handbook of analytical techniques in concrete science and technology principles techniques and applications pdf

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Foreword

Material sciences have always been a leading driver in construction
innovation. This is still true today with new technologies producing new
materials and advancing the performance and applications of old ones. Many
issues have to be addressed and resolved before such materials are
confidently accepted in practice. Issues such as durability and long-term
performance, design methods to allow their integration in construction, and
new or modified standards to facilitate acceptance in the marketplace are
often mentioned. This book on analytical techniques in concrete science and
technology is a valuable addition to the literature addressing these subjects.
Over the past three decades, material scientists at IRC have contributed extensively to the advances on construction research through the
development of experimental techniques. Their work is based on innovations
in many areas including differential thermal methods, x-ray diffractometry,
electron microscopy, petrography and design of special miniature techniques
for determining mechanical behavior of cement systems. It is, therefore,
natural to see that seven ofthe book' s chapters are written by IRC scientists.
To add to this strength, the other thirteen chapters are written by world-class
experts in their respective fields.
The book is the first of its kind addressing technologies associated with
the use ofboth organic and inorganic products and composite materials. The
principles of the techniques are explained and applications clearly described.
In addition, a wide selection of references are provided to give the reader
ready access to more detailed information should it be required.

~

Vll

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Foreword
The techniques described in the book are useful for analysis and
prediction of many material-related issues such as: (i) resolving durability
issues related to the mechanisms of reaction, (ii) determining parameters
that influence reaction kinetics of processes that affect material properties
and service life ofbuilding elements, (iii) developing and characterizing new
materials for durable structures, (iv) establishing reasons for structural
failures and conducting related forensic investigations, (v) providing a basis
for the development of relevant standards and methods for advancing
aspects of objective based codes, and (vi) validating numerical methods for
predicting long-term performance of construction materials.
The result is a handbook that presents up-to-date information in the
form that makes it valuable to read and come back to frequently. It should
become a valuable reference source for students and practitioners as well
as professionals engaged in standards writing.
Sherif Barakat
Director General
Institute for Research in Construction
National Research Council Canada
Ottawa, Canada

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Preface

Concrete is a composite material formed by mixing and curing
ingredients such as cement, fine and coarse aggregates, and water. Most
concretes, however, contain additional ingredients such as chemical admixtures including air-entraining admixtures, fly ash, fibers, slag, and other
products.
The physical, chemical and durability characteristics of concrete
depend on many factors such as the type and amount of the components,
temperature, pore and pore size distribution, surface area, interfacial
features, exposure conditions, etc. Consequently, a good understanding of
various processes occurring in cementitious systems necessitates the
application of diverse techniques.
Several physical, chemical, and mechanical techniques are applied in
concrete research and practice. They provide important information, including characterization of raw materials and cured concrete, quality control,
quantitative estimation of products, prediction ofperformance, development
of accelerated test methods, study of interrelationships amongst physical,
chemical, mechanical, and durability characteristics, development of new
materials, etc. In most instances, no single technique provides all the needed
information and hence application of several techniques becomes necessary. Information on the application of various techniques in concrete is
dispersed in literature, and few books are available that serve as a source or
reference. Hence a handbook incorporating the latest knowledge on the
application of various investigative techniques in concrete science and

/x

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x

Preface

technology has been prepared. Standard test methods are not covered in this
book as they are well described in publications ofnational and international
standards organizations.
The book is divided into twenty chapters. Each chapter describes the
technique and its application and limitations for the study ofconcrete,. Each
chapter also contains a list of important references that should serve as a
useful guide for further information.
The first chapter on concrete science describes the essential concepts
so that information presented in subsequent chapters can be easily followed.
The chapter deals with the formation of cement, its hydration behavior,
physicochemical processes related to the cement paste, and several important properties of concrete and durability aspects.
Chapter 2 deals with the description of a number of specialized
techniques used in conjunction with petrography for the evaluation and
analysis of aggregates of concrete.
Chemical analysis methods have been applied extensively to analyze
the components of concrete, chemical and mineral admixtures, raw materials for making cement and also to estimate cement contents. Modem
analytical tools enable much faster analysis than the wet chemical methods.
In Chapter 3, chemical analysis techniques reviewed include atomic absorption, x-ray emission and plasma spectroscopy. The chapter also contains
information on chemical (wet) methods of analysis.
Thermal analysis techniques based on the determination of physical,
chemical, and mechanical changes in a material as a function of temperature,
have been routinely used in concrete science and technology. Identification,
estimation of compounds, kinetics ofreactions, mechanisms of the action of
admixtures, synthesis of compounds, quality control and causes leading to
the deterioration of cementitious materials are investigated by these techniques. Various types of thermal techniques and their applications and
limitations are included in Chapter 4.
Although comparatively recent, IR spectroscopy is gaining importance,
especially with the development of user-friendly equipment as described in
the fifth chapter. This technique has been applied for identification of new
products and characterization of raw materials, hydrated materials, and

deteriorated products., Discussion on Raman spectroscopy, a complementary technique to IR, also forms a part of this chapter.
Nuclear Magnetic Resonance spectroscopy (NMR) is a effective tool
to probe atomic scale structure and dynamic behavior of cementing
materials. The application of NMR for determining the pore structure and

