What is Hydration of Cement? Know Heat of Hydration & Products of Cement Hydration Here

Hydration of Cement in Civil Engineering is basically the chemical reaction taking place when
water is added to the cement. The products formed as a result of the hydration of cement play an
important role in the strength gaining of concrete.

Hydration of Cement

Hydration of cement requires 38 % of water by total weight of cement on average.

The chemical reactions taking place between cement and water are termed as hydration of cement.

Anhydrous cement does not have adhesive property, hence it will only bind with coarse and fine aggregates when water is added. The hydration products have adhesive property which serves the main function of cement, i.e. to bind the materials together.

The active components of cement C₄AF, C₃A, C₃S, and C₂S react with water in the hydration of cement to give C-S-H gel and Ca(OH)₂.

Importance of Hydration of Cement

• The primary function of cement in concrete is that it acts as a binding material. Hence, it binds
  the aggregates together.
• Anhydrous cement (unhydrated cement) does not bind with fine or coarse aggregates. Adhesive
  property is gained by cement only after hydration. Hydration products discussed later on in this
  article possess this adhesive property.

Thus, the hydration of cement is of prime importance in concrete technology.

Hydration Process of Cement

Hydration starts in about one hour, and then hydrated products can be found too in the cement
paste. With time, the proportion of hydration products increases while cement paste decreases.

As the water is consumed in the hydration process, the products of hydration replace the water originally present. Thus, stiffening of mass takes place and as a result, the mass gains strength.

The products of hydration of cement start to deposit on the outer periphery of the nucleus of hydrated cement as soon as cement comes in contact of water.

As the hydration reaction of cement continues, more and more products of hydration are deposited on the cement particles. As a result, the water diffusion to the unhydrated nucleus of cement becomes less and less. As the water availability reduces, hydration reaction slows down. Thus, rate of hydration is reduced with time.

Following are the compounds present in cement paste at any time:

• Unreacted cement
• Water
• Gel- a fine grained product of hydration with large surface area
• Calcium hydroxide
• Some other minor compounds

Hydration of Major Products of Cement

As we know, cement constitutes two types of constituents, minor and major components. Major
Compounds of Cement / Bogue’s Compounds are responsible for strength gain as only they
undergo hydration.

All four major compounds of cement do not hydrate at the same rate. The rate of hydration of
aluminates is much more as compared to silicates. The stiffening of cement paste is determined
by the hydration of aluminates. The hardening of cement paste is determined by the hydration of
silicates.

Hydration of C₃S, C₂S, C₃A, and C₄AF is discussed in detail HERE.

Mechanism of Hydration of Cement

There are two mechanisms postulated explaining the process of hydration of cement as follows:

1. Through solution mechanism of cement hydration
2. Solid state mechanism of cement hydration

1. Through solution mechanism of cement hydration

According to through solution mechanism theory, the cement compounds dissolve when water is
added to form ionic constituents. Hydrates are thus formed in the solution making the solution
supersaturated.

From this supersaturated solution, products of hydration of cement are precipitated on account
of the low solubility of the hydrates.

Complete reorganization of original constituents takes place in through solution mechanism
during hydration.

Through mechanism can be presumed to be taking place during the early stages of hydration
when plenty of water is available.

2.Solid state mechanism of cement hydration [Topochemical]

As per solid state mechanism theory, the cement compounds are in the solid state instead of the
solution form. Water attacks these compounds in solid state and converts them into hydrated
products.

Hydration starts from the surface of the compounds. As time goes by, hydration continues to the
interior of the compounds.

Solid state mechanism continues in the later phase of hydration when the mobility of ions in the
solution is restricted.

Both the above mechanisms occur in the cement hydration reaction.

Complete hydration of cement particles is almost impossible.

For achieving complete hydration,

• Cement particles should be very finely ground
• It should be reground with excess water so that fresh surface is exposed to water again

For C₃S

For C₂S

For C₃A

Following observations can be made from the above reactions:

• C₃S hydration produces 61% C-S-H and 39% Ca(OH)₂. C₂S hydration produces 82% C-S-H and 18% Ca(OH)₂
• With the large amount of C-S-H produced by C2S, it can be concluded that the ultimate strength of C₂S will be higher as compared to that of C₃S.
• Ca(OH)₂ reduces the durability of concrete by decreasing its resistance to sulphate attack and acid attack.
• With the large amount of Ca(OH)₂ produced by C₃S, it can be concluded that the durability of C₃S will be higher as compared to that of C₃S.
• From the durability point of view, either of the following measures is taken-
  • C₃S content is limited
  • Pozzolana is added to remove the excess Ca(OH)₂
• C₃S requires more water content for complete hydration as compared to that of C₂S.

