of pozzolans as compared to the

pure OPC concrete.


Figure 6.13: The pore size distribution at normal conditions (dotted) and after autoclaving for 50

cycles (continuous)

6.6.3 Durability

6.6.3.1 Permeability

Two methods were used to measure the permeability of concrete; water penetration depth and air

permeability. The results of water penetration showed a significant increase in the depth of

penetrated water for all mixes due to autoclaving. The penetration for mix 18 reached about 80 mm

after autoclaving for 15 cycles, whereas it was about 18 mm before autoclaving, more than 4 times.

This is due to the increase in total porosity and the modification of the pores connectivity. Similar

trends but with lower values of water penetration depth can be apparently observed for all mixes

0

5

10

15

20

25

30

35

0.001 0.01 0.1 1 10 100

Cumulative volume (m

m3/g)

Pore radius (micron)

Mix 5 (III‐M20/10)

0

10

20

30

40

50

60

70

0.001 0.01 0.1 1 10 100


m3/g)


0

5

10

15

20

25

30

35

40

0.001 0.01 0.1 1 10 100


m3/g)


0

2

4

6

8

10

12

14

0.001 0.01 0.1 1 10 100


m3/g)


0

2

4

6

8

10

12

14

16

18

0.001 0.01 0.1 1 10 100


m3/g)


Mix 18 (I)

Mix 19 (I‐M20/10‐SF) Mix 21 (III‐M20/10‐SF)

Mix 23 (III‐M20/10)


even with low w/b ratio after hydrothermal treatment for 15 cycles. An observation here should be

clarified; in spite of the reduction of porosity of mix 23 (for example) with autoclaving, the water

penetration depth was increased. For this mix, the penetration depth increased from 5 to 22 mm

after autoclaving for 15 cycles. This could be interpreted as follow; the surfaces of pores are

normally rough and narrow for penetration while the heating causes smoothening of these surfaces,

thus the surface energy is decreased. This makes the average width of the necks governing the flow

to increase many times, and in particular the necks of gel pores to be widened [Bažant, 1997]. This

influence simplifies the passage of water through the necks, meanwhile, the pore volume of necks

still remains negligible, and therefore, no remarkable influence of the increased width of the pores

necks on the total porosity can be noticed. Another physical explanation was reported by Bažant,

the low density C‐S‐H phases are transformed to relatively high density C‐S‐H which shrinks and

therefore opens free spaces for the water penetration [Bažant, 2005]. Though the water

penetration depth was significantly increased after autoclaving for 15 cycles, some mixes such as 5,

21 and 23 could be evaluated as impermeable concrete under aggressive conditions, since the

penetration depth is less than 30 mm according to Neville [Neville, 2004].

In the contrary to the results of water penetration, were the results for air permeability, while the

only exception was mix 18. Spite all mixes showed reduction in air permeability, mix 18 with OPC

showed a significant increase at all levels of autoclaving. The reason for that as mention in the

previous paragraph was the significant increase in porosity, in particular the capillary porosity.

Partial replacement of OPC with silica fume and fly ash resulted in a noticeable decrease in air

permeability. This may be due to pozzolanic reaction of fly ash and silica fume with CH which

produces new pozzolanic C‐S‐H. This C‐S‐H blocks the pores and tightens the aggregate‐paste

interface which leads to a reduction in air permeability. At low w/b ratio, in spite of the increase of

water penetration depth, very low air permeability has been detected after autoclaving for 15 and

50 cycles. This could be explained as follow; although the high penetration of water into concrete

happened, that does not mean a continuous passage of water from one side to the other side

existed. On the other hand, for air permeability measurement, the passage should be continuous

from one surface to the other face of the concrete specimens.

6.6.3.2 Absorption (Capillary suction)

The absorption of concrete is strongly depending on the size and connectivity of capillary pores in

the concrete microstructure [Hilsdorf, 1995]. It is stated that if the total porosity is higher than 20 %,

the capillary pores become connective, while at lower porosity the transport of fluids and gases into

concrete is governed by the nano‐pores in the C‐S‐H microstructure [Bentz, 2000]. Therefore, the

characteristics of the formed C‐S‐H phases have an important role in controlling the movements of


gases and liquids through concrete. The experimental results showed that most mixes with w/b

ratio of 0.42 suffered an increase in the absorption coefficient, however some of these mixes

exhibited significant decrease in capillary porosity with autoclaving. The results of capillary suction

test of mix 18 showed a large increase in the absorption coefficient after autoclaving for 15 cycles

from0.32 to 0.50 kg/m2 h0.50. This is due to the presence of high concentration of calcium hydroxide

in the pore structure which could participate in the transformation of C‐S‐H gel into lime‐rich C‐S‐H

phases with high porosity. In spite of the addition of pozzolanic materials, mix 19 showed an

increase in absorption coefficient after hydrothermal treatment for 15 cycles from 0.1 to 0.2 kg/m2

h0.50. On the other hand, mixes with slag cement showed a little bit increase in absorption

coefficient after hydrothermal exposure. The reason for that may be lying in the influence of

autoclaving process in altering the pores sizes and modifying their opening without increasing the

total porosity volume. At low w/b ratio, the results showed very small (negligible) changes in the

absorption coefficient due to exposing to autoclaving for 15 cycles. This is totally agreed with the

results of capillary porosity. The large number of unhydrated cement and pozzolanic materials

particles of mixes with low w/b ratio (mix 21 and 23) affect the capillary porosity and transport

properties intensively. At w/b of 0.42, the microstructure and interfacial transition zone contains

more capillary pores and less unhydrated materials than those with low w/b (0.27). By autoclaving,

the widening of pores opening is more dominant with w/b ratio of 0.42, however, the refinement

and blocking of pores by accelerated hydration of non‐hydrated particles is more meaningful with w/b

ratio of 0.27. In addition, the phases transformation has an important role in this concern as will be

seen in the following chapter.

6.6.3.3 Chloride diffusion

The resistance of concrete to chloride ion ingress is a crucial factor influencing the concrete

durability [Yang, 2002]. The chloride penetration measurement gives indirectly a good indication

about the permeability and pore structure of concrete. The diffusion of chloride took place mainly

through the porous system. The rate of diffusion depends mainly not only on the porosity volume

but also on the physical characteristics of the capillary pore structure [Stanish, 2000]. The

experimental results of chloride ingress into concrete after hydrothermal exposure showed an

increase of chloride diffusion coefficient of most mixes; the only exception is mix 23. As expected

and as has been found from aforementioned experimental tests, mix 18 showed the highest

degradation due to hydrothermal treatment. After autoclaving for 50 cycles, the penetration of

chloride increased and reached the other face of the samples, which mean high rate of diffusion.

Significant enhancement was observed by addition of fly ash and silica fume as can be seen from the

results of mix 19 in both cases; at normal conditions and after autoclaving for 50 cycles. The same

trend can be easily noticed for mix 5. Compared to mix 18 with OPC cement, the use of slag cement


combined with fine fly ash (mix 5) resulted in denser microstructure and formation of silica‐rich

phases or at least reduces the formation of lime‐rich phases to a certain extent, which can explain

the high resistance to chloride diffusion. In addition, hydrothermal exposure accelerated the

pozzolanic reaction with calcium hydroxide which resulted in formation of pozzolanic C‐S‐H phases

with lower porosity compared to normal C‐S‐H phases. Nevertheless, the chloride penetration depth

was increased for mixes 5 and 19, though the total porosity and capillary porosity were significantly

reduced. The reason for that as previously mentioned may be due to the enlargement of the pores

openings which has a negligible effect on the total porosity, but have an important influence on

penetration of water into concrete. At low w/b ratio, silica fume concrete (mix 21) exhibited a

marginal increase in chloride diffusion with autoclaving for 50 cycles. This may be attributed to the

generation of some micro‐cracks associated with the use of silica fume at high temperature. The

chloride diffusion coefficient of mix 21 is still very low after autoclaving, about 2.47 x 10‐13 M2/sec.

However, mix 23 with fine fly ash and without silica fume showed very high resistance to ingress of

chloride before and after autoclaving. This is basically due to the very low content of capillary pores

as well as the dense and homogenous interfacial zone. Due to its spherical shape and its small size,

fine fly ash enhances the packing of particles, particularly near the aggregate surfaces. With

autoclaving, the hydration of cement and the pozzolanic reaction are significantly accelerated and

resulted in much more dense microstructure and homogeneous interfacial zone. In addition, the

hydration of unhydrated cement particles consumes more water, thus the capillary porosity is

reduced, the pore sizes are lowered and the capillary pores connectivity is interrupted. As a result,

concrete with fine fly ash and w/b ratio of 0.27 showed very low chloride diffusion coefficient even

after hydrothermal treatment, it was 4.9 x 10‐13 M2/sec.

Studying the influence of autoclaving on the properties of cement paste 125

7. Studying the influence of autoclaving on the properties of cement paste

7.1 General

Until now, very little information is available concerning the behavior of HPC with mineral admixture

under hydrothermal conditions according to the author knowledge. In chapter 6, the influence of

hydrothermal conditions up to 200 °C and 15.5 bars on the mechanical properties, porosity and

durability of densely packed high performance concrete has been studied. However, to obtain a

widespread information regarding the concrete behavior in these conditions, one should test large

variety of concrete because the presence of aggregate. Subsequently, for fundamental

understanding of the concrete performance, at the first step, it is reasonable to focus on the

ingredients of concrete which is mostly affected by these conditions, namely cement paste. So, this

chapter is focus on the effect of autoclaving with a temperature of 200 °C and saturated vapor

pressure of 15.5 bars for several cycles on the properties of cement paste incorporating various

cementitious materials.

In this chapter, several systems comprising OPC, CEM III/B, silica fume, normal and fine fly ash and

quartz powder have been prepared and tested to analyze their performance after hydrothermal

exposure. Because of the sensitivity of the testing conditions to micro‐cracking formation due to

liberated heat from cement hydration, isothermal calorimeter was used to measure and control the

hydration heat development of all cement pastes at normal conditions. In addition, compressive

strength and porosity of cement pastes have been implemented in order to study the influence of

autoclaving on the paste properties. Moreover, to investigate the microstructural changes due to

the hydrothermal treatment, scanning electron microscopy (SEM) has been performed.

Furthermore, an attempt to understand the chemistry of autoclaving process was carried out by the

help of thermogravimetric analysis (TGA) and energy dispersive X ray spectroscopy (EDX).

7.2 Mixes and tests

In order to study the behavior of cement paste after hydrothermal exposure, 10 mixes with different

compositions have been prepared and tested. Table 7.1 shows the composition and proportions of

different cement pastes. All cement pastes were named with the same names of the concrete mixes

in the previous chapters with the same cementitious materials compositions. Details of the

materials properties and tests procedures were explained in chapter 3. The hydration heat flow of

all mixes has been determined using isothermal calorimeter. On the other hand, the pastes were

mixed and the moulds were cast. After 24 hours the specimens were demoulded and cured under

water at 20 ± 1 °C for 56 days. After the designed curing period, samples were exposed to


autoclaving for 50 cycles at temperature of 200 °C and saturated vapor pressure of 15.5 bars. In

addition to compressive strength, porosity of different mixes was measured using helium

pycnometry at normal conditions and after hydrothermal treatment for 50 cycles. Helium

pycnometry was used also to determine the changes in the pastes density due to autoclaving. SEM,

EDX, and TGA were also used to study the microstructure changes and calcium hydroxide content in

both cases; at normal conditions and after autoclaving with 50 cycles.

