Analysis of alternative water sources for use in the manufacture of concrete

Queensland University of Technology
School of Physical and Chemical Sciences

Analysis of alternative water sources
for use in the manufacture of concrete

This thesis is submitted as partial fulfilment
of the requirements for the degree of
Maters of Applied Science
By
Leigh M. McCarthy B.Sc

Supervisor: Dr Serge Kokot
Assoc. Supervisor: Prof Ray L Frost


2


Abstract

In Australia and many other countries worldwide, water used in the manufacture of concrete
must be potable. At present, it is currently thought that concrete properties are highly
influenced by the water type used and its proportion in the concrete mix, but actually there is
little knowledge of the effects of different, alternative water sources used in concrete mix
design. Therefore, the identification of the level and nature of contamination in available
water sources and their subsequent influence on concrete properties is becoming
increasingly important. Of most interest, is the recycled washout water currently used by
batch plants as mixing water for concrete. Recycled washout water is the water used onsite
for a variety of purposes, including washing of truck agitator bowls, wetting down of
aggregate and run off.

This report presents current information on the quality of concrete mixing water in terms of
mandatory limits and guidelines on impurities as well as investigating the impact of recycled
washout water on concrete performance. It also explores new sources of recycled water in
terms of their quality and suitability for use in concrete production.

The complete recycling of washout water has been considered for use in concrete mixing
plants because of the great benefit in terms of reducing the cost of waste disposal cost and
environmental conservation. The objective of this study was to investigate the effects of
using washout water on the properties of fresh and hardened concrete. This was carried out
by utilizing a 10 week sampling program from three representative sites across South East
Queensland. The sample sites chosen represented a cross-section of plant recycling
methods, from most effective to least effective. The washout water samples collected from
each site were then analysed in accordance with Standards Association of Australia AS/NZS
5667.1 :1998. These tests revealed that, compared with tap water, the washout water was

3


higher in alkalinity, pH, and total dissolved solids content. However, washout water with a
total dissolved solids content of less than 6% could be used in the production of concrete
with acceptable strength and durability. These results were then interpreted using
chemometric techniques of Principal Component Analysis, SIMCA and the Multi-Criteria
Decision Making methods PROMETHEE and GAIA were used to rank the samples from
cleanest to unclean.

It was found that even the simplest purifying processes provided water suitable for the
manufacture of concrete form wash out water. These results were compared to a series of
alternative water sources. The water sources included treated effluent, sea water and dam
water and were subject to the same testing parameters as the reference set. Analysis of

these results also found that despite having higher levels of both organic and inorganic
properties, the waters complied with the parameter thresholds given in the American
Standard Test Method (ASTM) C913-08. All of the alternative sources were found to be
suitable sources of water for the manufacture of plain concrete.

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Statement of Originality

The work contained in this thesis has not been previously submitted to meet requirements
for an award at this or any other higher education institution. To the best of my knowledge
and belief, the thesis contains no material previously published or written by another person
except where due reference is made.

Signature

_____________________________________
Leigh M. McCarthy

Date

5

_________________________


Acknowledgements

This project would not have been possible without the support of many people. Many thanks

to Dr Serge Kokot and Prof. Ray Frost for their direction, assistance, and guidance. In
particular my supervisor, Dr Serge Kokot, who read my numerous revisions and helped
make some sense of the confusion. Thanks are also due to Mr. Glenn Carson, Dr Dak
Bakewash and Mr. Russel Gutsky from Readymix for their assistance and for providing me
with the financial means to complete this project. I would also like to thank Dr Wayde
Martens whose help was integral in the completion of this thesis. Also thanks to my fellow
postgraduate students, who sympathized with my complaints, understood my frustrations
and most of all offered guidance and support.

And finally, thanks go to my family and friends who endured this long process with me,
always offering support and love. Thanks to my parents who were unwavering in their
encouragement and support and who, through years of patience and hard work afforded me
a sense of ambition and self, allowing me to reach for my goals. To my sister Clare thanks
for your patience, understanding and tolerance of my disappointments and for sharing my
triumphs. Lastly, to my brother Sean whose own achievements served as a reminder that
only your best effort will do.