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Preface

xi

transport properties of cement and concrete via relaxation and imaging
methods and its application to anhydrous cement and hydrated cement
phases form some of the contents of Chapter 6.
Scanning Electron Microscopy and its adjunct, microanalytical unit,
known as Energy Dispersive X-ray Analyzer, have been accepted as
important investigative techniques in concrete technology. Chapter 7 comprises discussion on the microstructure of hydrated cement paste, C-S-H
phase, calcium hydroxide, aluminate hydrate phases, paste-aggregate interface, admixtures, slags, and fly ashes. Also included are studies on the
correlation ofmicrostructure with durability.
The eighth chapter on the application of x-ray diffraction focuses on
some of the fundamental aspects of the technique, the hardware and
software developments, and its applications to cement and concrete.
An understanding of the rheology of fresh cement paste and concrete
is essential for following the behavior of concrete in the fresh state.
Additions and admixtures in concrete alter its rheological behavior. Chapter
9 deals with rheological techniques and their application to fresh cement
paste and concrete.
Dimensional changes occur in cement paste and concrete due to
physical, chemical, and electrochemical processes. A discussion ofenergetics of surface adsorption and volume changes forms the scope of Chapter

10. Relevance of length changes to concrete deterioration is also highlighted
in this chapter.
The use of miniature specimens in cement science investigations has
proven to be very valuable because it assures a greater homogeneity of the
sample and increased sensitivity to the dimensional changes resulting from
physical and chemical processes. Chapter 11 provides results on compacted
powder used as a model system and includes discussion on creep and
shrinkage, volume stability, workability, and surface chemical changes.
Corrosion ofreinforced concrete is a major destructive process. Many
electrochemical techniques have been developed to study corrosion. Chapter 12 presents a comprehensive treatment of the principles of corrosion,
factors responsible for corrosion, and corrosion assessment techniques
relevant to concrete.
Surface area has an important influence on the rate of reaction of
cement to water and other chemicals. Many physical and mechanical
characteristics of cement and concrete are modified by changes in the
surface area. In Chapter 13, the techniques that are used for measuring
surface area are given with respect to their application to systems such as

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xt2

Preface

raw materials for cement, hydrated cement, concrete mix, and also to
durability studies.
The pore structure of hydrated cement systems influences significantly
the physicomechanical and chemical behavior of concrete. Several experimental techniques have been employed to evaluate the microstructure of the
cement paste. Chapter 14 presents a description of six techniques that have

been developed for the determination of pore structure. The relationship
between pore structure and strength/permeability is also included.
The application of silica polymerization analysis for an understanding
ofthe hydration process and structure of calcium silicate hydrates is detailed
in Chapter 15. Three major techniques used for polymerization studies are
described.
In concrete, the physical structure and the state of water in the matrix
influences the permeation process. In Chapter 16, test methods that are
employed to measure various transport characteristics of concrete are
evaluated. The applicability and limitations of these techniques is also
reviewed.
Inspection and testing of placed concrete may be carried out by
nondestructive testing methods. Sonic and pulse velocity techniques are
commonly used. Nondestructive methods are also applied to estimate
strength, surface hardness, pullout strength, etc. Details of various nondestructive techniques and their applications are included in Chapter 17.
There is evidence of a significant impact of computer and information
technologies on concrete science and technology. General development of
these technologies in recent years is reviewed in Chapter 18. The treatment
includes computer models, databases, artificial knowledge-based and computer-integrated systems.
In Chapter 19, entitled "Image Analysis," steps needed to identify
reactions of interest and extract quantitative information from digital images
are reviewed. In image analysis, multiple images are acquired and analyzed.
The prine ip 1e steps required for image analysis o fcemen ti ti ous materials are
described in this chapter.
Some of the more commonly used techniques in concrete studies are
presented in Chapters 2 to 19. There has been continued interest in
developing new techniques for the investigation of cement and concrete.
Chapter 20 comprises the description and application of fourteen of these
specialized techniques. They include such techniques as Auger Electron
Microscopy, Chromatography, Mass Spectrometry, X-Ray Absorption Fine

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Preface

xiii

Structure Analysis, Synchrotron Orbital Radiation Analysis, M6ssbauer
Spectrometry, Radio Tracer Technique, and Photoacoustic Spectroscopy.
Although every attempt has been made to cover the important
investigative techniques used in concrete technology, it is quite possible that
some information has been excluded or is missing. In addition, some
duplication of information occurs in some chapters. This was intentional
because some specific chapters may only be of interest to specialized
groups, and they provide enough self-contained information so that gleaning
through other chapters will not be needed.
This comprehensive handbook should serve as a reference material to
concrete technologists, materials scientists, analytical chemists, engineers,
architects, researchers, manufacturers of cement and concrete, standards
writing bodies, and users of concrete.
Ottawa, Canada
May 12, 2000