Table 7.1: Composition of cement pastes (%)

Mix Cement type Cement content FA Fine FA SF QP w/b SP

Mix 1 CEM III/B 90 10 0.42 0.7

Mix 2 CEM III/B 63 27 10 0.42 1.5

Mix 5 CEM III/B 63 27 10 0.42 1.4

Mix 7 CEM III/B 81 9 10 0.42 0.9

Mix 18 CEM I 90 10 0.42 0.87

Mix 19 CEM I 58.5 22.5 9 10 0.42 1.4

Mix 21 CEM III/B 60.3 22.5 7.2 10 0.27 4

Mix 23 CEM III/B 63 27 10 0.27 3

CEM I CEM I 100 0.30 0.8

CEMIII CEM III/B 100 0.30 0.4

7.3 Hydration of cement paste

7.3.1 Hydration heat

Cement hydration is accompanied with release of a considerable amount of heat and rise in

temperature (exothermic process). The rate and amount of hydration heat depend mainly on the

chemical composition of the mix. Because of the relatively low thermal conductivity of concrete, it

acts as insulator, a temperature gradient between inside and outside the concrete member may

exist. This could lead to thermal stresses and undesired thermal micro‐cracking [Wang, 2010]. In

addition, the significant heat liberation during cement hydration causes thermal shrinkage at early

ages. The thermal shrinkage is hindered by external restraints as well as by the internal restraint

which results from the difference in thermal expansion coefficient between cement and aggregate.

Thus, cracking possibility is very high because the low tensile strength of concrete and the high

thermal shrinkage [Liwu, 2006]. These detrimental effects associated with the release of hydration

heat have a crucial influence on durability and mechanical properties of concrete. It is also

responsible for loss of strength at latter ages and micro‐cracking [Sioulas, 2000]. The generated

micro‐cracks are very critical when concrete subjected to high temperature because it works as

cracks propagation. Therefore, it is important to exactly study the hydration heat development of

different cement mixes in order to select the appropriate mixture for a certain application.


In this investigation, isothermal calorimeter (MC‐CAL/100P) was used to determine the rate of

hydration heat flow as well as the cumulative heat of different cement pastes. A small amount of

each mix (about 10 g) was mixed for 90 seconds and immediately was put in the calorimeter at 20

°C. The measurement started directly after putting the specimen in the calorimeter and extended

for 7 days. Results of the hydration heat of all cement pastes are illustrated in Figure 7.1. It can be

observed that the trends of heat evolution of mixes with CEM I are similar. Two peaks can be

observed. The initial peak can be attributed to both exothermic wetting and the early reaction of

cement with water to form ettringite [Taylor, 1997]. A dormant period with very low heat evolution

followed the initial peak. Thereafter, the reaction is significantly accelerated and the released heat

was increased rapidly until it reaches the main peak, which attributed to the reaction of C3S with

water to form C‐S‐H and CH. After the main peak, the heat release is gradually decreased due to the

slow reaction at the late stage. Compared to CEM I, the addition of 10 % quartz to cement

postponed the peak of mix 18 for about 1.8 hours. The peak of mix 19 was a little bit higher than

that of CEM I and Mix 18. Nevertheless, the arrival time of the highest hydration exothermic rate

was markedly delayed by 3.56 hours due to the addition of 25 % of fine fly ash and 10 % silica fume.

It is also observed that, superplasticizer content caused a retardation effect on hydration. When

superplasticizer increases from 0.8 (CEM I paste) to 1.4 % (Mix 19), the dormant period was

extended by 4 hours. Cumulative heat hydration curves over a 7 days period are presented in

Figure 7.2. The total hydration heat of OPC pastes varied as supplementary materials added. It

reached about 350 J/g after 7 days for mix with neat OPC. Addition of 10 % quartz powder reduced

the total heat to about 306 J/g (mix 18). For mix 19, combination of 25 % fine fly ash and 10 % silica

fume resulted in additional decrease in the cumulative liberated heat to about 287 J/g.

Figure 7.1: The rate of hydration heat development of cement pastes

0

1

2

3

0 10 20 30 40

Heat flow (mW/g)

Time (hours)

CEMI

CEMIII

Mix 1

Mix2

Mix 5

Mix 7

Mix 18

Mix 19

Mix 21

Mix23 (III‐M20/10)

(III‐M20/10‐SF)

(I‐M20/10)

(I)

(III‐SF)

(III‐M20/10)

(III‐FA)

(III)


Figure 7.2: Cumulative hydration heat of different cement pastes

The measurement of hydration heat of slag cement pastes showed some interesting features as can

be observed from Figure 7.1. After the dormant period, the main peak of cement paste made with

pure CEM III/B was about 40 % of OPC. It was occurred approximately at the same time. Similar to

mix 18, the addition of 10 % quartz powder to slag cement (Mix 1) resulted in a decrease of the

height of the main peak, although it took place at the same time. Compared to mixes with fly ash

(mix 2 and mix 5), the addition of 10 % of silica fume (mix 7) accelerated the main peak and

significantly decrease the dormant period. Fly ash fineness seemed to have an important influence

on the hydration heat development. For mix 2 with normal fly ash, the main peak occurs earlier than

that of mix 5 with fine fly ash. Nevertheless, both mixes have approximately the same peak height.

At low w/b ratio, the hydration of cement is significantly retarded. In addition to the lack of water,

high dosage of superplasticizer has a crucial effect on retarding the hydration. The main peak of mix

21 and 23 was reached after about 21 and 27 hours respectively. The results of total liberated

hydration heat for slag cement pastes can be shown in Figure 7.2. The total heat of slag cement

paste after 7 days was 240 J/g which represents about 68 % of that of OPC. By partial replacement of

cement with quartz, silica fume and fly ash, gradual decrease of the cumulative heat was attained (mixes

1, 2, 5 and 7). Interestingly, mixes 21 and 23 showed very low cumulative heat after 7 days of hydration

(121 J/g), which represents only 35 % of that of OPC. These results agree with the conclusions reached

by some other researchers [Kolani, 2012; Merzouki, 2013].

7.3.2 Discussion

Predicting the potentiality of thermal cracking of cement‐based materials at early ages as well as the

development of thermal stresses due to hydration heat of cement requires a deep knowledge of

0

50

100

150

200

250

300

350

400

0 50 100 150 200

Cumulative heat (J/g)

Time (hours)

CEM I

CEM III

Mix 1

Mix 2

Mix 5

Mix 7

Mix 18

Mix 19

Mix21

Mix 23

(III‐M20/10)

(III‐M20/10)

(III‐M20/10‐SF)

(III‐SF)

(I)

(I‐M20/10‐SF)

(III‐FA)

(III)


thermal characteristics of cements. The hydration heat of cement depends mainly on the particle

size distribution, w/b ratio, temperature and the chemical composition of the mix [Taylor, 1997]. At

the first glance, the experimental results showed that the hydration heat of OPC is higher than that

of slag cement. The main peak of cement paste with 10 % quartz (Mix 18) is very close to that of

neat OPC. This may be attributed to the filler effect of quartz which may provide additional

nucleation sites for the hydrates from OPC (seeding effect) [Deschner, 2012]. However, partial

replacement of OPC with fine fly ash and silica fume (mix 19) resulted in a significant extension of

the dormant period and quite shifting of the main peak to the right side. For mix 19, the PH is

considerably decreased due to the absorption of calcium ions on both fly ash and silica fume

particles. Thus, the calcium concentration in the pore solution is reduced which retarded the

hydration, or by other words extended the dormant period. These results match the consequences

reported by [Langan, 2002]. Further explanation can be given by the increased amount of

superplasticizer for mix 19 which has significant retardation effect. Superplasticizer does not affect

the value of the peak, but it extended the dormant period, and thus the main peak is retarded,

shifted to the right direction. The total heat evolution of OPC pastes are approximately proportional

to the OPC content as can be clearly shown from Figure 7.2.

Compared to OPC, the blended cement hydration is more complex due to the presence of different

chemical and physical phenomena in addition to the hydration of cement such as filler effect and

pozzolanic reaction. Contrary to silica fume and fly ash, slag is a cementing material that can be

react by itself with water, but its hydration is very slow [Wang, 2010]. Because of its slow hydration

rate, slag is used in concrete mixes, mass concrete in particular, in order to alleviate the high early

strength and mitigate the risk of thermal cracking. The reactivity of slag depends basically on the

alkalinity of the pore solution [Lothenbach, 2011]. So, the presence of clinker with slag in CEM III/B

is of vital importance to produce suitable structural product. Because of the low reactivity of slag,

the hydration of the clinker portion in slag cement is accelerated due to the availability of sufficient

water for hydration [Merzouki, 2013]. The results of heat flow rate confirm this assumption. The

main peak of CEM III/B occurs roughly at the same time of that of OPC or even quite earlier. The

heat flow flux revealed that the length of dormant period is shortened and the main peak was

attained rapidly due to the addition of silica fume (mix 7) compared to CEM III paste. This may be

attributed to the very small size of silica fume particles which act as nucleation sites [Kadri, 2009].

However, by addition of fly ash, two main effects were coexisting. First, due to the spherical shape

of its particles, it works as excellent filler. Thus, more water liberated and available for accelerate

the hydration. The second effect is the low calcium ions concentration in the pore solution due to

the increased amount of water. Therefore, the dormant period is prolonged. Compared to fly ash

pastes, the main peak of silica fume paste is accelerated. The influences of all supplementary


materials in the first day are attributed to their filler effect only, because the pozzolanic reaction

with calcium hydroxide needs more time to take place [Lothenbach, 2011].

In the case of low w/b ratio, it is clear that the hydration rate is extremely reduced. This is to be

expected for several reasons. The free space for the growth of hydration products is limited because

of the low w/b ratio. Furthermore, due to the low w/b ratio, less water is available for hydration of

cement particles. Moreover, the use of pozzolanic materials along with low water content reduced

the contact area between cement particles and water, thus the hydration rate is decreased [Wang,

2010]. In addition, the use of low w/b ratio requires high amount of superplasticizer in order to

achieve desirable workability. This leads to notably retardation of the hydration. Results of mixes 21

and 23 with w/b ratio of 0.27 showed very long dormant periods and very low peaks. The low peaks

which mean low hydration rate can be attributed to the lake of water available for hydrating the

cement particles, whereas the extension of dormant period may be due to the increased amount of

superplasticizer as mentioned before. Compared to mix 23 with fine fly ash only, the dormant

period of mix 21 with both fine fly ash and silica fume was shortened. This may be due to the effect

of silica fume as discussed earlier. However, the main peak of mix 23 came very late, after about 27

hours. This is may be due to many parameters such as low w/b ratio, use of slag cement, high

amount of superplasticizer and the use of fly ash.

7.4 The influence of hydrothermal treatment on cement pastes

In this part, the effect of autoclaving for 50 cycles with temperature of 200 °C and saturated vapor

pressure of 15.5 bars on compressive strength, porosity, density and microstructure of cement paste

will be presented.