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Table of Contents

1

Introduction .................................................................................................................. 12
1.1

Prologue ............................................................................................................... 12

1.2

Concrete and its constituents ................................................................................ 15

1.3

Cement and aggregates........................................................................................ 17

1.3.1

Hydration reactions of cement ....................................................................... 21

1.3.2

Cement Hydration Products ........................................................................... 23

1.3.3

Admixtures..................................................................................................... 25

1.4
2

Water Quality, its properties and influence on concrete......................................... 26

Methodology ................................................................................................................ 29
2.1

Sample Guidelines ................................................................................................ 29

2.2

Sample Preparation .............................................................................................. 29

2.3

Equipment and Materials ...................................................................................... 31

2.4

Chemical Preservatives ........................................................................................ 31

2.5

Sampling Methods and Procedures ...................................................................... 31

2.6

Preparation of Concrete Cylinders ........................................................................ 33

2.7

Instrumental Analysis ............................................................................................ 34

2.8

Instrumentation used for the analysis of water samples ........................................ 35

2.8.1

Measurement of pH ....................................................................................... 35

2.8.2

Measurement of Relative Alkalinity ................................................................ 36

2.8.3

Measurement of Electrical Conductivity ......................................................... 37

2.8.4

Measurement of Total Dissolved Solids ......................................................... 38

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2.8.5

Measurement of Chloride ............................................................................... 39

2.8.6

Compressive strength Analysis of concrete samples ..................................... 40

2.9

Multi Criteria Decision Making Methods ................................................................ 42

2.9.1
2.10

Chemometric Analysis ................................................................................... 42

Multicriteria Decision Making (MCDM) .................................................................. 51

2.10.1

Preference

Ranking

Organisation

Method

for

Enrichment

Evaluation

(PROMETHEE) ............................................................................................................ 52
2.10.2
3

Compilation of baseline data for water quality .............................................................. 55
3.1

Washout Waters – Building a baseline .................................................................. 55

3.2

Concrete Plant sites throughout SE-Qld ................................................................ 56

3.2.1

Southport Concrete Plant ............................................................................... 56

3.2.2

Beenleigh Concrete Plant .............................................................................. 56

3.2.3

Murarrie Concrete Plant ................................................................................. 56

3.3

Analysis of Baseline Water ................................................................................... 58

3.4

Simple Analysis of Baseline Water Sample Results .............................................. 58

3.5

Chemometric interpretation of Water Quality data ................................................. 62

3.6

Chemometric Analysis of Baseline Samples ......................................................... 66

3.6.1

Principal Component Analysis ....................................................................... 66

3.6.2

PROMETHEE and GAIA ................................................................................ 72

3.7
4

Geometric Analysis for Interactive Aid (GAIA) ................................................ 54

Chapter Summary ................................................................................................. 80

Analysis of Alternative Water Sources for Comparison ................................................ 81
4.1

Location of alternative water source sampling sites .............................................. 81

8


4.2

Southport Sea Water ............................................................................................ 83

4.2.1

Southport Treated Effluent ............................................................................. 83

4.2.2

Kawana Treated Effluent................................................................................ 83

4.2.3

Coolum Bore Water ....................................................................................... 83

4.2.4

Gympie Bore water ........................................................................................ 84

4.2.5

Ipswich River Water ....................................................................................... 84

4.2.6

Murarrie Bore Water ...................................................................................... 84

4.2.7

Coomera Dam Water ..................................................................................... 84

4.2.8

Coomera Bore water ...................................................................................... 84

4.3

Analysis of Alternative Water Sources .................................................................. 86

4.4

Analysis according to water type ........................................................................... 88

4.4.1

Sea Water...................................................................................................... 88

4.4.2

Treated Effluent ............................................................................................. 89

4.4.3

Bore Water .................................................................................................... 91