V. S. Ramachandran
James J. Beaudoin

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Contributors

Xu Aimin

David Darwin

Sarkar and Associates
Houston, Texas

University of Kansas
Lawrence, Kansas

P. A. Muhammed Basheer

Geoffrey Frohnsdorff

The Queen's University of Belfast
Belfast, Northern Ireland

National Institute of Standards and
Technology
Gaithersburg, Maryland

James J. Beaudoin

National Research Council of
Canada
Ottawa, Canada

S. N. Ghosh

Structural Waterproofing Co.
P.O.- Shibpur, Howrah
West Bengal, India

A. K. Chatterjee

Cement House
Mumbai, India

P. E. Grattan-Bellew

National Research Council of
Canada
Ottawa, Canada

James R. Clifton

National Institute of Standards and
Technology
Gaithersburg, Maryland

XP

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xvi

Contributors

Gordon Ping Gu
National Research Council of
Canada
Ottawa, Canada

Vangi S. Ramachandran
National Research Council of
Canada
Ottawa, Canada

Nataliya Hearn
Consultant
Timonium, Maryland

Shondeep L. Sarkar
Sarkar and Associates
Houston, Texas

William G. ttime
Erlin, Hime and Associates
Northbrook, Illinois

Jan Skalny
Consultant
Timonium, Maryland

Dipayan Jana
Sarkar and Associates
Houston, Texas

Leslie J. Struble
University of Illinois
Urbana-Champaign, Illinois

Xihuang Ji
University of Illinois at UrbanaChampaign
Urbana, Illinois

Guokuang Sun
University of Illinois
Urbana, Illinois

R. James Kirkpatrick
University of Illinois
Urbana, Illinois
V. M. Malhotra
Canada Centre for Mineral and
Energy Technology
Ottawa, Canada

Hiroshi Uchikawa
Kanazawa Institute of Technology
Nonlchi, Ishikawa, Japan
J. Francis Young
University of Illinois
Urbana, Illinois

Jacques Marchand
Laval University

Quebec, Canada

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1
Concrete

Science

Vangi S. Ramachandran

1.0

INTRODUCTION

Concrete, made from cement, aggregates, chemical admixtures,
mineral admixtures, and water, comprises in quantity the largest of all
man-made materials. The active constituent of concrete is cement paste
and the performance of concrete is largely determined by the cement
paste. Admixtures in concrete confer some beneficial effects such as
acceleration, retardation, air entrainment, water reduction, plasticity, etc.,
and they are related to the cement-admixture interaction. Mineral admixtures such as blast furnace slag, fly ash, silica fume, and others, also
improve the quality of concrete.
The performance of concrete depends on the quality of the ingredients, their proportions, placement, and exposure conditions. For example,
the quality of the raw materials used for the manufacture of clinker, the
calcining conditions, the fineness and particle size of the cement, the
relative proportions of the phases, and the amount of the mixing water,
influence the physicochemical behavior of the hardened cement paste. In
the fabrication of concrete, amount and the type of cement, fine and coarse

aggregate, water, temperature of mixing, admixture, and the environment
to which it is exposed will determine its physical, chemical, and durability
behavior. Various analytical techniques are applied to study the effect of

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Analytical Techniques in Concrete Science and Technology
these parameters and for quality control purposes. The development of
standards and specifications are, in many instances, directly the result of
the work involving the use of analytical techniques. Discussion of the methods
employed in standard specifications is beyond the scope of this chapter.
In this chapter, basic aspects of the physical, chemical, durability,
and mechanical characteristics of cement paste and concrete are presented
because of their relevance to the application of various analytical techniques discussed in other chapters.

2.0

FORMATION OF PORTLAND CEMENT

According to ASTM C-150, portland cement is a hydraulic cement
produced by pulverizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more types of calcium sulfate, as
an interground addition.
The raw materials for the manufacture of portland cement contain,
in suitable proportions, silica, aluminum oxide, calcium oxide, and ferric
oxide. The source of lime is provided by calcareous ingredients such as
limestone or chalk and the source of silica and aluminum oxide are shales,
clays or slates. The iron bearing materials are iron and pyrites. Ferric
oxide not only serves as a flux, but also forms compounds with lime and
alumina. The raw materials also contain small amounts of other compounds such as magnesia, alkalis, phosphates, fluorine compounds, zinc

oxide, and sulfides. The cement clinker is produced by feeding the crushed,
ground, and screened raw mix into a rotary kiln and heating to a temperature of about 1300-1450~ Approximately 1100-1400 kcal/g of energy
is consumed in the formation of clinker. The sequence of reactions is as
follows: At a temperature of about 100~ (drying zone) free water is
expelled. In the preheating zone (750~ firmly bound water from the clay
is lost. In the calcining zone (750-1000~ calcium carbonate is dissociated. In the burning zone (1000-1450~ partial fusion of the mix occurs,
with the formation of C3S, C2S and clinker. In the cooling zone (14501300~ crystallization of melt occurs with the formation of calcium
aluminate and calcium aluminoferrite. After firing the raw materials for
the required period, the resultant clinker is cooled and ground with about
4-5% gypsum to a specified degree of fineness. Grinding aids, generally
polar compounds, are added to facilitate grinding.

