Hydration of Portland cement

Hydration of Portland Cement
Salahaddin University, Erbil, Iraq
College of Engineering
Civil Engineering Department

What is hydration of cement?
 It is a chemical reaction that occurs between the cement and water after addition
of water to the cement commonly to produce cement paste
 It is the product of reaction of cement with water. The chain of events that happen
is that when combined with water the individual cement compounds react with
water each in a particular way and produce hydration products also called
hydrates which in turn form a spongy mass known as gel
 It is a series of irreversible exothermic reactions. During this process a large amount
of heat is released
 The hydration produces the binding or “gluing” material that secures the
aggregates particles together in concrete
 The purpose is to achieve a firm, stiff and hard mass called the hardened cement
paste with the passing of time

Significance of hydration of cement
Understanding the hydration process is important for the following reasons;
 To achieve optimal strength and durability of cement paste and thus concrete
 To alleviate formation of thermal stresses that can lead to cracks
 To realize the importance of appropriate curing

Mechanisms of hydration of cement
 The are two ways in which the cement compounds may react with water;
1. By addition of water the compounds present in cement are dissolved to form
a super saturated solution from which hydrated products are precipitated. This
is the true hydration
2. By the reaction of hydrolysis
 The term hydration is usually applied to both hydration & hydrolysis

Products of hydration of cement
 Under similar conditions, the products of hydration of the individual cement
compounds are chemically the same as that of cement. This fact was observed
for the first time by Le Chatelier over a century ago
 The main hydrates are the calcium silicate hydrates and tricalcium aluminate
hydrate. The calcium silicate hydrates are the main cementitious compounds in
cement and the physical behavior of cement during hydration is similar to that
of these two compounds alone
 C4AF is considered to hydrate into tricalcium aluminate hydrate and some of it
becomes amorphous CaO.Fe2O3 in aqueous state
 Over a period of time, the products of hydration lead to stiffening, setting and
hardening of cement paste

Calcium silicate hydrates
 The C3S and C2S react with water to form calcium silicate hydrates known as the C-S-
H gel (once also called tobermorite gel) which constitutes the main binding
substance in concrete. They are also responsible for the final strength of the
hardened cement paste. C-S-H takes form in extreme small particles in the size range
colloidal matter
 70-80% of hydrated cement is comprised of calcium silicate hydrates
 As water added by a certain quantity, the C3S undergoes hydration first. Then the
C2S hydrates producing C3S2H3. (Both C3S & C2S is believed to approximately
produce the same final product C3S2H3, however there exists some uncertainty
about whether they both produce the same hydrate)
 The C-S-H formed by C3S and C2S has a typical Ca to Si ratio (~1.7 (1.5-2.0)) much
lower than that in C3S (3:1), so the excess Ca reacts to form Ca(OH)2 (CH) crystals.
C2S also forms CH which is an undesirable product of hydration having harmful
effects on concrete
 The C3S is much more reactive than C2S. The rate of reaction of C3S is moderately
fast while it is slow in C2S. Under standard temperature conditions, about half of C3S
will be hydrated by 3 days and 80% by 28 days. While C2S hydration takes longer by
a great deal. This is why C2S does not contribute to early strength

Calcium silicate hydrates (cont.)
 The hydration reactions can be shown by the following equations :
2C3S + 6H C3S2H3 + 3Ca(OH)2
2*[3*(40+16) + (28+2*16)] + 6*(2*1+16) [3*(40+16)+2*(28+2*16)+3*(2*1+16)] + 3*40+2*(16+1)
456 + 108 342 + 154 ** divide by 4.56 to express by weight of 100 units of C3S
100 + 23.68 75 + 48.68
Thus;
[100] + [24] [75] + [49]
For C2S;
2C2S + 4H C3S2H3 + Ca(OH)2
[100] + [21] [99] + [22]
 From molecular weight basis both silicates require almost the same amount of
water for their hydration whereas the CH produced by C3S is more than twice
the amount of CH produced by C2S