4.4.4

Dam and River Water .................................................................................... 91

4.5

Chemometric analysis of alternative water source samples .................................. 93

4.5.1

PCA analysis ................................................................................................. 93

4.5.2

SIMCA ........................................................................................................... 97

4.5.3

Fuzzy Clustering .......................................................................................... 103

4.5.4

PROMETHEE and GAIA .............................................................................. 105

4.6

Chapter Summary ............................................................................................... 116

5

Concluding Remarks .................................................................................................. 117

6

References ................................................................................................................ 119

9

Table of figures

Figure 2.1 Testing Apparatus used to determine Compressive Strength ............................. 40
Figure 2.2 Example of Principal Component Analysis ......................................................... 46
Figure 3.1 Biplot with baseline sample results with IRMV and compressive strength results67
Figure 3.2 PCA Biplot of PC1 vs. PC2 with all variables including compressive strength
results ................................................................................................................................. 69
Figure 3.3 GAIA plot showing reference variables and baseline sample sites

with

compressive strength results............................................................................................... 73
Figure 3.4 GAIA plot showing reference variables and baseline sample sites .................... 77
Figure 4.1 PC1 v PC2 for alternative water source results .................................................. 92
Figure 4.2 PC1 v PC2 all Alternative and all Baseline samples with compressive strength
results ................................................................................................................................. 94
Figure 4.3 Cooman Plot For Murarrie and Coomera Dam including RSD values ............... 100
Figure 4.4 Cooman Plot For Murarrie and Sea Water including RSD values ..................... 100
Figure 4.5 Plot of Discrimination power vs. variables for Southport & Murarrie.................. 106
Figure 4.6 GAIA plot for all Alternative and all Baseline samples ...................................... 109
Figure 4.7 GAIA Plot for all Alternative and all Baseline samples with compressive strength
results ............................................................................................................................... 114

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Table of tables

Table 1.1 - Chemical and physical composition of ordinary Portland cement ...................... 16
Table 2.1 - Specifications for sample preservation .............................................................. 30

Table 2.2 Preference functions in PROMETHEE ................................................................ 50
Table 2.3 – sample matrix ................................................................................................... 52
Table 3.1 Average Values over 10 week period for each plant and Tap Water compared to
the Tolerable Limits ............................................................................................................. 57
Table 3.2 Metal Concentrations from Southport, Beenleigh and Murarrie............................ 60
Table 3.3 Readymix Water Specification Guidelines ........................................................... 65
Table 3.4 PROMETHEE Net Ranking for Southport and Murarrie using Internal Readymix
Variables and Compressive strength results ....................................................................... 71
Table 3.5 PROMETHEE Net Ranking for the Baseline Data using Internal Readymix
Variables Only..................................................................................................................... 75
Table 4.1 Typical composition of sea water* ....................................................................... 82
Table 4.2 Comparison between Average Alternative Water Source Results and Compressive
strength Results with Baseline samples and Tap water....................................................... 85
Table 4.3 PCA models for SIMCA ....................................................................................... 96
Table 4.4 SIMCA fit to Southport RSDcrit=0.45, p=0.05........................................................ 98
Table 4.5 SIMCA fit to Murarrie where RSDcrit = 1.13, p= 0.05 ............................................ 98
Table 4.6 Hard clustering for Alternative Water Samples .................................................. 102
Table 4.7 Soft clustering for Alternative Water Samples .................................................... 104
Table 4.8 PROMETHEE Net ranking of alternative water sources .................................... 107
Table 4.9 PROMETHEE Net Ranking for all Alternative and all Baseline samples with
Compressive strength Results .......................................................................................... 112

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1

1.1

Introduction

Prologue

Second only to water, concrete is the most consumed substance, with three tonnes used per
person per year [1]. Twice as much concrete is used in construction as all other building
materials combined [2]. Thus, there is little doubt that concrete will remain in use well into
the future. As demand increases for this fundamental building material, studies such as the
one presented here will continue to be carried out in the hope of optimising the
characteristics and properties, ensuring that concrete remains environmentally friendly and
affordable. This study is aimed at understanding the role of water quality in concrete
manufacture. In order to accomplish this aim, this study has set out to identify key elements,
variables and characteristics necessary for a water source to be considered a viable option
within the concrete industry.