7.4.1 Compressive strength

Compressive strength tests were carried out on 20 mm cubes. After 24 hours, the cubes were

demoulded, and then cured for 91 days under water at temperature of 20 ± 1 °C. After 56 days of

adding the water to the pastes, 3 cubes from each mix were exposed to autoclaving for 50 cycles at

temperature of 200 °C and saturated vapor pressure of 15.5 bars. Then, the cubes were allowed to

naturally cool down. Thereafter, the compressive strength test was performed on the samples at

age of 91 days. Figure 7.3 shows the results of compressive strength of cement pastes without

autoclaving (Normal) and after autoclaving for 50 cycles. Without autoclaving, it is clear that all

mixes have compressive strength in the range of 70 to 80 MPa, the exception is mixes 21 and 23

with low w/b ratio which have high strength. At w/b ratio of 0.42, the addition of normal fly ash

reduces the compressive strength as can be seen for mix 2. On contrary, the addition of silica fume

significantly enhanced the compressive strength (mixes 7 and 19). Mix 21 with w/b ratio of 0.27 and

Studying

with bo

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After au

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with aut

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Figure 7

for 50 cy

0

20

40

60

80

100

120

Compressive stren

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94 MPa. Fo

uite similar.

The additio

he strength

56 MPa wit

of silica fum

not only pr

atio, the situ

to 96 MPa

e exhibited

94 to 98 MPa

ment pastes

7 Mix 18

I I‐M

s of cement p

he highest

he same com

Pa.

ues of comp

made with p

8 MPa at no

nce can be r

bout 65 %.

showed an

or mixes wi

The strengt

on of 10 % o

retrogressio

th autoclavi

e (about 33

evented the

uation is a lit

with autoc

small increa

a.

at normal

Mix 19 M

M20/10‐SF III‐M

paste

compressive

mposition bu

pressive stre

pure OPC (C

ormal condit

recognized f

In contrast

increase in

ith slag cem

th retrogres

of quartz po

on as can b

ng for 50 cy

3 %). Contra

e strength r

ttle bit confu

laving. How

ase in comp

conditions

Mix 21 Mix 2

M20/10‐SF I‐M20

e strength

ut without s

ength can be

CEM I) was

tions to 25

for mix 18 m

t, mix 19 w

compressiv

ment, the in

ssion in cem

owder to sla

e seen for m

ycles. Mix 7

ary to silica

retrogression

used. The co

wever, mix 2

pressive stre

and after a

23 CEMI

No

Au

0/10‐SF

131

at normal

silica fume

e obviously

extremely

MPa after

made with

with partial

ve strength

nfluence of

ment paste

ag cement

mix 1. The

showed a

fume, the

n, but also

ompressive

3 with the

ength with

utoclaving

CEM III

ormal

utoclaved


7.4.2 Porosity

The porosity of cement paste has been measured with two methods; helium pycnometry and water

porosity. The tests have been performed in two cases, at normal conditions and after autoclaving for

50 cycles. The pastes were prepared, mixed and cured under water at temperature of 20 ± 1 °C.

After the designed curing period, three samples from each mix were subjected to hydrothermal

treatment in the autoclave. Then, the samples were naturally cool down and transferred to the

porosity tests. All samples were freeze dried to a constant weight in order to preserve the

microstructure of the paste without any changes as possible. In addition to the autoclaved samples,

the porosity of normal specimens (without autoclaving) has been performed at the same age (91

days) in order to compare the results. The results of the measured porosity showed that both

methods (water porosity and helium pycnometry) have the same trend as can be seen in Figure 7.4

and Figure 7.5. Therefore, the following discussion will focus on the porosity measured with helium

pycnometry only. At normal conditions, the main factor control the porosity is the w/b ratio. Mixes

21 and 23 showed the lowest porosity without autoclaving which were about 21 and 24 %

respectively. Compared to mix 18 with OPC only, addition of fine fly ash and silica fume (mix 19)

significantly reduced the total porosity from 28 % to about 25 % at normal conditions.

With autoclaving for 50 cycles, the porosity of paste made with OPC (CEM I) was significantly

increased from 27 to 38 %. The same tendency can be also observed for mix 18 where the porosity

increased due to autoclaving by about 47 % of its original value at normal conditions. On the other

hand, mix 19 with partial replacement of OPC with pozzolanic materials showed an apparent

reduction in porosity after autoclaving for 50 cycles. The porosity reduced from about 25 to 20 %.

Slag cement showed a higher tendency to resist the porosity increase. The increase in porosity of

neat slag cement (CEM III) was small; it was increased from 36 to about 38 %. A slight increase in

porosity has been occurred for mix 1 with slag cement and 10 % quartz powder, it was increased

from 31 to 32 %. Similar to mix 19, the addition of pozzolanic materials resulted in reduction in the

porosity after exposing to hydrothermal treatment. Mixes 2, 5 and 7 showed reduction of porosity

by about 19, 4 and 10 % of their original values respectively. The same tendency has been revealed

for mixes 21 and 23 with low w/b ration. After hydrothermal exposure for 50 cycles, the porosity

reduced from 21 to about 15 % for mix 21 and from 24 to about 18 % for mix 23.

Studying

Figure 7

Figure 7

7.4.3 D

The den

without

order to

samples

measure

conditio

content

mix with

A small

been ob

0

5

10

15

20

25

30

35

40

45

Total porosity

0

10

20

30

40

50

Total porosity %

g the influen

7.4: Porosity

7.5: Water p

Density

nsity of all c

autoclaving

o minimize t

were finely

e the absolu

ns and afte

have the h

h OPC (CEM

increase in

bserved afte

Mix 1

Mix 1

III

III

nce of autocl

y of cement

porosity of d

cement pas

g (Normal) a

he microstr

y ground (<

ute density.

er autoclavin

highest den

I and mix 18

n density f

er autoclavin

Mix 2 Mi

Mix 2 Mix

III‐MIII‐FA

III‐M2III‐FA

laving on the

pastes mea

ifferent cem

stes has bee

and after au

uctural chan

0.063 mm).

Figure 7.6 i

ng for 50 cy

nsity at nor

8) revealed t

for paste m

ng for 50 cy

ix 5 Mix 7

x 5 Mix 7

III‐SFM20/10

III‐SF0/10

e properties

asured with

ment pastes

en determin

toclaving fo

nges due to

. Then, the

llustrates th

ycles. It is a

mal conditi

the highest d

made with n

ycles. Howev

7 Mix 18

Mix 18 M

I‐MI

I‐M2I

s of cement p

helium pyc

at 91 days

ned using h

or 50 cycles.

drying. Afte

samples we

he measured

pparently s

ions. Howev

density, abou

neat slag ce

ver, with pa

Mix 19 M

Normal

Mix 19 Mix

Normal

III‐MM20/10‐SF

III‐M2020/10‐SF

paste

cnometry

helium pycn

All samples

er drying to

ere sent to h

d density fo

hown that

ver, after a

ut 2.61 and

ement (CEM

artial replace

Mix 21 Mix 2

Autoclave

x 21 Mix 23

Autoclave

M20/10‐SF III‐M

0/10‐SF III‐M20

ometry in t

s were freez

a constant

helium pycn

or all pastes

mixes with

autoclaving

2.66 t/m3 re

M III) and

ement of sla

23 CEM I

ed

CEM I C

M20/10

0/10‐SF

133

two cases;

ze dried in

weight, all

nometry to

at normal

low water

exposure,

espectively.

mix 1 has

ag cement

CEM III

CEM III

Studying

with poz

and 2.52

consider

from 2.5

Figure 7

7.4.4 C

Calcium

on the p

measure

measure

after au

area of d

used for

under ni

the mas

hydroxid

highest

produce

addition

results o

CH, < 5 %

2.2

2.

2.3

2.

2.4

2.

2.5

2.

2.6

2.

Den

sity (t/m

3)

g the influen

zzolanic mat

2 to 2.49,

rable decrea

51 and 2.59 a

7.6: Specific

Calcium hy

hydroxide is

physical and

e the reacti

ed with the

toclaving fo

decompositi

r TGA analy

itrogen flow

ss loss betw

de content o

CH content

d about 16.

of high am

of mix 19 (ab

%. With auto

25

.3

35

.4

45

.5

55

.6

65

.7

Mix 1III

nce of autocl

terials, a sig

2.43 and 2

ase in densi

at normal co

c density (t/m

ydroxide co

s one of the

d chemical p

ivity of diff

help of ther

or 50 cycles.

ion of CH to

sis. The sam

w of 2.4 liters

ween 425 an

of all pastes.

as expecte

2 % of CH. L

ount of poz

bout 4.4 %).

oclaving, mo

Mix 2 MIIIIII‐FA

laving on the

gnificant red

2.41 t/m3 fo

ty also due

nditions to a

m3) of ceme

ontent

major prod

properties o

ferent pozzo

rmogravime

. It can estim

CaO and wa

mples have

s/hour. Calc

nd 550 °C [

. Without au

ed, about 18

Large reduct

zolanic mate

On the othe

ost mixes ga

Mix 5 MixIII‐SI‐M20/10

e properties

duction in de

or mixes 2,

to autoclav

about 2.39 a

ent pastes m

ucts of ceme

of the concre

olanic mate

tric analysis

mate CH in

ater. Approx

been heate

ium hydroxi

[Taylor, 199

utoclaving (n

8.35 %. Mix

tion of calciu

erials due to

er hand, mix

ained more C

x 7 Mix 18II SF

s of cement p

ensity took

5 and 7 r

ving process

nd 2.47 t/m3

measured w

ent hydratio

ete. Estimat

erials. Calciu

(TGA) for a

hydrated p

ximately 750

d to 900 °C

ide content

97]. Figure 7

normal), pas

x 18 with 10

um hydroxid

o their react

xes with slag

CH, the only

Mix 19 M

Norm

I‐M20/10‐SF III‐

paste

place. It red

espectively.

s took place3 for mixes 2

ith helium p

on which exe

tion of CH s

um hydroxid

all mixes at n

roducts by

0 mg of grou

C with heati

has been ca

7.7 shows t

ste with nea

0 % of quar

de content h

tion with CH

g cement pro

y exception w

Mix 21 Mix

mal Autocla

III‐M‐M20/10‐SF

duced from

At low w/

e. The densit

21 and 23 res

pycnometry

ert an impor

serves as a m

de content

normal cond

determining

und, dried pa

ng rate of 1

alculated by

he calculate

at OPC (CEM

rtz in additio

has been gain

H, as can be

oduced low

was mix 18.

23 CEM I

ave

M20/10

134

2.55, 2.49

/b ratio, a

ty reduced

spectively.

rtant effect

method to

has been

ditions and

g the peak

astes were

10 °C /min

the use of

ed calcium

M I) has the

on to OPC

ned by the

seen from

amount of

It reduced

CEM III

Studying

from 16

between

autoclav

to 12 %.

w/b ratio

Figure 7

7.4.5 C

In this p

results i

theoreti

ratio of o

Table 7.

Mix

C/S

The resu

hydrothe

tempera

Howeve

between

tobermo

0

5

10

15

20

25

Calcium hydroxide content %

g the influen

.2 to about

n CH and qu

ving for 50 c

On the oth

o of 0.42.