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Concrete Science

2.1

3

Composition of Portland Cement

The major phases of portland cement are tricalcium silicate
(3CaOoSiO2), diealeium silicate (2CaOoSiO2), tricalcium aluminate
(3CaO.A1203), and a ferrite phase of average composition
4CaOoA1203oFe203 . In a commercial clinker they do not exist in a pure
form. The 3CaOoSiO 2 phase is a solid solution containing Mg and A1 and
is called alite. In the clinker, it consists of monoelinic or trigonal forms
whereas synthesized 3CaO.SiO 2 is triclinic. The 2CaOoSiO 2 phase occurs

in the fl form, termed belite, and contains, in addition to A1 and Mg, some
K20. Four forms, o~, o~', ]3 and ),, of C2S are known although in clinker
only the [3 form with a monoclinic unit cell exists. The ferrite phase,
designated C4AF, is a solid solution of variable composition from C2F to
C6A2F. Potential components of this compound are C2F, C6AF 2, C4AF,
and C6A2F. In some clinkers small amounts of calcium aluminate of
formula NCsA 3 may also form.
ASTM C-150 describes five major types of portland cement. They
are: Normal Type I ~ w h e n special properties specified for any other type
are not required; Type II~moderate sulfate resistant or moderate heat of
hydration; Type III~high early strength; Type I V ~ l o w heat; and Type
V~sulfate resisting. The general composition, fineness, and compressive
strength characteristics of these cements are shown in Table 1.[ ]1
Portland cement may be blended with other ingredients to form
blended hydraulic cements. ASTM C-595 covers five kinds of blended
hydraulic cements. The portland blast furnace slag cement consists of an
intimately ground mixture of portland cement clinker and granulated blast
furnace slag or an intimate and uniform blend of portland cement and fine
granulated blast furnace slag in which the slag constituent is within
specified limits. The portland-pozzolan cement consists of an intimate and
uniform blend of portland cement or portland blast furnace slag cement
and fine pozzolan. The slag cement consists mostly of granulated blast
furnace slag and hydrated lime. The others are pozzolan-modified portland cement (pozzolan < 15%) and slag-modified portland cement (slag <
25%).

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Analytical Techniques in Concrete Science and Technology
Table 1. Compound Composition, Fineness and Compressive Strength

Characteristics of Some Commercial U.S. Cements
ASTM
Type

ASTM
Designation

Composition

Fineness
cmZ/g

C3S C2S C3A C4AF

Compressive Strength
% of Type I Cement*
1 day

2 days 28 days

I

General purpose

50

24

11

8

1800

100

100

100

II

Moderate sulfate
resistant-moderate
heat of hydration

42

33

5

13

1800

75

85

90

III

High early strength

60

13

9

8

2600

190

120

110

IV

Low heat

26

50

5

12

1900

55

55

75

V

Sulfate resisting

40

40

4

9

1900

65

75

85

*All cements attain almost the same strength at 90 days.

3.0

INDIVIDUAL CEMENT COMPOUNDS

3.1

Tricalcium Silicate

Hydration. A knowledge of the hydration behavior of individual
cement compounds and their mixtures forms a basis for interpreting the
complex reactions that occur when portland cement is hydrated under
various conditions.
Tricalcium silicate and dicalcium silicate together make up 75-80%
of portland cement (Table 1). In the presence of a limited amount of water,
the reaction of C3S with water is represented as follows:
3CaOoSiO 2 + xH20 --~ yCaOoSiO2o(x+y-3)H20 + (3-y)Ca(OH)2
or typically
213CaOoSiO2] + 7H20 --~ 3CaOo2SiO2o4H20 + 3Ca(OH)2
The above chemical equation is somewhat approximate because it is
not easy to estimate the composition of C-S-H (the C/S and S/H ratio) and
there are also problems associated with the determination of Ca(OH)2. In
a fully hydrated cement or C3S paste, about 60-70% of the solid comprises
C-S-H. The C-S-H phase is poorly crystallized containing particles of

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Concrete Science

5

colloidal size and gives only two very weak, diffuse peaks in XRD. The
degree of hydration of C3S can be measured by determining C3S or
Ca(OH)2 by XRD, the non-evaporable water by ignition, or Ca(OH)2 by
thermal or chemical methods. Each of these methods has limitations. The
Ca(OH)E estimated by XRD differs from that determined by chemical
analysis. For example, Pressler, et al.,[ a] found a value of 22% Ca(OH)2 by
XRD for portland cement pastes. The chemical extraction method gave
values 3-4% higher and this difference was attributed to the presence of
amorphous Ca(OH)E. Lehmann, et al.,[ 3] on the other hand, reported that
the extraction method yielded 30-90% Ca(OH)2 higher than that by XRD.
Thermogravimetric analysis gave identical values to those obtained by xray. Recently the technique of differential thermal analysis was applied by
Ramachandran[ 4] and Midgley[ 5] for estimating Ca(OH)E in hydrating
C3S.
The direct methods of determining C/S ratios are based on electron
optical methods such as electron microprobe or other attachments, or by
electron spectroscopy (ESCA). Although several values are reported, the
usual value for C/S ratio after a few hours of hydration of C3S is about 1.41.6. [6] The C/S ratio of the C-S-H phase may be influenced by admixtures.
There are problems associated with the determination of H20 chemically associated with C-S-H. It is difficult to differentiate this water from
that present in pores. The stoichiometry of C-S-H is determined by
assuming that little or no absorbed water remains in the sample at the ddry condition (the vapor pressure of water at the sublimation temperature
of solid CO a, i.e., -78~ In a recent investigation it has been shown that
higher hydrates may exist at humidities above the d-dry state.[7] It has been
proposed that drying to 11% RH is a good base for studying the stoichiometry of calcium silicate hydrate. At this condition, the estimate of adsorbed
water can be made with some confidence. This does not mean that higher
hydrates do not exist above 11% RH. Feldman and Ramachandran[ 8]