Concrete consists of aggregates, sand, cementitious material, admixtures and water which
are mixed together to provide a uniform plastic material [3]. This plastic material gradually
sets after a period of one to three hours which increases in strength, particularly over the first
month of its life [4]. Varying the mix of cement, sand and aggregate used in a concrete blend
consequently enables its use in a wide range of applications. Products can be designed,
coloured and shaped to accommodate a variety of environmental conditions, architectural
requirements and to withstand a wide range of loads, stresses and impacts. With the
increasing demand for high quality water, a large quantity of chemical agents must be used
in the water purification process, which in turn generates enormous amounts of waste wash
water [5]. Of all the options for wash water disposal, reuse has been considered most
economical and environmentally sound.

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This study evaluated the possibility of incorporating wash water in the making of concrete.

The goal was to search for the optimal specifications to maximize the replacement of potable
water with an alternative source.

Within this scenario, re-envisioning industrial wastes as alternative raw materials becomes of
interest, both economically and logistically, for a wide range of applications which includes
the fabrication of concrete. As such, there have been comprehensive studies detailing
possible alternative water sources suitable for the production of concrete.

Previous

research, such as that carried out by Al-Harty, Borger, and Chatveera has focused on what
effect minerals, salts and impurities contained in the water have on the properties and
performance of fresh and hardened concrete [1, 5, 6]. Results obtained in these studies
indicate that the use of non-potable water yields lower compressive strength in comparison
to concrete made with potable water.

Previous research in this field, carried out by Muszynski, Sandrolini and Su, has utilised
treated effluent water samples, water from streams, lakes and sea water for concrete
construction [2, 7, 8]. These studies however were not carried out within Australia, thus the
results found were not obtained in light of Australian Standards [9, 10]. Such studies also
imply that the concrete produced was not made of common aggregates and cement found in
Australia. And, whilst the results obtained will serve as a useful guideline to the behaviour of
wet, hardening and hardened concrete, the analytical results obtained in such studies have
not, as yet, been coupled with chemometrics methodology. Hence, this study aims to build
on previous investigations by combining instrumental and structural analysis with
chemometrics.

13

The novel application of Multi-Criteria Decision Making (MCDM) methods of data analysis
will also be carried out to complement chemometrics findings. Chemometrics was applied to
compare and discriminate individual samples, as this MCDM is primarily concerned with the
extraction of significant information from the data which has been characterised into
chemical, physico-chemical and structural components [11, 12].

This study sought to research and develop, the combination of instrumental analysis with
chemometrics to provide a rapid method for assessing the suitability of a variety of water
sources for use in concrete production. These would be developed such there would be no
lasting harmful effects to its properties and characteristics of the resultant concrete. This was
performed with the primary objectives:

•

To build a comprehensive water quality baseline with specific parameters outlining
the suitable elemental, physico-chemical and structural properties of water for use in
concrete manufacture.

•

To develop elemental, physico-chemical and structural guidelines with the aid of
chemometrics, and the novel application of Multi-Criteria Decision Making Methods
ensuring optimal water and concrete results.

•

To undertake an investigation into the suitability of alternative water sources,
employing chemometrics.

•

To report the results clearly, enabling industry to use the information to ensure that
changes are made to increase the cleanliness and quality of water and subsequently
improve the performance of the resultant concrete.

The remainder of this chapter will focus on the role of water in concrete production, as well
as provide an introduction to the composition of concrete.