7.7: Calcium

C-S-H pha

art, the tran

n 100 % tra

cal calculati

oxides and t

.2: C/S ratio

Mix 1 M

1.21

ults of calcu

ermal expos

ature level u

r, for CEM

n the lime‐r

orite, hillebr

Mix 1

III

nce of autocl

13 % with a

uartz powde

ycles. For pa

er hand, mi

m hydroxide

ses transfo

nsformation

ansformatio

on of C/S r

the chemica

o of cemet p

Mix 2 Mix

0.76 0.7

lating the C

sure. At this

up to 200 °

III/B, the C/

rich phases

randite and

Mix 2 M

III‐MIII‐FA

laving on the

autoclaving f

r. However,

aste made w

xes with low

content of c

ormation

of phases h

n of the inp

atio of all c

l compositio

pastes after

x 5 Mix

76 0.93

C/S ratio ind

s level and a

C are lime‐

/S ratio afte

and silica‐r

afwilite can

ix 5 Mix 7

No

M20/10 III‐SF

e properties

for 50 cycles

for pure OP

with CEM III

w w/b ratio

cement pas

has been ca

put materia

ement past

on of the bul

autoclaving

Mix 18

2.1

icated that

according to

rich phases

er hydrother

rich phases,

n be formed

7 Mix 18

ormal Aut

I‐MI

s of cement p

s. This may

PC paste (CE

only, the CH

showed low

stes measur

lculated ass

ls into new

es has been

lk system.

g for 50 cycl

Mix 19 M

0.96 0

CEM I paste

o Figure 2.13

such as of

rmal exposu

, therefore,

. The additio

Mix 19 M

toclaved

III‐MM20/10‐SF

paste

be due to th

EM I), it reac

H content in

wer content

red with TGA

uming that

hydrotherm

n carried ou

les with 200

Mix 21 Mix

0.66 0.7

e has C/S ra

3, the predo

α C2SH, jaf

ure is about

combinatio

on of some

Mix 21 Mix 2

III‐MM20/10‐SF

he pozzolan

ched about

ncreased fro

of CH than t

A

the autocla

mal C‐S‐H ph

ut based on

0 °C and 15

23 CEM I

76 3.2

atio of abou

ominant pha

ffeite and p

t 1.38. This

on of phase

quartz to C

23 CEM I

M20/10

135

ic reaction

21 % after

om about 5

those with

ve process

hases. The

the molar

.5 bars

CEM III

1.7

ut 3.2 after

ases in the

ortlandite.

value is in

es such as

CEM I as in

CEM III


mix 18 reduces the C/S ratio of the bulk to about 2.1. In this case, hillebrandite and α C2SH may be

the predominant phases because the C/S ratio is still high. The EDX measurements emphasized this

claim. The measured C/S ratio of mix 18 after hydrothermal exposure for 50 cycles was about 1.98.

On the other hand, for mix 19 which contains both fine fly ash and silica fume, the calculated C/S

ratio is 0.96. This low C/S ratio leads to formation of the low‐lime phases such as xonotlite and

tobermorite but the formation of tobermorite phase is more likely. The formation of xonotlite is

relatively unlikely because the relatively high alumina content (8.34 mass %) which hindered the

transformation of tobermorite to xonotlite [Bade, 1992]. In addition, the autoclave temperature of

200 °C is quite low for xonotlite formation in the presence of alumina and alkalis. Compared to CEM

III/B paste, the addition of 10 % quartz powder reduces the C/S ratio to about 1.21 as can be

calculated for mix 1 after complete transformation of the bulk materials. At this level, competition

between lime‐rich phases and silica‐rich phases occur. So, for this paste a mixture of αC2SH,

hillebrandite and tobermorite may be coexisted. By addition of fly ash or fine fly ash to CEM III/B as

in mixes 2, 5, and 23, the C/S ratio drops to about 0.76. Due to the low C/S ratio and the high

aluminum content of these mixes, the predominant phases could be the formation of tobermorite

type and gyrolite type. The EDX measurement approximately revealed similar results as can be seen

in Figure 7.9. For mix 5 after hydrothermal exposure for 50 cycles, the C/S was about 0.83. However,

the addition of silica fume significantly reduces the C/S ratio to about 0.93 (mix 7) which promotes

the formation of gyrolite‐type. The formation of gyrolite is more likely than tobermorite because the

low aluminum content in silica fume compared to fly ash. Finally, the calculated C/S ratio of mix 21

made with silica fume, fine fly ash in addition to CEM III/B is about 0.66 after complete hydration.

This very low C/S ratio encourages the formation of low‐lime phases such as tobermorite, gyrolite

and truscottite.

7.5 Discussion

At normal temperature, hydration of OPC produces about 50 % of C‐S‐H and about 15 ‐ 25 % of CH

[Taylor, 1997]. The produced C‐S‐H is approximately amorphous with C/S ratio of about 2 [Hong,

2004], which is strong binding materials (by Van der Waals force of attraction) responsible for

strength of cement and concrete at temperature below 110 °C. The experimental results confirm

these claims. The calcium hydroxide of CEM I paste was about 18 %, and increased with autoclaving

to 21 %. However, after hydrothermal treatment, C‐S‐H is no longer stable, it is substituted by more

stable phases with higher crystallinity and high specific density [Bezerra, 2011]. The composition and

properties of the formed phases depends mainly on the C/S ratio, water/solid ratio (W/S) and on the

period of exposing to hydrothermal treatment [Meducin, 2008]. As a result, the compressive

strength of CEM I paste was reduced (strength retrogression) from 78 to about 25 MPa. This is

attributed to the formation of lime‐rich phases such as hillebrandite, jaffeite and α C2SH. The


formed phases have high density as has been reported by experimental density tests (Figure 7.6).

The increased density leads to a reduction in the solid volume and accompanied by an increase in

porosity and reduction in strength. The high lime content of C‐S‐H comes from two main sources;

the initial C‐S‐H which has high C/S ratio due to hydration of clinker and the high content of CH

which offer a reservoir for the calcium ions [Ramachandran, 2003]. The specific density of CEM I

paste is increased from 2.42 to 2.61 t/m3 due to autoclaving for 50 cycles. The increased density can

be considered as an indication for the formation of deleterious α C2SH and hillebrandite phases,

which reported to have a density of about 2.7 t/m3 [Taylor, 1997]. As a result, large increase in

porosity took place due to the increased density. The experimental results of porosity agree with

this assumption very well. It has been increased from 29 to 41 % with autoclaving for 50 cycles.

In spite of addition of 10 % quartz powder to the system (mix 18), the compressive strength was

reduced with autoclaving. It was decreased from 77 to 27 MPa after hydrothermal exposure for 50

cycles. The added siliceous material is not sufficient to prevent the strength retrogression. The C/S

ratio is still high and suitable to form lime‐rich phases. Confirmation for this comes from the

calculation of C/S ratio as can be seen in Table 7.2 and the observation of EDX in Figure 7.9. The C/S

ratio of mix 18 was around 2 after autoclaving for 50 cycles. The formed phase at this level is alpha

dicalcium silicate hydrate and hillebrandite, which crystallize as dense rectangular tablets with high

porosity and low strength [Taylor, 1997]. The density of mix 18 increased from 2.45 to 2.66 t/m3

with autoclaving for 50 cycles as can be seen in Figure 7.6. The formed lime‐rich phases with high

density increased the shrinkage which resulted in huge increase in porosity as can be shown in the

results of porosity (Figure 7.4). The SEM images of the surface of the polished specimens at age of

91 days in both cases normal and after autoclaving for 50 cycles can be shown in Figure 7.8. At

normal conditions, higher concentration of ettringite, monosulphate and the oriented crystals of

calcium hydroxide are existed, which is generally considered a weaker component of the hydration

products. However, after autoclaving, SEM photos clearly showed very porous microstructure.

The incorporation of large amount of silica‐rich materials, fly ash and silica fume in addition to 10 %

quartz powder, resulted in not only preventing strength retrogression, but also enhancement in the

compressive strength after autoclaving as can be noticed from the results of mix 19. The

compressive strength increased from 83 to about 94 MPa after hydrothermal treatment for 50

cycles. The density measurement showed little change due to hydrothermal treatment. It reduced

from 2.45 to 2.44 t/m3. These are probably due to the low C/S ratio of the bulk (C/S equals 0.96

according Table 7.2), which prevents the formation of lime‐rich phases and instead silica‐rich phases

such as 1.1 tobermorite, xonotlite and gyrolite are formed. These phases have quite low density

which leads to dense microstructure with low porosity. The porosity of mix 19 is reduced from 25 %


to about 20 % after autoclaving. The low CH content also may have an influence on reducing the

porosity and enhancing the strength, because it was replaced with C‐S‐H. Thus, additional amount of

C‐S‐H is added to the system. Both low clinker content in the bulk and the incorporation of

pozzolanic materials are responsible for reducing the amount of CH which normally associated with

high porosity and weak strength. Compared to mix 18, mix 19 exhibited lower porosity after

hydrothermal exposure. The porosity of mix 19 is about the half of mix 18 after autoclaving.

Figure 7.8 shows the SEM images which revealed dense microstructure of mix 19 (normal), with

non‐reacted spherical fly ash particles embedded in the dense cement paste. After autoclaving, very

dense and homogeneous microstructure is formed with partially reacted fly ash covered with C‐S‐H

phases. These observations indicated how slow the pozzolanic reaction of fly ash with CH is. The

results of calcium hydroxide content of mix 19 showed that it was increased with autoclaving. This

may be because the slow reaction of pozzolanic materials as well as the low solubility of CH,

particularly with increasing the temperature [Taylor, 1997]. On the other hand, the transformation

of phases depends mainly on the C/S ratio of the bulk. Proof for that is from the results of mixes 18

and 19. At normal conditions, both mixes have a density of about 2.45 t/m3. However after

autoclaving, mix 18 with higher C/S ratio exhibited specific density of about 2.66 t/m3, whereas the

specific density of mix 19 with lower C/S ratio was reduced to about 2.44 t/m3. These results clarify the

important role of C/S ratio on the properties and transformation of C‐S‐H phases after autoclaving.

Similar results on the influence of siliceous materials on the strength of cement paste were reported by

Jing et al. [Jing, 2008]. Who attributed the enhancement of strength to the formation of tobermorite and

disappear of lime‐rich phases.

Pure slag cement paste (CEM III/B) showed apparent strength retrogression after autoclaving. It was

reduced from 71 to about 48 MPa after autoclaving for 50 cycles. Similar results were reported by Xi

[Xi, 1997]. He found that the compressive strength of slag cement with slag content of about 67 %

was reduced to about 50 % after autoclaving with 200 °C and 15.5 bars. Despite slag cement (CEM

III/B) contains relatively high amount of silica, the compressive strength is reduced with autoclaving.

The C‐S‐H formed from the hydration of slag cement has quite high C/S ratio (about 1.7). In this

level, lime‐rich phases such as afwilite, alpha dicalcium silicate and hillebrandite could be formed

[Meller, 2005; Bezerra, 2011]. The strength retrogression observed from the experimental results

indicated the major presence of the lime‐rich phases. The quite high C/S and the alkalinity of the

pore solution are such that αC2SH and hillebrandite are dominant. Furthermore, the density and

porosity measurements showed quite increase after hydrothermal exposure. This is because the

high density of the formed phases leads to coarsening the pore structure which increases the


porosity and reduces the strength. Similar results but with lower values were noticed for mix 1 with

addition of 10 % quartz powder to slag cement system. The compressive strength reduced from 74

to about 56 MPa. The reason for that may be the quite high C/S ratio, although it is lower than that

of paste made with pure slag cement. The calcium ions are supplied from two main sources: one

from clinker hydration and the second from the hydration of slag itself [Kolani, 2012]. Regarding CH

content, both pastes (CEM III and mix 1) have the similar initial CH content, about 5 %. While, after

hydrothermal exposure, the CH content was about 12 and 8 % for mixes CEM III and mix 1

respectively. The calcium hydroxide content relies mainly on both the hydration degree of the

clinker and on the progress of slag hydration. The first produces CH, whereas the latter consumes the

CH. The reduced amount of mix 1 is due to the addition of quartz powder. This effect emphasized the

pozzolanic reactivity of quartz under hydrothermal treatment when it is finely ground.