estimated that the bottled hydrated C-S-H equilibrated to 11% RH (approached from 100% RH) had a composition 3.28 CaO:2SiO2:3.92 H20.
Hydration Mechanism. The mechanism of hydration of individual
cement components and that of cement itself has been a subject of much
discussion and disagreement. In the earliest theory, Le Chatelier explained
the cementing action by dissolution of anhydrous compounds followed by
the precipitation of interlocking crystalline hydrated compounds. Michaelis considered that cohesion resulted from the formation and subsequent
desiccation of the gel.[ 9] In recent years, the topochemical or solid state
mechanism has been proposed.

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Analytical Techniques in Concrete Science and Technology
In spite of a large amount of work, even the mechanism of hydration
of C3S, the major phase of cement, is not clear. Any mechanism proposed
to explain the hydrating behavior of C3S should take into account the
following steps through which the hydration proceeds. Five steps can be
discerned from the isothermal conduction calorimetric studies (Fig. 1). In
the first stage, as soon as C3S comes into contact with water there is a rapid
evolution of heat and this ceases within 15-20 mins. This stage is called
the preinduction period. In the second stage, the reaction rate is very slow.
It is known as the dormant or induction period and may extend for a few
hours. At this stage, the cement remains plastic and is workable. In the
third stage, the reaction occurs actively and accelerates with time, reaching a maximum rate at the end of this accelerating period. Initial set
occurs at about the time when the rate of reaction becomes vigorous. The
final set occurs before the end of the third stage. In the fourth stage, there
is slow deceleration. An understanding of the first two stages of the
reaction has a very important bearing on the subsequent hydration behavior of the sample. The admixtures can influence these steps. The retarders,
such as sucrose, phosphonic acids, calcium gluconate, and sodium
heptonate, extend the induction period and also decrease the amplitude of

the acceleration peak.

STAGE

i
[]

~,

K

m

~

I
I

I
I

i

,

a

I

I

I

I

I

I

iI

II
I
I

I

I
I
I

I

I

l

'

IIlL

I
I/

J _1_

min

CEMENT

I

I!I

Y

~4
I

/~
I

\

/

;:

I

!

q

\

,\

'

I
LI_

I
_l

hours, days

hours

days

l"hDe

Figure 1. Rate of heat development during the hydration of tricalcium silicate and
portland cement.[69] (Reproduced with permission, Noyes Publications from Concrete
Admixtures Handbook, 2nd. Ed., 1995.)

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Concrete Science

7

The processes that occur during the five stages are as follows. In the
first stage, as soon as C3S comes into contact with water it releases
calcium and hydroxyl ions into the solution. In the second stage, the
dissolution continues and pH reaches a high value of 12.5. Not much silica
dissolution occurs at this stage. After a certain critical value of calcium
and hydroxide ions is reached, there is a rapid crystallization of CH and
C-S-H followed by a rapid reaction. In the fourth stage, there is a continuous formation of hydration products. At the final stage, there is only a slow
formation of products and at this stage the reaction is diffusion controlled.
It is generally thought that initially a reaction product forms on the
C3S surface that slows down the reaction. The renewed reaction is caused
by the disruption of the surface layer. According to Stein and Stevels,[l~
the first hydrate has a high C/S ratio of about 3 and it transforms into a
lower C/S ratio of about 0.8-1.5 through loss of calcium ions into solution.
The second product has the property of allowing ionic species to pass
through it thus enabling a rapid reaction. The conversion of the first to the
second hydrate is thought to be a nucleation and growth process. Although
this theory is consistent with many observations, there are others which do
not conform to this theory. They are: the C/S ratio of the product is lower
than what has been reported, the protective layer may not be continuous,
the product is a delicate film that easily peels away from the surface, and
the early dissolution may or may not be congruent.
The end of the induction period has been explained by the delayed
nucleation of CH. It is generally observed that the rapid growth of
crystalline CH and the fall of calcium ions in solution occur at the end of
the induction period. This suggests that the precipitation of CH is related

to the start of the acceleratory stage. If precipitation of CH triggers the
reaction, then additional Ca ions should accelerate the reaction unless it is
poisoned. Addition of saturated lime is known to retard the reaction. Also,
it does not explain the accelerated formation of C-S-H. Tadros, et al.,[ 1~1
found the zeta potential of the hydrating C3S to be positive, indicating the
possibility of the chemisorption of Ca ions on the surface resulting in a
layer that could serve as a barrier between C3S and water. During the
precipitation of Ca(OH)2 it is thought that Ca 2+ from the solution is
removed (which will in turn trigger the removal of Ca 2+ from the barrier)
and the reaction is accelerated.