It will also introduce the

chemometric techniques utilised for this study and conclude with an examination of the

14


instrumental techniques utilised in this project. Chapter 2 describes the materials,
procedures and chemometrics aspects utilised in this work. Chapter 3 is concerned with the
instrumental study of water samples and concrete test cylinders. Chapter 4 focuses on the
chemometric modelling abilities of water from a cross-section of treatment plants and will
concentrate on a novel investigation of alternative water sources through instrumental and
chemometric modelling.

1.2

Concrete and its constituents

Concrete is one of the most widely used construction materials [4, 13]. It is a durable and
high strength material that has a low permeability. Both the fresh and hardened states of
concrete must fulfil the intended purpose of its use. Consistency and cohesiveness are the
two most important properties when concrete is in its fresh state, as they must facilitate

compaction and transportation without segregation [14]. When in the hardened state, it is
imperative that the compressive strength of concrete lies within the required limits. The
compressive strength affects density, impermeability, tensile strength and chemical
resistance.

Concrete is a construction material that is made from cement, aggregate such as gravel and
sand, water and admixtures [15, 16]. It solidifies and hardens after mixing and placement
due to a chemical process known as hydration. In order for concrete to be manufactured,
water must react with the cement, which bonds the other components together, eventually
creating a stone-like material. Whilst concrete is the final monolithic product, cement is a
vital component and acts as the bonding material [17].

15


Table 1.1 - Chemical and physical composition of ordinary Portland cement
*H. F. W. Taylor, Cement Chemistry, 2nd Ed., Academic Press, London (1997).

Component

Ordinary
Portland Cement (%)

SiO2

21.95

Al2O3

4.95

Fe2O3

3.74

CaO

62.33

MgO

2.08

SO3

2.22

K2O

0.56

Na2O

0.32

TiO2

0.17

Mn2O3

0.05

Cl-

0.01

Initial setting time (min)

110

16


1.3

Cement and aggregates

Cement is a basic ingredient of concrete, mortar and plaster. The most common type of
hydraulic cement, Portland cement, consists of a mixture of oxides of calcium, silicon and
aluminium can be seen in Table 1.1. Discovered by an English engineer Joseph Aspdin in
1824, it is manufactured primarily from limestone, clay minerals and gypsum in a high
temperature process that drives off carbon dioxide and chemically combines the primary
ingredients into new compounds [18, 19]. Hydraulic cements harden and set after the
addition of water as a result of chemical reactions with the mixing water and after hardening,
retain strength and stability even under water [18-20].

Cement and water form a paste coating each particle of stone and sand. The hydration
reaction causes cement paste to harden and gain strength [21, 22]. This reaction is vital for
the properties attained in the final concrete mix, and as such, the characteristics of the

concrete are determined by the quality of the paste. The strength of the paste, in turn,
depends on the ratio of water to cement which is measured but the weight of mixing [23-27].
This is the weight of the mixing water divided by the weight of the cement. High-quality
concrete is produced by lowering the water-cement ratio as much as possible but always
trying to retain enough water to ensure the workability of fresh concrete. A higher quality
concrete is produced if less water is used, provided the concrete is properly placed,
consolidated, and cured.

17


A low water-to-cement (w/c) ratio is needed to achieve strong concrete. It would seem
therefore that by merely keeping the cement content high, one could use enough water for
good workability and still have a low w/c ratio [6, 24, 27]. The problem is that cement is the
most costly of the basic ingredients. Thus, in order to ensure an economical and practical
concrete mix, both fine and coarse aggregates are utilised to make up the bulk of the
concrete mixture. And as such the quality of aggregates is very important [28, 29]. Fine
aggregate (sand) is made up of particles which can pass through a 3/8 inch sieve whilst
coarse aggregates are larger than 3/8 inch in size [28]. It is however becoming common for
recycled aggregates from construction, demolition and excavation waste to be used as
partial replacements of natural aggregates, while a number of manufactured aggregates,
including air-cooled blast furnace slag and bottom ash are also permitted [30, 31]. While
recycling building materials is important, the shape, size, density and strength of such
aggregate particles can vary significantly, and can therefore adversely influence the
properties of the concrete.