Products of Cement Hydration

The products of hydration of cement are:

• calcium silicate hydrates
• calcium hydroxide
• calcium aluminate hydrates

Mechanical properties of concrete depend on the physical structure of hydration products, rather
than the chemical composition of cement.

1. Calcium Silicate Hydrates (C-S-H gel)

Calcium silicate hydrate (C-S-H gel) and calcium hydroxide (Ca(OH)2) are formed during the
reaction of C3S and C2S with water.

Calcium silicate hydrate (C-S-H gel) is the most important hydration product of Portland cement.

Structure: The hyphen between C-S-H represents the non-defined product of calcium silicate
hydrate. It is a poorly crystalline fibrous mass.

As properties in concrete are determined by the structure, the difference in composition in terms
of calcium and silica proportion is not of much significance.

Tobermorite gel: C-S-H gel is also called tobermorite gel as its structure is similar to the naturally
occurring mineral tobermorite.

C-S-H gel is the most important product formed during the hydration reaction as it determines
the favorable properties of concrete.

Initially, there was no surety that the product of both C₃S and C₂S is the same, but later on, it was
proved true.

50-60 % of the volume of solids in fully hydrated cement paste comprises C-S-H gel alone.

2. Calcium Hydroxide

Structure: Distinct hexagonal prism morphology.

20-25 % of the volume of solids in fully hydrated cement paste comprises Ca(OH)₂.

Sulphate Attack due to Ca(OH)₂: Ca(OH)₂ reacts with the sulfur present in soils or water and
calcium sulfate is produced. Calcium sulfate reacts with C₃A, a major compound of cement, and
results in the deterioration of concrete.

5+ Measures to Prevent Sulphate Attack are described HERE.

Ca(OH)2 is not advantageous owing to the following reasons-

• It is soluble in water
• It undergoes leaching making concrete porous (especially in hydraulic structures)
• It decreases the durability of concrete
• It causes the deterioration of concrete
• It promotes sulfur attack on concrete

As seen above, Ca(OH)₂ is disadvantageous to concrete. The harmful effects are overcome by
the use of blending materials like fly ash, silica fume, and other pozzolanic materials.

Advantage: Ca(OH)₂ is of alkaline nature so the pH is maintained around 13, hence it can provide
corrosion resistance.

3. Calcium Aluminate Hydrates

Hydration of C₃A: Calcium aluminate system (CaO-Al₂O₃-H₂O) is formed upon the hydration of
aluminates. C₃AH₆ is a stable cubic hydrate formed during hydration, which can remain the same
till 225 °C. Other crystalline hydrates formed are C₄AH₁₉ and C₂AH₈.

C₃A reacts with water immediately causing a flash set. However, gypsum added during the
manufacture of portland cement acts as a retarder to slow down its reaction. If this step is not
done, portland cement cannot be used for most of the applications.

Hence, not hydration of C₃A, but hydration of C₃A in presence of gypsum is of significance.
Gypsum dissolves in water forming calcium sulphoaluminate, which is insoluble. It forms a
colloidal membrane around C₃A by depositing on its surface, and its hydration is retarded.

Hydrated aluminates give no contribution to the strength of cement. Rapid hydration of C₃A may
help in contributing little to the early strength of cement.

However, the hydration of aluminates makes the concrete prone to sulphate attack. Thus, it
affects the durability of concrete and is rather not desirable.

Hydration of C₄AF: Hydration of C₄AF forms a system of (CaO-Fe₂O₃-H₂O). C₃FH₆ is a hydrated
product, which is comparatively more stable.

Even this does not contribute to the strength. But, hydrates of C₃AH₆ show more resistance to
sulphate attack as compared to the hydrates of C3A.

Gypsum and alkalies decrease the solubility of C3A.