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Analytical Techniques in Concrete Science and Technology
There are other mechanisms, based on the delayed nucleation of
C-S-H, to explain the end of this induction period. One of them suggests
that the stabilization action of the C3S surface by a thin layer of water is
removed when a high Ca 2+ concentration in the solution causes the
precipitation of C-S-H nuclei. According to Maycock, et al.,[ 121 the solid
state diffusion within the C3S grain controls the length of the induction
period. The defects enhance diffusion and thereby promote the C-S-H
nucleation. According to Fierens and Verhaegen,[13] the chemisorption of
H20 and dissolution of some C3S occur in the induction period. The end of
the induction period, according to them, corresponds to the growth of a
critical size of C-S-H nuclei.
There are other theories which have been proposed to fit most of the
observations. Although they appear to be separate theories, they have
many common features. They have been discussed by Pratt and Jennings.[ 141
A detailed discussion of the mechanisms of hydration of cement and C3S

has been presented by Gartner and Gaidis.[15]
The hydration of C2S proceeds in a similar way to that of C3S, but is
much slower. As the amount of heat liberated by C2S is very low compared to that of C3S, the conduction calorimetric curve will not show the
well defined peaks as in Fig. 1. Accelerators will enhance the reaction rate
of CzS. The reaction of CzS and water has been studied much less than that
involving C3S.

3.2

Dicalcium Silicate

Just as in the hydration process of C3S , there are uncertainties
involved in determining the stoichiometry of the C-S-H phase found in the
hydration of C2S. The hydration of dicalcium silicate phase can be represented by the equation.
2 [2CaOoSiOz] + 5H20 --. 3CaOo2SiO 2o4H20 + Ca(OH)2
The amount of Ca(OH)2 formed in this reaction is less than that
produced in the hydration of C3S. The dicalcium silicate phase hydrates
much more slowly than the tricalcium silicate phase.
Figure 2 compares the rates of hydration of C3S and C2S. The
absolute rates differ from one sample to the other; for example, C3S is
much more reactive than C2S. Several explanations have been offered to
interpret the increased reactivity of C3S. Proposed explanations include:
the coordination number of Ca is higher than 6, coordination of Ca is

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Concrete Science

9

irregular, holes exist in the crystal lattice, and differences o c c u r in the
position of the Fermi level. Some preliminary work has been done to test
the relative reactivities of Ca 2+ in CaO, Ca(OH)2, C3S, and C2S by mixing
each of them with known amounts of AgNO3 .[16] By heating them, it was
found that the reaction of AgNO 3 with CaO, Ca(OH) 2, and hydrated C3S,
was stoichiometric with respect to Ca. Only 27% Ca present in C3S and
6% Ca from C2S reacted with AgNO 3. Possibly C3S and C2S structures are
such that some Ca 2+ ions are relatively more reactive owing to structural
imperfections. There is evidence that if one mol of labeled Ca is reacted
with C2S to form C3S, the hydration of C3S would show that the initial
reaction product contains mainly the labeled Ca ions. Further work would
be necessary before definite conclusions can be drawn.

100

.........

I

I . . . . .

I

"'

I

....

-

'

3 CaO. SiO

8O
60
40
20
~,

1

'

,

I ....
3

I
7

,I,
14

l
28

gO

Period of Hydration, Days
Figure 2. The relative rates of hydration of 3 CaOoSiO 2, and 2 CaO~

.[69] (With

permission, Noyes Publications, Concrete Admixtures Handbook, 2nd Ed., 1995.)

The rate of strength development of individual cement compounds
was determined by Bogue and Lereh in 1934. [17] The comparison of
reaetivities and strength development of these compounds was not based
on adequate control of certain parameters, such as particle size distribution, water:solid ratio, specimen geometry, method of estimation of the
degree of hydration, etc. Beaudoin and Ramaehandran[ 181have reassessed
the strength development in cement mineral pastes, both in terms of time
and degree of hydration. Figure 3 compares the results of Bogue and Lerch

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10

Analytical Techniques in Concrete Science and Technology

with those of Beaudoin and Ramachandran.[ 18] Significant differences in
the relative values of strengths developed by various phases were found.
At ten days of hydration the strength values were ranked as follows by
Beaudoin and Ramachandran: CaAF > C3S > C2S > C3A. At fourteen days
the relative values were in the order C3S > CaAF > C2S > C3A. The BogueLerch strength values both at ten and fourteen days were: C3S > C2S > C3A
> CaAF. At one year, the corresponding values were C3S > C2S > CaAF >

C3A (Beaudoin-Ramachandran) and C3S = C2S > C3A > CaAF (BogueLerch). Beaudoin and Ramachandran found that compressive strength vs.
porosity curves on a semilog plot showed a linear relationship for all
pastes (Fig. 4). The lines seem to merge to the same value of a strength of
500 MPa at zero porosity. This would indicate that all the pastes have the
same inherent strength. Comparison of strengths as a function of the
degree of hydration revealed that at a hydration degree of 70-100%, the
strength was in the decreasing order C3S > CaAF > C3A.