Concrete is a blend of natural materials, and often has natural imperfections. The
performance of exterior concrete slabs is significantly influenced by the entrainment of
microscopic air bubbles into the concrete [32]. An air entrainment admixture causes
microscopic air bubbles to form throughout the concrete that function as relief valves when

water in the concrete freezes, helping to prevent surface deterioration.

During hydration and hardening, concrete needs to develop certain physical and chemical
properties. Among other qualities, mechanical strength, low moisture permeability, and
chemical and volumetric stability are necessary. There are many characteristics that affect
concrete and its properties, all of which depend on the specific mix being used [30]

18


Workability is the ability of a fresh or plastic concrete mix to fill the mould properly without
reducing the concrete's quality. Workability depends on water content, aggregate size,
cementitious content and level of hydration, but can also be modified by adding chemical
admixtures [33]. Raising the water content or adding chemical admixtures will increase a
concrete‟s workability. Excessive water will lead to increased bleeding and/or segregation of
aggregates with the resulting concrete having reduced quality. The use of an aggregate with
an undesirable gradation can result in a very harsh mix design with a very low slump, which
cannot be readily made more workable by addition of reasonable amounts of water [16].

Cement requires time to fully hydrate before it acquires strength and hardness, thus
concrete must be cured once it has been placed. Curing is the process of keeping concrete
under a specific environmental condition until hydration is relatively complete [16]. Good
curing is usually undertaken in a moist environment with a controlled temperature. This is
necessary as a moist environment promotes hydration, since increased hydration lowers
permeability and increases strength resulting in a higher quality material [34]. Improper
curing can lead to several serviceability problems including cracking, increased scaling, and
reduced abrasion resistance.

Compressive strength of concrete determines how much pressure concrete can withstand
before cracking and weakening. This compressive strength depends mainly on the

properties and quality of the cement paste and the aggregate [35, 36]. If the aggregate
consists of a soft or weak material, the concrete will also be weak. The strength of the
concrete can be controlled by choosing the mix proportions provided that good quality
aggregates are used and the correct manufacturing procedures are followed. If not enough
water was added to the mix, the cement paste remains too dry and stiff and the concrete will
be weak. If too much water was added, making the cement paste too thin, the concrete will
again be weak [37].

19


Concrete has relatively high compressive strength, but significantly lower tensile strength
and as a result, concrete always fails from tensile stresses. The practical implication of this is
that concrete elements subjected to tensile stresses must be reinforced. Concrete is most
often constructed with the addition of steel or fibre reinforcement. The reinforcement can be
by bars, mesh, or fibres [38].

As concrete is a liquid which hydrates to a solid, plastic shrinkage cracks can occur soon
after placement; but if the evaporation rate is high, they often can occur during finishing
operations. Aggregate interlock and steel reinforcement in structural members often negates
the effects of plastic shrinkage cracks, rendering them aesthetic in nature [38]. Properly
tooled control joints or saw cuts in slabs provide a plane of weakness so that cracks occur
unseen inside the joint, making a more aesthetic presentation. In very high strength concrete
mixtures, the strength of the aggregate can be a limiting factor to the ultimate compressive
strength. In concretes with a high water-cement ratio the use of coarse aggregate with a
round shape may reduce aggregate interlock [39, 40].

Concrete has a very low coefficient of thermal expansion. However, if no provision is made
for expansion very large forces can be created, causing cracks in parts of the structure not
capable of withstanding the force or the repeated cycles of expansion and contraction [41].

As concrete matures it continues to shrink, due to the ongoing reaction taking place in the
material, although the rate of shrinkage falls relatively quickly and keeps reducing over time.
The relative shrinkage and expansion of concrete and brickwork require careful
accommodation when the two forms of construction interface.

Since the hydration of cement is so significant, the following section examines in detail the
process of hydration, explaining the reactions that occur and their influence on the final
concrete product.