The incorporation of fly ash in combination with slag cement significantly enhanced the compressive

strength after hydrothermal exposure. The compressive strength increased by about 18 % and 10 %

for mixes 2 and 5 respectively. This may be attributed to the formation of 1.1 tobermorite which has

been accelerated with increasing the aluminum content [Matsui, 2011]. This phase is known to

enhance and stabilize compressive strength [Kyritsis, 2009]. In addition, the strength reducing phase

αC2SH will not be formed anymore because of the low C/S ratio. Beaudoin reported that fly ash

performance at high temperature is similar to that of pure silica, can apparently reduce the C/S ratio

and prevent the formation of αC2SH [Beaudoin, 1979]. Figure 7.8 shows the SEM of mix 5 before and

after autoclaving. The non‐reacted fly ash sphere can be clearly seen. This confirms the slow

hydration of slag cement and the very low rate of pozzolanic reaction. This is attributed to the low

alkalinity in the pore structure, since the reaction of slag is activated by the liberated CH. The results

from TGA for CH content showed that the amount of liberated CH was about 4 % before autoclaving

and increased to about 6.5 % after hydrothermal exposure. After autoclaving, SEM of mix 5 showed

the formation of C‐S‐H, probably tobermorite phase as reported in literature [Garbev, 2004].

Another evidence for the possibility of tobermorite formation can be easily detected from the

results of EDX. The C/S of pore solution was about 0.94 and reduced to 0.83 after autoclaving which

is suitable for the formation of silica‐rich phases such as tobermorite. Taylor stated that, C/S ratio in

the range of 0.8 ‐ 1 is optimum for formation 1.1 tobermorite [Taylor, 1997]. In addition, the density

of slag cement paste was apparently reduced by addition of fly ash, either normal or fine. The

density of mix 2 and 5 after hydrothermal treatment was 2.49 and 2.43 t/m3. This could be considered

an evidence for formation of tobermorite which has a specific density of around 2.44 [Taylor, 1986].

These results reflected the ability of fly ash to prevent the strength retrogression by formation of

more stable and strong phases. The main benefit of fly ash comes from the high content of silica and

aluminum, that encourage the formation of 1.1 tobermorite which responsible for reducing the


porosity and permeability [Kyritsis, 2009]. These results are in good agreement with Hilsdorf

[Hilsdorf, 1986], who confirmed the ability of fly ash to prevent strength retrogression. On the other

hand, conflict results were obtained about the behavior of silica fume. Despite the porosity and

density of paste made with silica fume was reduced, strength retrogression took place. Similar

results about the behavior of silica fume was found by Luke [Luke, 2004], who reported that

undefined phases were formed with the use of silica fume. In addition, alpha dicalcium silicate were

detected which could explain the strength retrogression in the case of silica fume. These effects can be

attributed to the low amount of aluminum in the bulk solution, and the possibility of microcracking.

At low w/b ratio, interesting results were obtained. Considerable strength retrogression for mix 21

was observed. The strength was decreased from 109 to 96 MPa (about 13 %) due to autoclaving for

50 cycles. On contrary, mix 23 with the same composition but without silica fume exhibited very

good resistance to strength retrogression. Not only the strength retrogression is prevented, but also

a little bit increase in the compressive strength was achieved after exposed to 50 autoclaving cycles.

It was increased from 94 to 98 MPa with autoclaving. This difference in strength can be attributed to

the formation of different phases in both cases (mixes 21 and 23). At hydrothermal conditions, the

system and the formed phases were very sensitive to any change in the chemical composition of the

pore solution. The high content of aluminum incorporated by the addition of fly ash enforced the

formation of more stable phases (mix 23). However, the addition of silica fume resulted in

significant strength retrogression which can be attributed to the low aluminum content in the

system. In addition, it can be also attributed to the high probability of micro‐cracking formation in

the case of silica fume, particularly with hydrothermal exposure. Although both mixes (21 and 23)

exhibited difference performance concerning the compressive strength, the density and porosity are

similar. The density of mix 21 and 23 was reduced from 2.51 and 2.59 to 2.39 and 2.47 t/m3

respectively after hydrothermal exposure. Similar trend can be observed for porosity results. At

normal conditions, very low porosity was attained due to the low w/b ratio. The low water content

offered small space for the cement to hydrate. The C‐S‐H can form only if enough free space is

available [Taylor, 1997]. In addition, the added pozzolanic materials work as filler, thus more

reduction in porosity can be achieved. After autoclaving, a significant decrease in porosity took

place. The hydrothermal treatment lead to formation of silica‐rich C‐S‐H phases which resulted in

dense microstructure as can be observed from the results of mixes 21 and 23. The porosity of mixes

21 and 23 reduced from 21 and 25 % at normal conditions to 15 and 18 % respectively after

autoclaving for 50 cycles. On the other hand, the results of calcium hydroxide content of mixes 21

and 23 shows that liberated calcium hydroxide in the pore solution was small, which indicated the

low hydration degree. However after hydrothermal treatment, small increase in the CH content of

both mixes with low w/b ratio occurred.

Studying

Figure

autoclav

g the influen

7.8: SEM

ving for 50 c

nce of autocl

measurem

cycles (right

laving on the

Mix

Mix 19

ents for ce

t)

e properties

5 (CEMIII‐M

Mix 18 (CEM

9 (CEMI‐M20

ement pas

s of cement p

20/10)

MI)

0/10‐SF)

tes at nor

paste

rmal condit

ions (left)

141

and after

Studying

Figure 7

g the influen

7.9: EXD an

nce of autocl

alysis of ce

laving on the

ment at nor

M

e properties

rmal conditio

Mix 5 (CEMII

Mix 5 (CEM

M

Mix

s of cement p

ons and afte

I‐M20/10) n

MIII‐M20/10)

Mix 18 (CEMI

x 18 (CEMI) a

paste

er autoclavi

ormal

) autoclaved

I) normal

autoclaved

ing for 50 cy

142

ycles

Applying the optimized concrete in hot water tank 143

8. Applying the optimized concrete in hot water tank

8.1 Introduction

In this investigation and as previously discussed, optimized concrete mixes have been prepared and

tested in the laboratory and they showed stable properties regarding concrete strength and

durability after hydrothermal exposure. However, for hot water tank, the concrete has to be tested

in the real conditions, where it will be implemented and subjected to different loading conditions. In

addition, it is important from the safety point of view to study the failure mode of concrete tank in

such aggressive conditions. This chapter presents the experimental results of the preliminary stage

of producing a concrete tank to store hot water and steam at temperature higher than 100 °C based

on the results of this thesis.

8.2 Concrete mixture

In these preliminary experiments, mix 23 with small modifications has been used for constructing

the concrete tank models. This mix has been chosen because of the low amount of hydration heat

as well as its high durability and stability after hydrothermal exposure for 50 cycles. Table 8.1 shows

the mix composition and the properties of the used concrete. Figure 8.1 and Figure 8.2 show the

concrete mixing and the measurement of fresh concrete properties.

Table 8.1: Composition and properties of the used concrete for prliminary experiments

Composition

CEM III /B 32.5 (kg/m3) 224

Fine fly ash (kg/m3) 96

Aggregate (4 – 8) (kg/m3) 885

Aggregate (2 – 4) (kg/m3) 546

Sand (0 – 2) (kg/m3) 485

Quartz sand (kg/m3) 82

Quartz powder (kg/m3) 45

Superplasticizer (kg/m3) 6

Water (kg/m3) 96

Total (kg/m3) 2465

Rheological properties

Fresh concrete density (kg/m3) 2470

Flow diameter (cm) 47

Temperature of fresh concrete (°C) 18.3

Air content (%) 1

Applying

Hardene

Compre

Splitting

Water p

Total po

Figure 8

Figure 8

g the optimiz

ed concrete p

essive strengt

g tensile stren

penetration d

orosity (91 da

8.1: Prepara

8.2: Flow dia

zed concrete

properties

th (91 days)

ngth (91 days

epth (91 day

ays)

ation for con

ameter and

e in hot wat

(M

) (M

s) (m

(%)

ncrete mixin

air content

er tank

Pa)

Pa)

m)

)

g

of the usedd concrete

85

5.

6

6.

5

.2

.9

144

Applying

8.3 C

A cylindr

has been

As can b

are fixed

surface a

Figure 8

8.4 Fi

In the e

installing

tank and

outside

capacity

been use

heating

surface

parallel

tempera

moment

the conc

and out

g the optimiz

Concrete ta

rical tank m

n built as ca

be seen from

d in both sid

and steel pla

8.3: Hot wat

irst experi

experiment,

g the upper

d on the ou

the tank. In

y of 10 KW to

ed to measu

rate (1 ‐ 2 °C

have been o

to each oth

ature differe

t with high te

crete tensile

tside the co

zed concrete

ank model

odel with di

an be seen in

m the figure,

des. In addit

ates in order

er concrete

iment

the concre

steel plate,

uter surface

n addition, a

o heat up th

ure the temp

C/min). The

observed an

her as can b

ences throug

ensile stresse

strength, th

oncrete tank

e in hot wat

l

iameter of 8

n Figure 8.3

both sides

tion, a dens

r to prevent

tank mode

ete tank mo

thermocoup

e as well in

a pressure g

he water has

perature dis

results of th

nd the wate

be shown fro

gh the tank

es on the co

erefore crac

k reached 7

er tank

850 mm, hei

. This tank h

of the tank

sifying mate

any escape

l

odel has be

ples have be

order to m

gauge has b

s been insta

stribution in

he test show

er began to

om Figure 8

wall. This t

ncrete wall.

cking occurre

72 °C as m

ght of 600 m

has been co

are opened

rial has bee

of water an

een partly f

een fixed on

measure the

been also im

lled. On the

the concret

wed that at 9

escape. All

8.5. These cr

temperature

The generat

ed. The temp

measured wi

mm and wal

onstructed w

. Therefore,

en installed b

nd vapour.

filled with w

the bottom

temperatur

mplemented.

other hand

te wall. The

90 °C some

these crack

racks are fo

e difference

ted tensile st

perature diff

th thermoc

ll thickness o

without reinf

, two thick s

between the

water (20 °

m and on the

re changes

. Electric h

, infra‐red c

test started

cracks at th

s are longit

rmed due t

leads to te

tresses are h

ference betw

couples. The

145

of 125 mm

forcement.

steel plates

e concrete

C). Before

top of the

inside and

eater with

camera has

d with high

e concrete

udinal and

o the high

emperature

higher than

ween inside

e Infra‐red

Applying

camera

as can b

been de

been ob

Figure 8

Figure 8

g the optimiz

measureme

be seen in fig

etected. Mor

served.