Either of the following precipitates is obtained-

1. Ettringite: calcium aluminate trisulphate hydrate (C₆AS₃H₂₂)
2. Monosulphate: calcium aluminate monosulphate hydrate (C₄ASH₁₈)

Ettringite

• Ettringite hydrates and crystallises first owing to the high sulphate/aluminate ratio in the
• solution phase, i.e. during the first hour of hydration.
• When sulphate concentration is reduced, aluminate concentration increases owing to the
• hydration of C3A and C4AF. Now, ettringite becomes unstable.
• Gradually, ettringite forms monosulphate when sulphate concentration declines and that of
• aluminate rises.
• Cement containing >5 % C4A will have the final product of monosulphate.

Is it Correct to Call Hydration Products as Gel?

Not exactly.

Le Chatelier gave crystalline theory in which he stated that the hydration products are
precipitates resembling crystals interlocked with each other.

Michaelis gave colloidal theory in which he mentioned precipitates as a colloidal mass of
gelatinous nature.

Now, it is accepted that the products of hydration are more like gel in which poorly formed thin
fibrous crystals of infinitely small size are present.

Gel makes the concrete porous. Its porosity is 28%. The gel pores are filled with water.

Structure of Hydrated Cement

To understand the behaviour of concrete, the study of the structure of hydrated cement becomes
important.

At the macro-phase level, concrete is a 2-phase material comprising –

• paste phase
• aggregate phase

Aggregate particles are dispersed in cement paste. Paste structure is more important as it
governs the following properties of concrete –

• strength
• permeability
• durability
• drying shrinkage
• elastic properties
• creep
• volume change properties

At the microscopic level, 3rd phase is also seen – the transition zone.

Transition Zone

• It is visible only at a microscopic level, presumably the third phase of concrete.
• It is the region between the particles of coarse aggregates and hardened cement paste observed in the vicinity of large aggregate particles.
• Water accumulates below the elongated and flaky aggregates due to internal bleeding. This reduces the bond strength in this region.
• Transition zone is important, as it is a plane of weakness in concrete. Internal bleeding and such factors make the quality of cement paste poor in this region.
• Even before loading, micro-cracks develop in this region because of drying shrinkage or temperature variation. When the structure is loaded, these cracks propagate into bigger cracks.

Thus, the transition zone is the strength limiting phase in concrete. This is because concrete may
fail at lower stress than that resisted by bulk paste or aggregates.

Unhydrated Core

In cement grains, the hydration of cement compounds adheres firmly to its unhydrated core.

However, the unhydrated portion left in the cement grain does not affect the strength gain of the
cement mortar or concrete. The only condition being the mortar or concrete should be well
compacted.

To prove this, Abrams obtained strength of 280 MPa at only a 0.08 w/c ratio. At such a low w/c
ratio, hydration will occur only at the surface and most of the cement particles at the core would
remain unhydrated. Still, high strength could be obtained.

Present day high performance concrete is obtained at a w/c ratio of 0.25. In this too, the core
remains unhydrated.

The unhydrated cement core works like fine aggregates in the system.

Importance of Products of Hydration of Cement

Adhesive or cementing property in cement is attributed to the products of hydration only.

Following aspects of hydration products of cement are important:

• quality
• quantity
• continuity of formation
• stability
• rate of formation

Chemistry of Cement Hydration

Chemistry of cement hydration describes the chemistry of the reactions occurring between cement and water.

Anhydrous cement components react with each other upon addition of water and hydrated compounds are formed. These hydrated products have very low value of solubility.

Through Solution Mechanism

This theory states

• The cement compounds dissolve in water forming a supersaturated solution
• Various hydrated products precipitate from this supersaturated solution

In early stages of hydration, when plenty of water is available, true solution mechanism may be operated.

Solid State Mechanism

This theory states

• Cement compounds remain in solid state as per this theory
• Water attacks these compounds in solid state only
• Hydrated products start forming from the surface
• Hydration reaction continues to the inner region of the compounds gradually

In later stages of hydration, when water availability is reduced, solid state mechanism of hydration may operate.

Hydration Reaction of Cement

C₃S produces less C-S-H as compared to C₂S. It reacts with water quickly and is responsible for early strength gain of cement. Quality and density of Ca(OH)₂ formed by C₃S is inferior as compared to C₂S.