20


1.3.1

Hydration reactions of cement

The formation of water-containing compounds facilitates the hardening and setting of
hydraulic cements [42]. The reaction and the reaction products are referred to as hydration
and hydrates respectively. When cement and water are mixed together, the reactions which
occur are mostly exothermic. An indication of the rate at which the minerals are reacting, is
given by monitoring the rate at which heat is evolved using a technique called conduction
calorimetry.

During the process of heat evolution three principal reactions occur [6]. Firstly, during
hydration and hardening, concrete develops certain physical and chemical qualities including
mechanical strength, low moisture permeability, and chemical and volumetric stability. Each
of these characteristics effect the produced concrete however, the water to cement ratio
has the greatest effect on the quality of concrete [1, 43].

Almost immediately on adding water, some of the clinker sulphates and gypsum dissolve,

producing an alkaline, sulfate-rich solution. Soon after mixing, the crystals of calcium
aluminate (Ca3Al2O6), hereby annotated as (C3A), reacts with the water to form an
aluminate-rich gel [44]. The gel reacts with sulfate in solution to form small rod-like crystals
of ettringite ((CaO)6(Al2O3)(SO3)3·32 H2O). (C3A) hydration is a strongly exothermic reaction
but it does not last long, typically only a few minutes and is followed by a period of a few
hours of relatively low heat evolution. This is called the dormant or induction period.

The first part of the dormant period corresponds to the time when it is most beneficial for the
concrete to be placed. As the dormant period progresses, the paste becomes too stiff to be
workable. At the end of the dormant period, the alite (Ca3SiO5) and belite (Ca2SiO4) in the
cement start to hydrate, with the formation of calcium silicate hydrate and calcium hydroxide.

21


During this phase, an increase of calcium hydroxide occurs as a result of hydrolysis of
tricalcium silicate Equation 1.1.

Equation 1.1
2Ca3SiO5 + 6H2O  3 Ca(OH)2 + Ca3Si2O7.3H2O

Thus, on the addition of water, calcium silicate rapidly reacts to release calcium ions,
hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the
release of hydroxide (OH-) ions. This initial hydrolysis slows down quickly after heat
evolution begins to decrease. The reaction slowly continues producing calcium and
hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide
starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions are
precipitated out of solution accelerating the reaction of tricalcium silicate to calcium and
hydroxide ions. This corresponds to the main period of cement hydration, during which time
concrete strength increases. The cement grains react from the surface inwards, and the

anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals
become accessible to water.

The cement paste immediately stiffens and increases with time. After reaching a certain level
of hardness, a second reaction takes place which promotes the immediate set of the
concrete. .

Equation 1.2
Ca3(AlO3)2 + 6H2O  Ca6(AlO3)2.6H2O

When gypsum (CaSO4 2H2O) is added to the cement, as it is in most hydraulic cements, the
hydration reaction of tricalcium aluminate is altered (Equation 1.3).

The reaction of

tricalcium aluminate with water forms calcium aluminate trisulphate hydrate (ettringite). Once

22


the system is free of gypsum, calcium aluminate monosulphate hydrate (monosulphate)
forms as in Equation 1.4.