Calcium silicate hydrates (cont.)
 By observation, it has been realized that the hydration of C3S does not continue
in a constant rate after the initial rapid reaction with water on first mixing. The
rapid release of CH into the solution creates an outer layer of CSH(considered
10nm thick). This layer somewhat blocks further hydration so that very little
reaction occurs. This period is commonly called as dormant period
 After some time (an hour or two), the surface layer ruptures due to pressure and
rate of hydration rises again. Setting takes place afterwards
 The dormant period is practically significant since it is workable during that
period allowing concrete to be placed and compacted before setting
 Considerable strength already developed before the hydration process is
finished and a small amount of hydrated cement particles are believed to bind
together the still remaining unhydrated particles. Long-term hydration results in
comparatively little rise in strength
 Further, it has been observed that strength development of Portland cement is
similar to that of the calcium silicate hydrates

Stages of hydration of C3S
 Stage 1: correlates to a period of somewhat rapid evolution of heat, which lasts for short time
(about 15 min.)
 Stage 2: correlates to a period of dormancy (dormant period) lasting several hours. This is why
concrete remains plastic for several hours
 Stage 3: correlates to a period of acceleration when dormant period ends, lasting (about 4-8
hrs) during which the rate of heat evolution reaches its maximum value. By the end of this
period, the final set has passed and hardening has begun
 Stage 4: correlates to period of deceleration during which the rate of heat evolution declines to
a very low value
 Stage 5: after the rate of heat development reaches a very low value, it continues in a low
steady rate
Rate of heat evolution during
Hydration of C3S

Effect of temperature on hydration of C3S
 The hydration of C3S is highly sensitive to temperature i.e. with increase in
temperature there is increase in the rate of hydration
 Once the hydration is in stage 5, it is much less temperature affected
 The effect of temperature on the hydration of C3S is shown below;

Tri-calcium aluminate hydrate
 The amount of C3A is relatively small in cement but regardless it is the behavior of C3A
that is of immense significance to us when hydrated
 C3A in pure form (in absence of calcium sulfate, CaSO4) is wild when reaction with
water takes place causing almost immediate stiffening of the resulting paste (flash set).
 C3A reacts rapidly to form C2AH8 and C4AH19 which are unstable and therefore
convert to C2AH6. This reaction with water is very rapid and involves high amounts of
heat;
C3A + 6H C2AH8 & C4AH19 (unstable) C2AH6 (stable)
[100] + [40]
 If small amount of gypsum (dihydrate, CaSO4.2H2O) or hemihydrate (CaSO4.0.5H2O)
added to the cement, gypsum and C3A react to form insoluble calcium
sulfoaluminate i. e. before mixing with water the initial wild reactions are restricted by
formation of a protective layer called ettringite on the surface of C3A crystals. But
ultimately tricalcium aluminate hydrate is formed. The reaction is as follows;
 C3A + 3CaSO4 + 32H C3A.CaSO4.32H (ettringite)
C3A + dissolved calcium & sulfate ions + water ettringite
 Initial peak rate of heat development shows that adding water to cement causes
formation of some calcium aluminate hydrate directly

Tri-calcium aluminate hydrate (cont.)
 Dehydrated form of gypsum preferred due to rapid dissolution that ensures proper supply of
dissolved calcium and sulfate ions and will be more efficient in controlling highly reactive forms of
C3A. However, if level of dehydrated gypsum is too high then the gypsum may crystallize from
solution causing false set which is premature stiffening. When mixing continues the initial
workability is recovered
 Hydration of C3A requires much more water than that of calcium silicates
 The hydration of C3A is also actually retarded by CH liberated by hydration of C3S. This is due to
reaction of CH with C3A and water to form C4AH19 which forms a coat (layer) on the surface of
unhydrated C3A grains. It is also possible that CH decreases the concentration of aluminate ions in
the solution thus impeding rate of C3A hydration
 Hardened cement paste may be attacked by sulfates causing disruptions in cement paste due to
formation of sulfoaluminate from C3A. The protective layer created by formation of ettringite is
broken down during conversion of ettringite to monosulfoaluminate (giving way for C3A to react
rapidly again). When monosulfoaluminates come into contact with a new source of sulfate ions(
external source of sulfate ions) ettringite can be reformed. This possible reforming of ettringite is the
basis for sulfate of PC when exposed to external source of sulfates
 The high rate of heat development may also cause cracks in concrete which we would like to
avoid in all circumstances