As the hydration reaction occurs faster in C₃S, heat of hydration is liberated earlier if cement has higher content of C₃S. Also, it produces more heat of hydration than that of C₂S. Hence, cement with higher C₃S is preferred for cold weather concreting.

The corresponding weights of the compounds are written as superscript with the compounds.

2 C₃S¹⁰⁰ + 6 H₂O²⁴ → C₃S₂H*₆⁷⁵ + 3 Ca(OH)₂ ⁴⁹           { H* = H₂O}

C₂S hydrates slowly and liberates less heat of hydration. Density of  Ca(OH)₂ produced by C₂S is more and its specific surface is also higher.

C₂S¹⁰⁰ + H₂O²¹ →C-S-H⁹⁹ + Ca(OH)₂²²

As seen from both the equations, C₂S produces less Ca(OH)₂ as compared to C₃S.

C₃A reacts with water quickly and promotes flash setting of cement. As the cement sets earlier, hydration of C₃S and C₂S is prevented. To prevent the flash setting of cement, retarders like gypsum are added to cement clinkers at the time of grinding.

C₃A + H₂O → C₃AH₆

Besides, calcium sulphate present in the clinker dissolves in water forming calcium sulphoaluminate, which is insoluble in water. It deposits on the outer periphery of C₃A and forms a colloidal membrane. This membrane halts the hydration of C₃A.

C₃A + H₂O + CaSO₄ →CACṠH₁₂   {Ṡ represents SO₃}

The colloidal membrane breaks as the pressure of compounds formed during the hydration increases and C₃A is again available for hydration.

C₃A catalyses the hardening of C₃S.

Hydrated aluminates do not contribute to significant strength to concrete. Its early hydration provides a little strength to cement, but it is not much. If the concrete is prone to sulphate attack, then the aluminates can greatly affect the durability of concrete as they promote sulphate attack.

Hydration Products of Cement

Two important products of hydration of cement are discussed below:

1. C-S-H gel (Calcium Silicate Hydrate Gel)

It is also called tobermorite gel. It is the most important product of hydration as it determines the good properties of concrete.

C-S-H is written with hyphen, as it is not a proper, well-defined compound. It is in the form of poor crystalline fibrous mass.

Surface area of C-S-H: 100-700 m²/g

Solid to solid distance in C-S-H: 18

A completely hydrated Portland cement paste possesses 50-60 % of C-S-H gel phase of the total volume of solids. Hence, we can conclude that the C-S-H gel is crucial in determining the properties of paste.

2. Ca(OH)₂ (Calcium Hydroxide Crystals)

Calcium hydroxide is liberated in silicate phase. It is soluble in water and leaches out of concrete making it porous, hence it is not a desirable product of hydration.

The calcium hydroxide crystallises in the free space available. Its crystals have distinct hexagonal prism morphology. The crystals are also called portlandite.

Ca(OH)₂ forms 20-25 % of the total volume of solids in the hydrated cement paste.

The surface area of calcium hydroxide crystals is less as compared to C-S-H gel. The strength contributing potential of calcium hydroxide is also less.

Merit of Ca(OH)₂

Calcium hydroxide crystals start dissolving in water releasing hydroxyl ions in water. Thus, it is alkaline in nature and helps in maintaining the pH around 13. This prevents corrosion of reinforcement in concrete.

Demerits of Ca(OH)₂ in Concrete:

• It decreases durability of concrete
• It reacts with sulphates present in soil or water and promotes the phenomenon called sulphate attack. (Calcium sulphate formed reacts with C₃A component of cement causing deterioration of concrete) This further reduces durability
• As it is soluble in water, it can leach out of concrete

Blending materials like fly ash, silica fume, and various pozzolanic materials are used to curb the negative effects of Ca(OH)_2.

Water Requirement for Hydration of Cement

For complete hydration of Portland cement, 38 % of water by weight of cement is required. Presence of excess water may lead to capillary cavities.

Out of this, 23% is bound water while 15% is gel water. Water participating in the hydration
reaction is not available and is called bound water. Water filling up the gel pores is called gel
water.

Both bound water and gel water are complementary to each other. If enough water is not
available to fill the gel pores, then the formation of the gel will be stopped.

With a w/c ratio of 0.38, complete hydration will take place with no excess water. Excess water, if
present, forms the capillary cavities in the concrete. The capillary cavities increase porosity and
are undesirable.