8.4: Prepara

8.5: Longitud

zed concrete

ents showed

gure 6. Rega

reover, no m

ation for the

dinal cracks i

e in hot wat

also the hig

arding concr

micro‐cracki

first experim

in the wall of

er tank

gh temperat

rete perform

ng or any d

ment

f the concre

ture differe

mance, no d

etrimental i

ete tank mod

nce through

eterioration

nfluence on

del due to tem

hout the con

n in concrete

n concrete s

mperature g

146

ncrete wall

e itself has

surface has

gradient

Applying

Figure 8

8.5 Se

To cope

two‐laye

23) with

addition,

tempera

thermoc

of heatin

continuo

that poin

widened

plate cre

energy (t

steam w

stored e

cracking

vapour h

g the optimiz

8.6: Temper

econd exp

with the pro

er tank mode

out reinforce

, a thermal i

ature gradie

couples. In th

ng, the temp

ous heating,

nt, the steel

d. In addition

eated. Cons

the height o

was continuo

energy inside

has been ob

has been occ

zed concrete

rature distrib

periment

oblem of tem

el has been

ement, whil

nsulating ma

nt. The tem

his experime

perature of w

the tempera

l plate was d

n, the densify

equently, st

of the steam

ously escapi

e the tank.

bserved in in

curred.

e in hot wat

bution on th

mperature g

developed.

e the outer

aterial has b

mperature b

ent, low hea

water inside

ature was ra

deformed an

ying materia

team with h

column reac

ng for more

After coolin

nternal or ext

er tank

e concrete

gradient (tem

The inner la

layer was m

been fixed on

between the

ting rate has

the tank rea

aised slowly

nd the dista

l has expand

high pressur

ched about 1

e than 15 m

ng down, th

ternal concr

tank measu

mperature m

ayer was ma

ade of norm

n the surface

e two layers

s been appli

ached 155 °C

to 162 °C an

nce betwee

ded and a ga

re is sudden

15 meters as

minutes whic

he concrete

rete layers. In

ured using th

moment) and

de of the op

mal concrete

e of the tank

s has been

ed (10 °C/ho

C as can be s

nd the press

en it and the

ap between c

nly escaped

s can be sho

ch reflect th

tank has be

n addition, n

he Infra-red

d longitudina

ptimized con

with reinfor

k in order to

measured

our). After s

een in Figure

sure reached

e concrete su

concrete wa

with large a

own in Figure

he high amo

een examin

no diffusion o

147

d camera

al cracking,

ncrete (mix

rcement. In

reduce the

also using

ome hours

e 8.7. With

d 6 bars. At

urface was

ll and steel

amount of

e 8.7). The

ount of the

ed and no

of water or

Applying

g the optimiz

The ta

The te

Figure 8.7

zed concrete

ank is totally

mperature w

Large

: The secon

e in hot wat

y insulated f

was continu

amount of e

nd prelimina

er tank

rom all sides

ously increa

energy and s

ary test on tw

s to reduce t

ased with he

steam has b

wo-layer ho

temperature

ating and re

een released

ot water con

e difference

eached 155 °

d

crete tank m

148

s

°C

model

Applying

8.6 Fu

Based o

hot wate

large cyl

hot wate

several w

tank, the

problem

optimize

prestres

totally in

the curr

can be s

seen in F

Figure 8

g the optimiz

uture work

n the result

er tank mod

lindrical tank

er tank mod

weeks. It ca

e steel plate

m of the stee

ed concrete

sed steel in

nsulated fro

ent stage, t

een in Figur

Figure 8.9.

8.8: Internal

zed concrete

k

ts of this inv

del, a new re

k model (pro

del will be su

an be also u

will be repl

el plate. The

without re

n order to k

m outside in

he internal

e 8.8. Moreo

layer of the

e in hot wat

vestigation a

esearch proje

ototype) wit

ufficient to f

used for pro

aced with a

e tank will b

inforcement

keep the co

n order to re

layer of the

over, the mo

e future conc

er tank

and on the r

ect has been

th dimensio

feed one ho

oducing elec

concrete do

be also two‐

t, while the

oncrete wall

educe the te

e tank has b

ould for the

crete tank m

results of th

n planned an

n of 1800 x

ouse (100 m2

tricity by us

ome (half sp

layered; the

e outer one

ls under com

emperature

been constru

concrete do

model made

he prelimina

nd already s

1250 mm w

2) with the r

sing Peltier

phere) becau

e internal on

will be of

mpression.

difference a

ucted with t

ome has bee

e of the optim

ry experime

started. In th

will be perfo

required hot

effect. In th

use of the de

ne will be m

normal con

The tank w

and therma

thickness of

en fabricated

mized conc

149

ents of the

his project,

rmed. This

t water for

he planned

eformation

ade of the

crete with

will be also

l losses. At

70 mm as

d as can be

rete

Applying

Figure 8

g the optimiz

8.9: The fab

zed concrete

ricated mou

e in hot wat

uld for concr

er tank

rete model tto be used iinstead of ssteel plate

150

Conclusions and recommendations 151

9. Conclusions and recommendations

9.1 Conclusion

The research project presented within this PhD‐thesis was focused mainly on studying the possibility

of developing a dense high performance concrete mixture for a concrete tank to store solar thermal

energy in hot water and steam at high temperature up 200 °C and pressure of 15.5 bars. In the

targeted tank, the leakage of water and vapour is prevented by implementing a very dense high

performance concrete. The optimized concrete should exhibited high efficiency and stability after

exposing to various autoclaving cycles. An important subject in this respect is the stability of C‐S‐H

phases after hydrothermal exposure with several cycles. Many factors influence the behaviour of

concrete under these hydrothermal conditions. Regarding transport properties through the

optimized concrete, the mineralogical properties of concrete constituents are of more concern.

However, chemical composition of the concrete mixture is considered the basic parameter with high

impact on the stability of C‐S‐H phases.

The first step of this investigation is to develop a high density concrete mixture. Increasing the

packing density is considered an excellent strategy to get an optimized concrete mix. This can

achieved via three main mechanisms. The first one is maximizing the packing density of solid

particles by applying the Ideal Fuller curve for aggregate and fine materials grading. The second

mechanism is designing a dense cement matrix taking into consideration several items such as

physical and chemical properties of fine materials. The third mechanism is densifying the interfacial

transition zone which is the locus of micro‐cracking and affects the concrete durability and

mechanical properties.

The experimental work has been divided into three main parts. In the first one, a high density

concrete mixture has been optimized and tested. In this concern, 23 mixes with more than 560

standard cubes and 90 standard cylinders have been prepared and tested. The second part of this

investigation focused on the effect of autoclaving with 200 °C and 15.5 bars on properties of the

optimized concrete. Mechanical properties including compressive strength, tensile strength and

rebound number as well as concrete porosity have been measured at normal conditions and after

autoclaving with numerous cycles. Regarding the concrete durability, the main three mechanisms of

ingress of gases and liquids through concrete; permeability, absorption and diffusion, have been

measured before and after autoclaving for 50 cycles. In the third part, 10 different cement pastes

have been prepared and tested in order to deeply understand the effect of hydrothermal conditions on

characteristics and stability of C‐S‐H phases. In this concern, EDX, TGA and SEM measurements have

been used to study the changes in pastes morphology and properties due to autoclaving for 50 cycles.


Based on the experimental results, the following conclusions can be drawn:

Concrete mixture optimization:

The Ideal Fuller curve is different from the well‐known Fuller parabola. It has lower coarse

aggregate content and higher fine materials content.

By applying the Ideal grading curve, it is possible to produce HPC with superior properties using

only 312 kg/m3 of cementitious materials.

In spite of the use of low amount of cementitious materials (312 kg/m3), with w/b ratio of 0.27,

the compressive strength reached about 100 MPa, while the elastic modulus was more than 50

GPa. This can be directly related to the high packing density of the mixture.

The maximum porosity of concrete mixes was 10 % for mix with OPC and without any

supplementary materials, which reveal the important role of packing density on concrete

porosity. However, with replacing part of OPC with 25 % fine fly ash and 10 % silica fume (mix

19), the total porosity reduced to 6 %. On the other hand, a total porosity of 3 % has been

achieved by combination of high packing density and low w/b ratio.

A very low capillary porosity was obtained for all mixes compared to tradition concrete and

HPC. In particular, mix 21 with w/b ratio of 0.27 has a capillary porosity of lower than 2 % which

is in the range of UHPC.

According to Neville, all concrete mixes can be considered impermeable under aggressive

conditions (water penetration depth < 20 mm). For mix 23, it reached 3 mm only.

The use of supplementary materials with low w/b ratio has reduced the chloride diffusion

coefficient of concrete 20 times lower than that of OPC.

A strong relationship has been found between the measured total porosity and mechanical

properties of concrete. However, the durability aspects were found to have clear relations to

the capillary porosity of concrete.

Generally, silica fume is more effective in improving the durability and strength of concrete in

systems with low w/b ratio than systems with high w/b ratio.

Effect of autoclaving with 200 °C and 15.5 bars on the properties of concrete:

Mixes with pure cement either OPC or slag cement showed a large decrease in compressive

strength after autoclaving for 50 cycles. However, the addition of fly ash stabilized the concrete

and prevented the strength retrogression.

At low w/b ratio, the addition of supplementary materials not only prevented the strength

retrogression, but also it increased the compressive strength to 130 % of its original value after

autoclaving for 50 cycles.


The porosity of OPC concrete was increased by about 30 % after autoclaving. However, partial

replacement of cement with fine fly ash and silica fume reduced the increase in the porosity

after autoclaving for 50 cycles.

Combination of fine fly ash and slag cement exhibited a decrease in total porosity after

autoclaving. It reduced from 7 to 5 % with w/b ratio of 0.42. However, at low w/b ratio, it has

been decreased from 5 to 2.6 % after hydrothermal treatment for 50 cycles (mix 23).

The pore size distribution of concrete showed strong alteration due to autoclaving. The volume

of pores > 10 nm is increased with autoclaving for OPC concrete, while slag cement concrete

showed an increase in fine pore volume and reduction in big pores volume.

The air permeability test showed a decrease in permeability of most concrete mixes after

autoclaving for 15 and 50 cycles. The only exception is the OPC concrete mix (mix 18) which

exhibited a significant increase in air permeability with autoclaving.

A slight increase in water absorption coefficient of most concrete mixes was occurred due to

autoclaving for 50 cycles. Mix 23 with fine fly ash and low w/b ratio was the only exception. It

showed a slight decrease in absorption coefficient after autoclaving for 15 cycles.

The chloride diffusion coefficient of OPC concrete exhibited a significant increase after

autoclaving. However, mix 23 showed a good resistance to chloride penetration before and

after autoclaving. It exhibited a chloride diffusion coefficient of 4.29 x 10‐13 m2/s after

autoclaving for 50 cycles. Although mix 21 with both fine fly ash and silica fume showed the

lowest value before autoclaving (2.47 x 10‐13 m2/s), the chloride diffusion started to increase

after autoclaving and reached 4 x 10‐13 m2/s.

Effect of autoclaving with 200 °C and 15.5 bars on properties of cement pastes:

Similar to the results of concrete, large changes in compressive strength of cement pastes have

been occurred due to autoclaving for 50 cycles. Significant strength retrogression was found for

OPC and CEM III pastes. However, replacing part of cement with fine fly ash and silica fume

resulted in increase in compressive strength after hydrothermal treatment for 50 cycles. It

increased from 83 MPa at normal conditions to 94 MPa after treatment (mix 19).