3.3

Tricalcium Aluminate

Although the average C3A content in portland cement is about 4-11%,
it significantly influences the early reactions. The phenomenon of flash
set, the formation of various calcium aluminate hydrates and calcium
carbo- and sulfo-aluminates, involves the reactions of C3A. Higher amounts
of C3A in portland cement may pose durability problems. For example, a
cement which is exposed to sulfate solutions should not contain more than
5% C3A.
Tricalcium aluminate reacts with water to form C2AH 8 and C4AH13
(hexagonal phases). These products are thermodynamically unstable so
that without stabilizers or admixtures they convert to the C3AH 6 phase
(cubic phase). The relevant equations for these reactions are:
2C3A + 21H ~ C4AH13 -I- C2AH 8
C4AHI3 + C2AH 8 ~ 2C3AH 6 + 9H

In saturated Ca(OH)2 solutions, C2AH 8 reacts with Ca(OH)2 to form
C4AHI3 or C3AH6, depending on the condition of formation. The cubic
form (C3AH6) can also form directly by hydrating C3A at temperatures of
80~ or above.[19][2~

C3A + 6H --+ C3AH 6

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100

11

Bogue-Lerch

=l 8O

Ii
c 60
-.o-

4o

-.o-

e~

-o- c,~l
n. 20
E

8

100

0

200
Time, days

300

400

100

Beaudoln-Ramaohandran

,e,

5

8O

41b

c 60
f~

'6

0

tO ~

J

,,-20
E

8

0

0

100

200
Time, days

300

400

Figure 3. Compressive strength of hydrated cement compounds. (With permission,
Noyes Publications, Concrete Admixtures Handbook, 2nd Ed., 1995.)

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12

Analytical Techniques in Concrete Science and Technology

1000
=f
.~
c

1~

J
2
a.
E
0

o

100
<>

10

1

i

.~. c=~s
9o . c.~s
,.O- C, AF

-O- C3A

0.1

0.01
0

_

_

20

40
Porosity, %

60

Figure 4. Porosity vs. strength relationships for cement compounds.[]8]
The C3A pastes exhibit lower strengths than do the silicate phases
under normal conditions of hydration. This is attributed to the formation
of the cubic phase. Under certain conditions of hydration of C3A, i.e., at
lower water/solid ratios and high temperatures, the direct formation of
C3AH 6 (resulting in the direct bond formation between the particles) can
improve the strength of the body substantially.
In portland cement, the hydration of the C3A phase is controlled by
the addition of gypsum. The flash set is thus avoided. The C3A phase
reacts with gypsum in a few minutes to form ettringite as follows:
C3A + 3CSH 2 + 26H ~ C3Ao3CSH32
After all gypsum is converted to ettringite, the excess C3A will react

with ettringite to form the low sulfo-aluminate hydrate.
C3A~

+ 2C3A + 4H ~

3[C3A~

]

Gypsum is a more effective retarder than lime for C3A hydration and
together they are even more effective than either of them. The common
view for the explanation of the retardation of C3A hydration by gypsum is
that a fine grained ettringite forming on C3A retards the hydration. This
layer thickens, bursts, and reforms during the induction period. When all
sulfate is consumed, the ettringite reacts with C3A with the formation of
monosulfo-aluminate hydrate. This conversion will occur in cements within

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Concrete Science

13

12-36 hrs with an exothermic peak. Addition of some admixtures may
accelerate or delay this conversion. It has also been suggested that ettringite
may not, per se, influence the induction period[21][22] and that adsorption
of sulfate ions on the positively charged C3A retards the hydration. It has
also been suggested that osmotic pressure may be involved in the rupture
of ettringite needles. This theory is based on the observation of hollow

needles in the C3A-gypsum-H20 system. Rupture of ettringite allows
transfer of A1 ions into the aqueous phase with the quick formation of
hollow needles through which more A13§ can travel.[ 14]

3.4

The Ferrite Phase

The ferrite phase constitutes about 8-13% of an average portland
cement. In portland cement the ferrite phase may have a variable composition that can be expressed as C 2 (AnFI_n) where O < n < 0.7.
Of the cement minerals, the ferrite phase has received much less
attention than others with regard to its hydration and physico-mechanical
characteristics. This may partly be ascribed to the assumption that the
ferrite phase and the C3A phase behave in a similar manner. There is
evidence, however, that significant differences exist.
The CaAF phase is known to yield the same sequence of products as
C3A , however, the reactions are slower. In the presence of water, CnAF
reacts as follows:
CaAF + 16H ~ 2C2(A,F)H 8

C4AF + 16H --~ C4(A,F)H13 + (A,F)H 3
Amorphous hydroxides of Fe and A1 form in the reaction of C4mF.
The thermodynamically stable product is C3(A,F)H 6 and this is the conversion product of the hexagonal hydrates. Seldom does the formation of
these hydrates cause flash set in cements.
Hydration of CaAF at low water:solid ratios and high temperatures
may enhance the direct formation of the cubic phase.[ a3] Microhardness
measurement results show that at a w/s = 0.13, the samples hydrated at 23
and 80~ exhibit microhardness values of 87.4 and 177 kg/mm 2 respectively. The higher strengths at higher temperatures may be attributed to
the direct formation of the cubic phase on the original sites of CaAF. This
results in a closely welded, continuous network with enhanced mechanical strength.