Full hydration with a 0.38 w/c ratio is based on the assumption that hydration takes place in an
airtight system, where the exchange of moisture is not possible.

In general practice, complete hydration of cement is never achieved, hence w/c ratio less than 0.38
is also used. Especially for high performance concrete, a w/c ratio < 0.38 is used to avoid capillary
activities.

Now, the volume of gel is twice the volume of unhydrated products. Production of gel increases
with the hydration of cement. The gel fills up the space occupied by water earlier.

However, at a w/c ratio above 0.7, the gel will never be enough to fill up the space occupied by
water. Hence, the concrete will remain a porous mass.

Excess water in the concrete is undesirable. Effects of Excess Water in Concrete are described
HERE.

Let us also understand bound water and gel water in some more depth.

Bound water-

The water, which actually chemically combines with the cement compounds for hydration, is bound water.

For hydration of Portland cement, an average of 23 % water by weight of cement is required, which chemically combines with water.

For hydration of C₃S component of cement, 24 % of water is required while for hydration of C₂S component of cement, 21 % of water is required.

Gel water-

The water filling the gel pores of cement is gel water.

About 15 % of water by weight of cement is required as gel water.

If enough water is not available for filling up gel pores, than formation of gel itself will cease.

If more than 38 % of water is present in cement paste, the excess water will create undesirable cavities of capillary.

Rate of Hydration of Different Components of Cement

Development of strength of each component of cement

C₃S gains strength in early days. Most of its strength is gained in 28 days itself. At the end of 28 days, hydration of C₃S almost comes to an end.

C₂S starts gaining strength later, after 28 days. In initial stage, the gain of strength of C₂S is only about 15 % of that of C₃S.

C₃A catalyses the hydration reaction. Hence, it is used in quick hardening cement so that hydration of C₃S and C₂S is promoted and strength is attained.

Graph = age vs strength

Rate of hydration of each compound of cement

Graph = age vs fraction hydrated

From the graph, following conclusions can be made:

• C₄AF hydrates to nearly 90 % of its volume present in the initial stage, which is the highest
• C₃A hydrates to around 80 % of volume present in the initial stage
• C₃S hydrates to about 40 % of volume present in the initial stage
• C₂S does not show significant hydration in the initial stage

When water is added to cement, the sequence of components in order of their hydration reaction is as follows:

1. C₃A (Hydrates earliest)
2. C₄AF
3. C₃S
4. C₂S

Heat of Hydration of Cement

The hydration of cement, i.e., the reaction between cement and water is an exothermic reaction and hence considerable amount of heat is liberated. The liberation of heat upon hydration of cement is called heat of hydration.

Definition of Heat of Hydration: Heat of hydration can be defined as the quantity of heat evolved in joules per gram of cement upon complete hydration at a particular temperature.

(Heat of hydration can be proved by the following- Take freshly mixed concrete in a vacuum flask
and note its temperature at different intervals. Note the rise in temperature.)

The heat of hydration liberated during the complete hydration of cement remains constant. The amount of heat liberated during hydration depends on the relative proportions of the major components of cement.

The depth of hydration at 28 days is only 4 μ.

Heat of Hydration of Different Cement Components

Heat of hydration of OPC cement at different time intervals is tabulated below:

Heat of hydration of OPC cement

C₃A > C₃S > C₄AF > C₂S

Importance of Heat of Hydration

In Mass Concreting:

• Control of heat of hydration is particularly important in mass concreting as in concrete dam
• construction.
• The temperature at the inside of the concrete mass is 50 times more than that of the original
• temperature at the time of placing of concrete. it continues to remain so for a prolonged time.
• This temperature rise is due to the fact that the mass concrete cools only from the surfaces
• exposed to the atmosphere.
• The interior parts remain heated as the heat is released during the hydration of cement is
• trapped inside.
• If this heat is not allowed to escape, a rapid increase in the strength of interior concrete will be
• observed.
• Cooling of outer surfaces, while the inner concrete remains heated, induces stresses in
• concrete resulting in shrinkage cracks.
• Hence, the study of the heat of hydration and its control measures is important.

In Cold Weather Concreting:

Heat of hydration is desirable in cold weather concreting when the ambient temperature is too
low to activate the hydration reactions.