Equation 1.3
Ca3(AlO3)2 + 3CaSO4.2H2O + 26H2O  Ca6[Al(OH)6]2(SO4)3.26H2O

Equation 1.4
Ca6[Al(OH)6]2(SO4)3.26H2O  3CaO.Al2O3.CaSO4.12H2O

1.3.2

Cement Hydration Products

The products of the reaction between cement and water are termed 'hydration products.'
When concrete is manufactured using Portland cement as the cementitious material there
are four main types of hydration product:
Calcium silicate hydrate: this is the main hydration product and is the main source of
concrete strength. It is often abbreviated, using notation, to 'C-S-H,' the dashes indicating
that no strict ratio of SiO2 to CaO is inferred. The Si/Ca ratio is somewhat variable but
typically approximately 0.45-0.50.
Calcium hydroxide - Ca(OH)2: often abbreviated, as 'CH.' CH is formed mainly from alite
hydration. Alite has a Ca/Si ratio of 3:1 and C-S-H has a Si/Ca ratio of approximately 2:1, so
excess lime is available from alite hydration and this produces CH.
Ettringite: Ettringite is present as rod-like crystals in the early stages of cement hydration.
The chemical formula for ettringite is Ca 6[Al(OH)6]2(SO4)3.26H2O.
Monosulfate: Monosulfate tends to occur in the later stages of hydration, after a few days.
Usually, it replaces ettringite, either fully or partly. The chemical formula for monosulfate is
C3A.CaSO4.12H2O. Both ettringite and monosulfate are compounds of C 3A, CaSO4
(anhydrite) and water, in different proportions.

23


Al-Fe-tri (Aft) and Al-Fe-mono (AFm) phases: Ettringite is a member of a group known as
AFt phases. The general definitions of these phases are somewhat technical [4, 40], but
ettringite is an AFt phase because it contains three (tri) molecules of anhydrite when written
as C3A.3CaSO4.32H2O and monosulfate is an AFm phase because it contains one (mmono) molecule of anhydrite when written as C 3A.CaSO4.12H2O[45].
Important points to note about AFm and AFt phases are that:


They contain large amounts of water, especially the AFt phases.



They contain different ratios of sulfur to aluminium.



Aluminium can be partly-replaced by iron in both AFm and AFt phases.



Sulfate ion in the AFm phases can be replaced by other anions; a one-for-one
substitution if the anion is doubly-charged (e.g.: carbonate, CO2-) or one-for-two if the
substituent anion is singly-charged (e.g.: hydroxyl, OH- or chloride, Cl-). The sulfate in
ettringite can be replaced by carbonate or, probably, partly replaced by two hydroxyl
ions [4, 40].

Monosulfate gradually replaces ettringite in many concretes because the ratio of available
alumina to sulfate increases with continued cement hydration. On mixing cement with water,
most of the sulfate is readily available to dissolve, but much of the C 3A is contained inside
cement grains with no initial access to water. Continued hydration gradually releases
alumina and the proportion of ettringite decreases as that of monosulfate increases.
If there is eventually more alumina than sulfate available, the entire sulfate will exist as
monosulfate, with any additional alumina present as the hydroxyl-substituted AFm phase. If
there is an excess of sulfate, the cement paste will contain a mixture of monosulfate and
ettringite. Near the concrete surface, carbonation will release sulfate as carbonate ions
replace sulfate in the ettringite and monosulfate phases.

24

1.3.3

Admixtures

Admixtures often strengthen, speed up or slow down the setting time, and help to protect
concrete against the effects of temperature changes and exposure. Therefore admixtures
tend to counteract any forces that negatively affect concrete [46]. Often in the form of
powder or fluids, admixtures are added to the concrete to give it certain characteristics not
obtainable with plain concrete mixes [46]. In normal use, admixture dosages are less than
5% by mass of cement, and are added to the concrete at the time of batching or mixing. The
most common types of admixtures are [46-48]:


Accelerators speed up the hydration (hardening) of the concrete.



Retarders slow the hydration of concrete, and are used in large or difficult pours
where partial setting before the pour is complete is undesirable.



Air-entrainers add and distribute tiny air bubbles in the concrete, which will reduce
damage during freeze-thaw cycles thereby increasing the concrete's durability.



Plasticizers (water-reducing admixtures) increase the workability of plastic or "fresh"

concrete, allowing it to be placed more easily, with less consolidating effort.



Superplasticisers (high-range water-reducing plasticizers) which have fewer
deleterious effects when used to significantly increase workability. Alternatively,
plasticizers can be used to reduce the water content of a concrete (and have been
called water reducers due to this application) while maintaining workability. This
improves its strength and durability characteristics.



Pigments can be used to change the colour of concrete, for aesthetics.



Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in
concrete.



Bonding agents are used to create a bond between old and new concrete.



Pumping aids improve pumpability, thicken the paste, and reduce thinning of the
paste.

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