The rate of heat of hydration of C3A
 The hydration curve looks similar to that of C3S
 The first peak occurs within minutes of hydration then reduced to a lower value due t
formation of ettringite coat. The heat of hydration stays low until the ettringite breaks and
converts into monosulfoaluminate after all gypsum has been used to create ettringite
 The rate of heat evolution increases with the beginning of ettringite conversion to
monosulfoaluminate and reaches second peak and afterwards starts decreasing to a
steady state
Rate of heat evolution during hydration of C3A
with gypsum

Hydration of C4AF
 Hydration of C4AF is comparable to that of C3A but proceeds much slower and
involves less heat
 C4AF does not hydrate fast enough to encourage flash set and gypsum retards
hydration of C4AF even more than C3A
 Gypsum doesn’t react with C3A only, it reacts with C4AF forming calcium
sulfoferrites and also calcium sulfoaluminate. Its presence may also accelerate
hydration of silicates
 Practical observation has shown that cement low in C3A and higher in C4AF are
more resistant to sulfate attack. This is explained in the way that re-formation of
ettringite from monosulfoaluminate does not take place in case of C4AF due to
presence of iron in it
 The water bound chemically in C3A and C4AF are 40 and 37 respectively. The
latter is calculated on the assumption that final reaction of hydration of C4AF is;
C4AF + 2CH + 10H C3AH6 + C3FH6

Calcium Hydroxide, Ca(OH)2 or CH
 Created during hydration of calcium silicates
 It may compose nearly 25% of volume of solids in hydrated paste and causes
the concrete to become porous, weak and undurable
 Has the ability to react with sulfates present in water or soil to form calcium
sulfates further reacting with C3A and causes deterioration of concrete
 Its effect can be mitigated by use of pozzolans
 Its single benefit is that due to being alkaline, it keeps the pH value of water at
around 13 in the concrete thus decreasing corrosion of steel reinforcement
 Takes form in crystalline material

Typical development of hydration of pure compounds
Hydration of calcium silicates
Hydration of aluminates
 The hydration of cement compounds
are summarized in the tables and rate
of hydration shown in the curve

 With Time :
- The rate of hydration decreases continuously.
- The size of unhydrated cement particles decrease. For instance,
after 28 days in contact with water, grains of cement have been
found to have hydrated to a depth of only 4 μm, and 8 μm after
a year.
 This is due to:
1) Accumulation of hydration products around the unhydrated
cement grains which lead to prevent water from channeling to
them.
2) Reduction of the amount of water either due to chemical
reaction or evaporation.
3) Reduction of the amount of cement due to reaction.

Water requirement of hydration of cement
 For full hydration of Portland cement, C3S needs 24% of water by weight of
cement and C2S needs 21%. On average PC needs 23% of water by weight of
cement
 This 23% chemical combines with cement and can be called as bound water
 Aside from the bound water, a certain amount of water is found within the pores
of the gel called gel water
 The bound water and gel water are complementary to each other. If the
amount of water is insufficient to fill the gel pores, there is no question of the gel
pores presence.
 It has been estimated that about 15% water by weight of cement is needed to
fill the gel pores. Thus for completion of hydration reaction a total of
approximately 23%+15%=38% of water by weight of cement is estimated to be
required so that no excess water is left
 However, if more water is used than required for full hydration, then the excess
water will form undesirable capillary cavities

Structure of hydrated cement
 Fresh cement paste is plastic network of cement
particles in water but once it has set, its gross volume
appears approximately constant
 At any stage of hydration, the hardened paste consists
of poorly crystallized hydrates of compounds, refereed
to as gel, of CH crystals, some minor components,
unhydrated cement and the residue of water-filled
spaces in fresh paste called capillary pores. Their
volume is reduced with the progress of hydration. Water
present in these pores is called capillary water
 Within the gel itself also exists interstitial voids called gel
pores
 Since most of products of hydration are colloidal, during
hydration the surface area of the solid phase increases
enormously, and a large quantity of free water
becomes adsorbed on the surface area
 If no water movement to or from the cement paste is
allowed, the reactions of hydration consume the water
until too little is left to saturate the solid surfaces and the
humidity within the paste declines. This is called self-
desiccation.
Simplified model of cement paste structure.
Solid dots represent gel particles. The interstitial
Spaces are gel pores. Spaces marked by C
represent capillary pores
 Thus the water added to cement
can be classified as:
1. Gel water
2. Combined water
3. Capillary water