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14

Analytical Techniques in Concrete Science and Technology

In cements, C4AF reacts much slower than C3Ain the presence of
gypsum. In other words, gypsum retards the hydration of CaAF more
efficiently than it does C3A. The rate of hydration depends on the composition of the ferrite phase; that containing higher amounts of Fe exhibits
lower rates of hydration. The reaction of CaAF with gypsum proceeds as
follows:[ 24]
3C4AF + 12CSH 2 + 11 OH --~ 4[C6(A,F)SH32]+ 2(A,F)H 3
The low sulfo-aluminate phase can form by the reaction of excess
CaAF with the high sulfo-aluminate phase.
3C4AF + 2[C6(A,F)SH32 ] ~ 6[C4A,F)SH12] + 2(A,F)H 3
At low water/solid ratios and high temperatures the low sulfoaluminate may form directly.[ 25]
The above equations involve formation of hydroxides of A1 and Fe
because of insufficient lime in CaAF. In these products, F can substitute
for A. The ratio of A to F need not be the same as in the starting material.
Although cements high in C3A are prone to sulfate attack, those with high
CaAF are not. In high C4AF cements, ettringite may not form from the low
sulfo-aluminate, possibly because of the substitution of iron in the
monosulfate. It is also possible that amorphous (A, F)3 prevents such a
reaction. Another possibility is that the sulfo-aluminate phase that forms is
produced in such a way that it does not create crystalline growth pressures.

4.0

PORTLAND CEMENT

Although hydration studies of the pure cement compounds are very
useful in following the hydration processes of portland cement itself, they
cannot be directly applied to cements, because of complex interactions. In
portland cement, the compounds do not exist in a pure form, but are solid
solutions containing A1, Mg, Na, etc. The rate of hydration of alites
containing different amounts of A1, Mg, or Fe, has shown that, at the same
degree of hydration, Fe-alite shows the greatest strength. There is
evidence the C-S-H formed in different alites is not the same.[ 26] The
hydration of C3A, CaAF, and C2S in cement are affected because of
changes in the amounts of Ca 2+ and OH- in the hydrating solution. The
reactivity of CaAF can be influenced by the amount of SO42- ions consumed by C3A. Some SO42 ions may be depleted by being absorbed by the
C-S-H phase. Gypsum is also known to affect the rate of hydration of

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15

calcium silicates. Significant amounts of A1 and Fe are incorporated into
the C-S-H structure. The presence of alkalis in portland cement also has an
influence on the hydration of the individual phases.
As a general rule, the rate of hydration in the first few days of
cement compounds in cements proceeds in the order C3A > C3S > CaAF >
C2S. The rate of hydration of the compounds depends on the crystal size,
imperfections, particle size, particle size distribution, the rate of cooling,
surface area, the presence of admixtures, the temperature, etc.

In a mature hydrated portland cement, the products formed are C-S-H
gel, Ca(OH)2, ettringite (AFt phases), monosulfate (AFm phases), hydrogamet phases, and possibly amorphous phases high in AP § and SO4 ions.[6]
The C-S-H phase in cement paste is amorphous or semicrystaUine
calcium silicate hydrate and the hyphens denote that the gel does not
necessarily consist of 1:1 molar CaO:SiO 2. The C-S-H of cement pastes
gives powder patterns very similar to that of C3S pastes. The composition
of C-S-H (in terms of C/S ratio) is variable depending on the time of
hydration. At one day, the C/S ratio is about 2.0 and becomes 1.4-1.6 after
several years. The C-S-H can take up substantial amounts of A13+, Fe 3§
and SO42- ions.
Recent investigations have shown that in both C3S and portland
cement pastes, the monomer present in the C3S and C2S compounds
(SiO4 4" tetrahedra) polymerizes to form dimers and larger silicate ions as
hydration progresses. The gas liquid chromatographic analysis of the
trimethyl silylation derivatives has shown that anions with three or four Si
atoms are absent. The polymer content with five or more Si atoms
increases as the hydration proceeds and the amount of dimer decreases. In
C3S pastes, the disappearance of monomer results in the formation of
polymers. In cement pastes, even after the disappearance of all C3S and
C2S, some monomer is detected possibly because of the modification of
the anion structure of C-S-H through replacement of some Si atoms by A1,
Fe, or S.[6] Admixtures can influence the rate at which the polymerization
proceeds in portland cement and C3S pastes.
The minimum water:cement ratio for attaining complete hydration
of cement has been variously given from 0.35 to 0.40, although complete
hydration has been reported to have been achieved at a water:cement ratio
of 0.22. [27]
In a fully hydrated portland cement, Ca(OH) 2 constitutes about 2025% of the solid content. The crystals are platy or prismatic and cleave
readily. They may be intimately intergrown with C-S-H. The density of
Ca(OH)2 is 2.24 g/cm 3. The crystalline Ca(OH)2 gives sharp XRD

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