Early heat of hydration is attributed to the hydration of C3S. As hydration of C3A is controlled by
the retarders added to the cement for the purpose of preventing the flash setting of cement.

Heat of Hydration	Time
50 %	1 & 3 Days
90 %	6 Months

Total Heat of Hydration in cement depends on the relative quantity of the major compounds of
cement.

Total heat of hydration of cement is independent of the fineness of the cement.

Heat of Hydration Test of Cement

The nature of hydration can be deduced by measuring the heat evolved during the hydration
reaction.

Almost half of the total heat is liberated in the first 3 days.

To know the complete test procedure, check out Heat of Hydration of Cement Test.

Rate of Hydration of Cement

Different compounds are present in cement hydrate at different rates. The heat of hydration
liberated by different compounds varies too.

Heat liberation after the setting of cement is described in the graph below:

Ascending peak A: When water is added to cement, heat is evolved for few minutes. This heat evolution is because of reaction of aluminates and sulphates in the solution. This is represented by ascending peak A.

However, the peak does not last long as gypsum present in cement decreases the solubility of
aluminates.

Descending peak A: Gypsum depresses the solubility of eliminate and hence initial heat evolution decreases very fast.

Ascending peak B:

Ettringite formation in cement results in the ascending peak B after 4-8 hours of hydration. This
heat is liberated by the reaction of C₃S (ettringite).

Fineness of cement affects the rate of hydration of cement; but not the total heat of hydration.

Rate of hydration is the fastest in C₄AF as seen from the above graph. The sequence of Bogue’s
compounds in ascending order of rate of hydration is: C₄AF > C₃A > C₃S > C₂S.

Reaction of C₃S ensures heat evolution and peak can be observed in graph.

Values of Heat of Hydration of Different Components of Cement

Heat of hydration of OPC cement

Ordinary cement liberates 89-90 cal/g heat in 7 days and 90-100 cal/g in 28 days upon hydration.

Significance of Heat of Hydration

As water is added in cement, considerable amount of heat is evolved.

In case of mass concreting, this heat of hydration may rise the temperature by 50 degree Celsius to the original temperature of concrete. The high temperature remains for some time and hence in structures like dam, heat of hydration needs to be considered.

As large heat of hydration is released, a temperature difference is established between outer and inner concrete mass. Hence, temperature stresses are induced which results into cracks in concrete.

Increase and decrease in heat of hydration may result into expansion and contraction of foamed concrete.

The rate of heat evolution of the cement compounds, if their equal amount is considered, is listed below:

1. C₃A (maximum)
2. C₃S
3. C₄AF
4. C₂S

Does Fineness of Cement Affect Rate of Hydration?

As the fineness of cement increases, the rate of hydration also increases.

This is because the finer the cement is, the more surface area its particles have for the hydration reaction to progress.

As the reaction progresses faster if the cement particles are finer, more heat of hydration is liberated in the early stage. However, the total heat of hydration remains the same.

How long Hydration of Cement last?

The hydration reaction is not an instantaneous reaction, it continues infinitely. However, the rate of reaction is faster in earlier stage, but it continues at reduced rate.

Does Unhydrated Particles in Cement Reduce the Strength of Concrete?

As seen above, hydration reaction is a continuous one and therefore not all the particles might be hydrated. So does it affect the strength of concrete? Absolutely no.

The hydrated products formed from hydration of cement remains adhered to the unhydrated particles of cement. Hence, the unhydrated particles will not reduce the strength of concrete if the particles are well compacted.

When low water cement ratio is used, then only the surface particles of cement are hydrated while the core particles remain unhydrated. The unhydrated cement particles work as very fine aggregates.

Hydration of Cement used in Hydraulic Structures

Porous concrete is not recommended for hydraulic structures.

Calcium hydroxide is soluble in water and it leaches out making the concrete porous. Hence, concrete should have less Ca(OH)₂ to prevent the concrete from becoming porous.

Now, C₂S component of cement produces less Ca(OH)₂ and more C-S-H as compared to C₃S upon hydration.

Hence, cement with more C₂S is recommended for hydraulic structures.

Note- Hydration of cement is related to the gain of strength of cement upon the chemical reaction. It is different from setting of cement.

FAQ