Around 50 % increase of porosity of OPC paste has been measured after autoclaving. The

addition of pozzolanic materials significantly prevented the increase in porosity.

For OPC pastes, the autoclaving process resulted in transformation of phases to lime‐rich

phases such as portlandite, α C2SH and hillebrandite. These phases have high density, high

porosity and very low strength as revealed by the experimental measurements. In addition, the

EDX measurement confirmed the formation of these phases because of the high C/S ratio of the

OPC systems.


Systems with high amount of supplementary materials have low C/S ratio of the bulk (< 1). The

formed phases in these systems after hydrothermal exposure are the silica‐rich phases such as

tobermorite and xonotlite which have low density, small porosity and high strength.

Fly ash and fine fly ash exhibited very high efficiency in reducing the porosity and increasing the

compressive strength of cement paste after autoclaving for 50 cycles.

SEM measurements of OPC pastes revealed the high concentration of CH, monosulphate and

ettringite before autoclaving, while a porous microstructure has been formed after autoclaving.

However, dense microstructure has been detected for mixes with high content of silica‐rich

materials even after hydrothermal treatment.

The experimental results showed the importance role of C/S ratio on the stability and properties of

the system. When it increases more than 1, weak and porous product is formed which can be

considered as a detrimental for concrete durability and sustainability. In contrast, the addition of

silica‐rich materials such as quartz, slag and fly ash reduce the C/S ratio to lower than 1, which can

be accounted as an advantageous for concrete under hydrothermal conditions.

The aluminium content in the system has an important role on phase transformation. Materials

such as fly ash which contain high amount of silica and aluminium provoke the formation of

silica‐rich phases with high strength and low porosity such as tobermorite.

9.2 Recommendations

For the sake of optimizing a high dense high performance concrete, three mechanisms are of main

concern. The first one is maximizing the packing density of solid particles. This can be achieved by

applying the Ideal Fuller curve (Figure 4.1) to enhance the packing of the system and to reduce the

required cement content (as discussed in section 4.2). Basically, increasing the packing density of

aggregate would decrease the volume of paste needed to fill up the voids and increase the amount

of additional paste that could be utilized to improve the workability. Additionally, the concrete will

have less durability problems such as permeability, shrinkage, and thermal degradation. Aggregate

as they come from the quarry do not normally have size distributions that fit the dense packing

curve, so it is important to modify the aggregate grading to meet the dense packing curve. Fine

materials such as quartz powder and quartz sand can be used to modify the grading of aggregate

especially in the fine zone under the grading curve. These materials are required also to produce

stable concrete in fresh state with good cohesion and low segregation. Increasing the range of

particle size helps to enhance the packing because systems with wide range of particle sizes have

higher packing density than systems with narrow range.


The second mechanism to produce dense concrete is to make the cement matrix as dense as

possible (as discussed in section 4.3). In this regard, three parameters are very important. First, the

particle size distribution of cementitious materials. Cement alone has low packing density and high

porosity because of the narrow particle size distribution which makes the inter‐particle voids bigger.

However, the use of various fine materials with different particles size distribution can largely

reduce the voids content within the cementitious materials mixture. The efficiency of these

materials in filling up the voids and improving the packing depends mainly on their fineness and

particle shape. Generally, a broader range of particle size distribution would yield systems with high

packing density. The second parameter is the chemical composition of the bulk materials. Many of

concrete problems especially at high temperature come from the high content of calcium hydroxide

because it is soluble and very weak compared to calcium silicate hydrates. Therefore, it is important

to use blended cement such as slag cement as well as using pozzolanic materials to densify the

microstructure, reduce the porosity and to decrease the generated hydration heat. The third

parameter is controlling the w/b ratio to achieve both the required workability and reduce the

capillary porosity. It is recommended to take the w/b ratio lower than 0.42 to reduce the porosity

while the required consistency can be achieved by the use of superplasticizer.

The third mechanism to optimize a durable concrete is to densify the transition zone which is the

weakest zone in concrete and affects all concrete properties (as discussed in section 4.4). This zone

is formed mainly due to low packing of cement particles on the aggregate surface and also due to

the one‐sided growth of cement hydration products. To cope with the first problem, low packing

density on the aggregate surface, very fine materials such as silica fume and fine fly ash can be used

to enhance the packing on the aggregate surface. However, for one‐sided growth, we suggested a

certain sequence of adding the materials to the concrete mixer. After addition of the aggregate,

some of the mixing water should be added to make the aggregate surface wet (in the site, the

aggregate is normally wet), and then the fine materials will be added. By this sequence, fine

materials can stick well on the aggregate surface and reduce the one‐sided growth effect. On the

other hand, during the compaction of fresh concrete, segregation may occur and a thick water film

could be formed beneath the aggregate particles. Moreover, local bleeding is favoured to happen in

the transition zone and around the aggregate surface. So, it is important to use the sufficient amount

of fine materials to prevent these harmful effects by enhancing the packing due to their filler effect.


To summarize, the following steps are recommended to design a dense concrete mixture:

Determination of the maximum aggregate size considering its effect on water and binder

content as well as on the hardened concrete properties (as discussed in page 56).

Calculating the solid materials fractions using the Ideal Fuller curve which applied for both

aggregate and binder (Figure 4.1).

Choose the aggregate grading that fit the curve to achieve the maximum packing density (Figure 4.3).

Selecting the cement type to control chemistry of the hydration products (as discussed in page 60).

Optimization of the binder proportioning (< 0.125 mm) by using various supplementary

materials, type and content, in such a way that maximum packing can be attained (granulometric

viewpoint), meanwhile, dense and durable cement matrix microstructure as well as thin and dense

interfacial transition zone can be achieved (chemical viewpoint) (section 4.3.2).

Determination of the required fillers addition (quartz sand and quartz powder) to fill the gaps

between the mixture and the targeted curve (section 4.2).

Calculating the water content from rheological, physical and chemical points of view (w/b 0.42, in order to reduce the capillary porosity as low as possible).

Adapting the superplasticizer dosage, that is compatible with the used cement, to get the

required consistency, and to enhance the packing density of fine materials.

Regarding the stability of concrete under high pressure and temperature, the chemical composition of

the bulk materials is very important. The bulk materials should be selected in such a way that the formed

phases after hydrothermal exposure must be stable and strong. For example, when ordinary Portland

cement pastes exposed to autoclaving, lime‐rich phases such as alpha dicalcium silicate and hillebrandite

predominated which increase the porosity and reduce the strength. The formation of these detrimental

phases can be avoided by adding fine silica (about 30‐40 %). So, silica‐rich materials such as fly ash and

slag can help to produce stable phases with high strength and low porosity such as tobermorite,

xonotlite and gyrolite. The calcium/silicon ratio (C/S) of the formed phases is very important and

depends mainly on the chemical composition of the input materials and on its fineness.

The aluminium content plays an important role in the formation of tobermorite and extending its

stability up to higher than 200 °C. The role of aluminium can be clarified by comparing two cases;

systems with fly ash which contains high aluminium with system of silica fume which has lower

aluminium content. In the first case, phases with high strength and low porosity are formed, while

for the second phases the direction is not clear. Some other studies detected alpha dicalcium silicate

hydrate in systems with silica fume. So, more research is needed to deeply understand the

behaviour of the systems containing silica fume under hydrothermal treatment. However, the

important role of fly ash on enhancing the concrete properties has been improved by many studied

as well as by the current investigation.

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Shekarchi, M., Debicki, G., Granger, L. and Billard, Y. (2002). Study of leaktightness integrity of containment wall without liner in high performance concrete under accidental conditions ‐ I. Experimentation. Nuclear Engineering and Design, Vol. 213 (1): pp. 1‐9.

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Sioulas, B. and Sanjayan, J.G. (2000). Hydration temperatures in large high‐strength concrete columns incorporating slag. Cement and Concrete Research, Vol. 30 (11): pp. 1791‐1799.

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Standards

DIN EN 197‐1:2011: Zement – Teil 1: Zusammensetzung, Anforderungen und

Konformitätskriterien von Normalzement; Deutsche Fassung EN 197‐

1:2011.

DIN EN 450‐1:2012: Flugasche für Beton – Teil 1: Definition, Anforderungen und

Konformitätskriterien; Deutsche Fassung EN 450‐1:2012.

DIN EN 13263‐1:2009: Silikastaub für Beton – Teil 1: Definitionen, Anforderungen und

Konformitätskriterien; Deutsche Fassung EN 13263‐1: 2005+A1:2009.

References 169

DIN EN 12620:2013: Gesteinskörnungen für Beton; Deutsche Fassung EN 12620:2013.

DIN EN 934‐2:2012: Zusatzmittel für Beton, Mörtel und Einpressmörtel – Teil 2:

Betonzusatzmittel – Definitionen; Anforderungen, Konformität,

Kennzeichnung und Beschriftung; Deutsche Fassung EN 934‐

2:2009+A1:2012.

DIN EN 12390‐2:2009: Prüfung von Festbeton – Teil 2: Herstellung und Lagerung von

Probekörpern für Festigkeitsprüfungen; Deutsche Fassung EN 12390‐

2:2009.

DIN 1045‐2:2008: Tragwerke aus Beton, Stahlbeton und Spannbeton – Teil 2: beton –

Festlegung, Eigenschaften, Herstellung und Konformität‐

Anwendungsregeln zu DIN EN 206‐1.

DIN EN 12390‐1:2012: Prüfung von Festbeton – Teil 1: Form, Maße und andere Anforderungen

für Probekörper und Formen; Deutsche Fassung EN 12390‐1:2012.

DIN EN 12350‐5:2009: Prüfung von Frischbeton – Teil 5: Ausbreitmaß; Deutsche Fassung EN

12350‐5:2009.

DIN EN 12350‐7:2009: Prüfung von Frischbeton – Teil 7: Luftgehalt – Druckverfahren; Deutsche

Fassung EN 12350‐7:2009.

DIN EN 12350‐6:2009: Prüfung von Frischbeton – Teil 6: Frischbetonrohdichte; Deutsche Fassung

EN 12350‐6:2009.

DIN EN 12390‐3:2009: Prüfung von Festbeton – Teil 3: Druckfestigkeit von Probekörpern;

Deutsche Fassung EN 12390‐3:2009.

DIN 1048‐5:1991: Prüfverfahren für Beton, Festbeton, gesondert hergestellte Probekörper.

DIN EN 12390‐6:2010: Prüfung von Festbeton – Teil 6: Spaltzugfestigkeit von Probekörpern;


DIN EN 12390‐7:2009: Prüfung von Festbeton – Teil 7: Dichte von Festbeton; Deutsche Fassung

EN 12390‐7:2009.

DIN EN 12504‐2: Prüfung von Beton in Bauwerken – Teil 2: Zerstörungsfreie Prüfung –

Bestimmung der Rückprallzahl; Deutsche Fassung EN 12504‐2:2012.

DIN EN 12390‐8:2009: Prüfung von Festbeton – Teil 8: Wassereindringtiefe unter Druck;


DIN EN ISO 15148:2002: Bestimmung des Wasseraufnahmekoeffizienten bei teilweisem

Eintauchen; Deutsche Fassung EN ISO 15148:2002

DIN EN 206‐1:2001: Beton – Teil 1: Festlegung, Eigenschaften, Herstellung und Konformität;


TGL 21094‐12:1975: Prüfung des erhärteten Betons, Bestimmung der spezifischen

Gasdurchlässigkeit.