Volume of products of hydration
 the space available for products of hydration
consists of volume of dry cement and the
volume of water added to the mix
 Non-evaporable water, determined under a
particular condition, is taken as 23% of mass
of anhydrous cement. The porosity is
considered about 28% i.e. about 28% of total
volume of gel occupied by gel pores
 The specific gravity of products of hydration
of cement is such that they take up a larger
volume than the absolute volume of
unhydrated cement but smaller than the sum
of volumes of dry cement and non-
evaporable water by approximately 0.254 of
the volume of non-evaporable water. Hence;
V(hydration products) = V(dry cement) +
V(non-evaporable water, 23%*mass of cement) –
0.254*V(non-evaporable water)

Cement paste at different stages of hydration

Heat of hydration
 It is the quantity of heat, joules per gram of unhydrated cement evolved upon complete hydration at a
certain temperature
 For practical purposes, the rate of heat evolution matters, not necessarily the total heat evolved
 The actual value of heat of hydration depends on the chemical composition of cement and is nearly the
same as the sum of the heats of hydration of individual compounds when hydrated separately
 Because in early stages of hydration the different compounds hydrate at different rates, the rate of heat
evolution as well as the total heat evolved depends upon the compounds composition of cement. For this
reason, lessening the proportions of the compounds that hydrate most rapidly (C3A & C3S) the rate of
heat evolution in early phase of concrete an be lowered
 Fineness of cement also has an influence on the rate of heat release. Increase in fineness speeds up the
hydration reactions and therefore higher rate of heat evolution. Early rate of hydration of each
compound in cement is proportional to the surface area of cement. However, the effect of surface area
is negligible in later stages of hydration and total heat evolved is not affected by fineness of cement
 A controlled heat evolution is beneficial for many uses of concrete as suitable types of cement have
been developed such as low heat cement. It is thus advisable to understand heat of hydration of different
cements in order to choose the most suitable cement for a specific purpose
 The heat release is actually advantageous in cold weathers and in precast production where temp rise
accelerates strength development, in large-scale pours (mass concrete) the temperature difference
between core of the concrete and surroundings can create stresses leading to thermal cracking
 The temperature rise is dependent on: cement type (fineness, C3S & C3A contents), pouring temperature,
pour dimension, type of formwork, etc..

Heat of hydration (cont.)
Heat of hydration developed after 72 hrs at different
temperatures
Heat of hydration of compounds Influence of C3A on heat evolution (C3S
approximately constant)
Influence of C3S on heat evolution (C3A
Approximately constant)

Heat of hydration (cont.)
Development of heat of hydration of different types of Portland cement
(cured at 21°C, w/c:0.4)
 BS & ASTM describe method
of determining the heat of hydration by
measuring heats of solution of hydrated
and unhydrated cement in a mixture of
nitric and hydrofluoric acids (HNO3 & HF).
The difference between the two values
represents heat of hydration

Heat evolution curve
Heat of hydration of cement paste (by conduction calorimetry at 20°C)

Setting and hardening of cement
 Setting is used to describe the stiffening phase of cement. This happens after adequate addition
of water which leads to formation of the paste which in turn gradually becomes less plastic with
time
 Setting refers to change of cement paste from fluid to a rigid state
 The paste is said to have set when it becomes stiff enough to resist pressures
 The setting period is divided into two stages;
1. initial set 2. final set
 After the final set, the cement paste is believed further develop in strength and rigidity
 The initial set corresponds to rapid rise in temperature and final set corresponds to peak
temperature
 The setting time decreases with rise in temperature. But at low temperature the reverse effect is
observed (setting is retarded).
 Setting of cement can be further divided into;
1. flash set 2. false set
 False set can occur due to added water reaction with anhydrate CaSO4 (gypsum) forming
hydrate forms of gypsum which can cause stiffening, presence of alkalis during storage can form
alkali carbonates, these react with CH liberated by hydration of C3S to form CaCO3 which
precipitates and creates rigidity, and aeration of C3S at moderately high temperature