References 170

Appendices 171

11. Appendices

Composition of concrete mixes

Mix

Cementitious materials composition (wt.%)

SP (wt.%)

Aggregatekg/m3

QP kg/m3

QS kg/m3

w/b ratio

Cement FA M20 M10 SF

Type %

1 CEM III/B 100 0.7 1854 46 84 0.42

2 CEM III/B 70 30 1 1854 46 84 0.42

3 CEM III/B 70 30 1 1854 46 84 0.42

4 CEM III/B 70 30 1 1854 46 84 0.42

5 CEM III/B 70 15 15 1 1854 46 84 0.42

6 CEM III/B 70 20 5 5 0.9 1854 46 84 0.42

7 CEM III/B 90 10 0.7 1854 46 84 0.42

8 CEM III/B 65 25 10 1.1 1854 46 84 0.42

9 CEM III/B 65 12.5 12.5 10 1 1854 46 84 0.42

10 CEM III/B 65 15 5 5 10 1 1854 46 84 0.42

11 CEM I 42.5 N 100 0.87 1854 46 84 0.42

12 CEM I 42.5 N 70 20 5 5 1 1854 46 84 0.42

13 CEM I 42.5 N 70 15 15 1.8 1854 46 84 0.42

14 CEM I 42.5 N 65 12.5 12.5 10 2.2 1854 46 84 0.42

15 CEM I 42.5 N 65 15 5 5 10 1.9 1854 46 84 0.42

16 CEM I 42.5 N 90 10 1.2 1854 46 84 0.42

17 CEM III/A 65 12.5 12.5 10 1.7 1854 46 84 0.42

18 CEM I 32.5 R 100 0.87 1854 46 84 0.42

19 CEM I 32.5 R 65 12.5 12.5 10 1.4 1854 46 84 0.42

20 CEM III/B 67 12.5 12.5 8 1.8 1887 45 86 0.36

21 CEM III/B 67 12.5 12.5 8 4 1947 48 88 0.27

22 CEM III/B 70 15 15 3 1885 47 86 0.36

23 CEM III/B 70 15 15 4 1947 48 88 0.27

FA: fly ash M20: fine fly ash M10: fine fly ash SF: silica fume SP: superplasticizer

Appendi

Append

several

A‐1: Co

cycle

Compressive stren

gth (Mpa)

Compressive strength (Mpa)

ices

dix A: Mech

cycles

ompressive

0

10

20

30

40

50

60

70

80

90

100

Mix 1

0

10

20

30

40

50

60

70

80

90

100

Mix 1

hanical prop

strength of

Mix 2

1 Mix 12

0

20

40

60

80

100

120

Compressive stren

gth (MPa)

perties of co

f concrete m

Mix 3 Mix 4

Mix 13 M

Mix 20

oncrete at n

mixes at nor

4 Mix 5

Mix 14 Mix 1

Mix 21

normal cond

rmal condit

Mix 6 Mix

15 Mix 16

Mix 22

ditions and

tions and af

7 Mix 8

Mix 17

Mix 23

28 days

after 1 cycle

after autoc

fter autocla

Mix 9 Mix

28

afte

Mix 18 Mix

28 day

after 1

e

172

claving for

aving for 1

x 10

days

er 1 cycle

x 19

ys

1 cycle

Appendices 173

A‐2: Compressive strength of concrete samples at normal conditions (0 cycles) and after

autoclaving for several cycles up to 15

50

55

60

65

70

0 5 10 15


(Mpa)


Mix 1

60

70

80

90

100

110

0 5 10 15


(Mpa)


Mix 2

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 3

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 4

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 5

60

70

80

90

100

110

0 5 10 15

Compressive stren

gth

(Mpa)


Mix 6

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 7

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 8

50

60

70

80

90

100

0 5 10 15

Compressive stren

gth

(Mpa)


Mix 9

60

70

80

90

100

110

0 5 10 15

Compressive stren

gth

(Mpa)


Mix 10

Appendices 174

50

60

70

80

90

100

0 5 10 15


(MPa)


Mix 11

60

70

80

90

100

110

0 5 10 15


(Mpa)


Mix 12

70

75

80

85

0 5 10 15


(Mpa)


Mix 13

0

50

100

150

0 5 10 15Compressive strength

(Mpa)


Mix 14

60

70

80

90

100

110

0 5 10 15

Compressive stren

gth

(Mpa)


Mix 15

50

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 16

70

80

90

100

110

120

0 5 10 15


(Mpa)


Mix 17

60

70

80

90

100

0 5 10 15


(Mpa)


Mix 18

60

70

80

90

100

110

0 5 10 15


(Mpa)


Mix 19

70

80

90

100

110

120

0 5 10 15


(Mpa)


Mix 20

Appendices 175

A‐3: Results of rebound hammer for concrete mixes at normal conditions and autoclaving for several cycles

70

80

90

100

110

120

0 5 10 15


(Mpa)


Mix 21

60

70

80

90

100

110

0 5 10 15


(Mpa)


Mix 22

70

80

90

100

110

120

0 5 10 15


(Mpa)


Mix 23

36

37

38

39

40

41

42

43

44

0 5 10 15

Reb

ound number


Mix 1

38

40

42

44

46

48

0 5 10 15

Reb

ound number


Mix 2

38

40

42

44

46

48

50

0 5 10 15

Reb

ound number


Mix 3

40

42

44

46

48

50

52

54

0 5 10 15

Reb

ound number


Mix 4

363840424446485052

0 5 10 15

Reb

ound number


Mix 5

38

40

42

44

46

48

50

52

0 5 10 15

Reb

ound number


Mix 6

Appendices 176

38

40

42

44

46

48

50

52

0 5 10 15 20

Reb

ound number


Mix 7

38

40

42

44

46

48

50

0 5 10 15

Reb

ound number


Mix 8

42

44

46

48

50

52

54

0 5 10 15

rebound number


Mix 13

42

44

46

48

50

52

54

56

0 5 10 15

Reb

ound number


Mix 14

40

42

44

46

48

50

52

54

0 5 10 15

Reb

ound number


Mix 15

40

42

44

46

48

50

52

0 5 10 15

Reb

ound number


Mix 16

44

46

48

50

52

0 5 10 15

Reb

ound number


Mix 17

40

42

44

46

48

50

0 5 10 15

Reb

ound number


Mix 18

42

44

46

48

50

52

0 5 10 15

Reb

oumd number


Mix 19

40

42

44

46

48

50

52

54

0 5 10 15

Reb

ound number


Mix 20

Appendices 177

A‐4: Splitting tensile strength at 91 days at normal conditions and after autoclaving for

several cycles in MPa

Mix Normal 1 cycle 15 cycles

1 3.4 3.2 2

2 4.5 4.5 4.2

3 4.6 4.6 ‐

4 5 5.1 ‐

5 5 4.8 4.5

6 3.6 3.6 ‐

7 4.3 2.6 2.57

8 4.4 3.1 ‐

9 4.8 3.7 3.5

10 4.7 3.1 ‐

11 4.2 3 ‐

12 5.2 4.2 ‐

13 5.1 5.1 ‐

14 5.7 5.5 ‐

15 3.8 4.2 ‐

16 5.5 4.2 ‐

17 4.4 3.8 ‐

18 3.8 3.4 2.8

19 4.7 5 5.3

20 5.5 5.6 ‐

21 5.9 5.6 5.5

22 5.7 5.7 ‐

23 5.9 5.7 5.9

42

44

46

48

50

52

54

0 5 10 15

Reb

ound number


Mix 21

42

44

46

48

50

52

0 5 10 15

Reb

ound number


Mix 22

40

42

44

46

48

50

52

54

0 5 10 15

Reb

ound number


Mix 23

Appendices 178

Appendix B: Results of concrete durability

B1: Capillary suction of concrete mixes

1‐ The increase in concrete mass with time due to capillary suction

At normal conditions (mixes 1 – 10)

After autoclaving for 15 cycles (mixes 1 – 10)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5

Increase in

concrete mass (kg/m

2)

Square root of time (hours)

Mix 1 Mix 2

Mix 3 Mix 4

Mix 5 Mix 6

Mix 7 Mix 8

Mix 9 Mix 10

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5

Incraese in


2)


Mix 1 Mix 2

Mix 3 Mix 4

Mix 5 Mix 6

Mix 7 Mix 8

Mix 9 Mix 10

Appendices 179

At normal conditions (mixes 11 ‐ 23)

After autoclaving for 15 cycles (mixes 11 – 23)

0

0.5

1

1.5

2

0 1 2 3 4 5

Increase in concreate mass (kg/m

2)


Mix 11 Mix 12Mix 13 Mix 14Mix 15 Mix 16Mix 17 Mix 18Mix 19 Mix 20Mix 21 Mix 22Mix 23

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 1 2 3 4 5

Increase in m

ass per unit area (kg/m

2)


Mix 11 Mix 12Mix 13 Mix 14Mix 15 Mix 16Mix 17 Mix 18Mix 19 Mix 20Mix 21 Mix 22Mix 23

Appendi

2‐ Wat

15 cy


t (kg/m

2hr0

.5)


t (kg/m

2hr0

.5)

ices

er absorptio

ycles at age

0

0.1

0.2

0.3

0.4

Mix 1

0

0.1

0.2

0.3

0.4

0.5

0.6

Mix 11

Absorption coefficient

(kg/m

2hr0

.5)

on coefficien

of 91 days

Mix 2

1 Mix 12

0

0.04

0.08

0.12

0.16

0.2

(kg/m

2hr0

.5)

nt of concre

Mix 3 Mix

Mix 13

Mix 20

ete mixes at

x 4 Mix 5

Mix 14 Mix

Mix 21

normal con

Mix 6 M

x 15 Mix 16

Mix 22

ditions and

Mix 7 Mix

6 Mix 17

Mix 23

Norm

15 cy

after autoc

8 Mix 9

Normal

Mix 18

N

1

3

mal

cles

180

laving for

Mix 10

15 cycles

Mix 19

Normal

15 cycles

Appendices 181

Appendix C: Thermogravimetric analysis of cement pastes at normal conditions and after

autoclaving for 50 cycles at the age of 91 days

74767880828486889092949698

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

NormalAutoclaved

82

84

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (° c)

NormalAutoclaved

80828486889092949698100102

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

NormalAutoclaved

82

84

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

Normal

Autoclaved

82

84

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (° C)

Normal

Autoclaved

82

84

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (° C)

Normal

Autoclaved

767880828486889092949698100102

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (° C)

Normal

Autoclaved

70

75

80

85

90

95

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

NormalAutoclaved

84

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

Normal

Autoclaved

86

88

90

92

94

96

98

100

0 100 200 300 400 500 600 700 800 900

wgt %

Temperature (°C)

Normal

Autoclaved

CEM I CEM III/B

Mix 1 Mix 2

Mix 5

Mix 18

Mix 7

Mix 19

Mix 21 Mix 23

Appendi

Append

ices

dix D: SEM fo

Normal

Normal

Normal

or cement p

pastes at no

ormal cond

Mix 5

Auto

Mix 11

Auto

Mix 14

Auto

itions and a

oclaved

oclaved

oclaved

after autoclaving for 50

182

0 cycles

Appendi

Append

ices

dix E: Rene LLCPC calculaations of th

e dry mixtuures porositty

183

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