Strength build-up
On adding water to the cement:
 C3A is the fastest to hydrate and contributes to immediate strength (1-3 days) but at
advanced age it has little to no contribution to strength development
 C4AF does not have any noticeable effect of strength development
 C3S hydrates almost immediately next to C3A and has an immense effect on early strength
 C2S has been found to be responsible to strength development 28 days onwards
contributing to ultimate strength
Development of strength in pure
compounds

Factors affecting hydration
Main factors :
 Chemical composition of cement
The mineral composition of cement and their ratios are the main factors affecting the hydration of cement. Various mineral
components will reveal different characteristics when reacting with water. For example, the increase of C3A can speed up the
hydration, setting and hardening rate of cement, and the heat of hydration is high at the same.
 Cement type
each type of cement has a particular chemical composition to serve the purpose for which it is to be used so increasing or
decreasing some of the compounds (esp. C3A & C3S) will lead to direct increase or decrease in rate of hydration i.e. type III
cement has rapid hydration characteristics with increased early-age hardening whereas type II & type IV have retarded hydration
rates due to the fact that its purpose is served when hydration proceeds slowly
 Sulfate content
If the content of SO3 is too little, the retardation affect will be unobvious. Too much SO3 will accelerate the hydration of cement.
The appropriate amount of SO3 depends on the gypsum content and of C3A in the cement and it also related to the fineness of
cement and the content of SO3 in clinker. If the content of SO3 exceeds the limit, it will lower the strength of cement and it can
even lead to poor soundness, which will cause the expanded destruction of cement paste.
 Fineness
The finer the cement particles are, the larger the total surface area is and the bigger the area contacting with water is. Thus, the
hydration will be quick, the setting and hardening will be accelerated correspondingly, and the early strength will be high.
 Water/cement ratio
If the cement consumption is unchanged, the increase of the mixing water content will enhance the amount of capillary
porosities, lower the strength of cement paste, and extend the setting time. Therefore, in practical projects, the amount of water
and cement will be changed without modifying the water-cement ratio.

Factors affecting hydration (cont.)
 Curing temperature
Usually, the temperature rises at the time of curing, and the hydration of cement and the development of early strength
become fast. If the hardening process occurs at a low temperature, the final strength won’t be affected though the
development of the strength is slow.
 Addition of supplementary cementing substances, additives and admixtures
Hydration, setting, and hardening of Portland cement are constrained by C3S, C3A. And all the admixtures that affect the
hydration of C3S, C3A can change the performance of the hydration, the setting and hardening of Portland cement. For
example, the accelerator agents (such as CaC12, Na2S04) can accelerate the hydration and the hardening of cement
and improve its strength. On the contrary, the retarding agents (such as calcium lignosulphonate) can delay hydration
and hardening of cement and affect the development of the early strength.

Approaches used to study hydration of cement
Following methods are used to investigate cement hydration:
 Thermal analysis
 X-ray diffraction
 Scanning electron microscopy

References
 Properties of concrete, A. M. Neville
 Advanced concrete technology, John Newman & Ban Seng Choo
 Concrete technology, B. L. Gupta & Amit Gupta
 Hydration of cement, Rizwan Riaz, Muhammad Safdar & Fatima Mehvish
https://www2.slideshare.net/rizwansamor/hydration-of-cement?qid=179af270-
64dc-4673-bb4d-dfab983e3b54&v=&b=&from_search=5
 Concrete technology, Hydration of cement, Sukhvinder Singh
https://www2.slideshare.net/SukhvinderSingh89/hydration-of-cement-
73655061?qid=179af270-64dc-4673-bb4d-dfab983e3b54&v=&b=&from_search=2
 Concrete technology, A. M. Neville & J. J. Brooks

Hydration of Portland cement

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