HYDRATION OF CEMENT

On adding water to cement, the silicates and aluminates present in the cement start a chemical reaction and form a spongy gel. The chemical reaction that takes place between cement and water is referred to as hydration of cement. During this process, a large quantity of heat is evolved. The quantity of heat in calories, liberated on complete hydration of cement is called heat of hydration. The different cement compounds hydrate at different rates and liberate different quantities of heat. The quantity of heat liberated depends upon the amount of different constituents in the cement. There are two ways in which the compounds present in the cement may react with water. In the first case, on addition of water, cement compounds dissolve to produce a super saturated solution from which different hydrated products are precipitated. In the second type of reaction the water is hydrolyzed i.e. the water attracts the cement compounds in the solid state converting the compounds into hydrated products.

HYDRATION PRODUCTS

The following are the important products of hydration of cement:

1. Calcium Silicate Hydrate (C-S-H)
2. Calcium Aluminate Hydrates
3. Calcium Hydroxide [Ca(OH)₂]

Calcium Silicate Hydrate (C-S-H): The main products of hydration of C₃S and C₂S with water are calcium silicate hydrate (C-S-H) gel and calcium hydroxide, Ca(OH)₂. Calcium silicate hydrates are the most important products of hydration of cement. It makes up 50 to 60 % of the volume of solids in a completely hydrated cement paste.

C₃S gives a faster rate of reaction accompanied by greater heat evolution which contributes to the early strength of cement. A cement having higher quantity of C₃S content is better for cold weather concreting. Making the approximate assumption that both C₃S and C₂S produce C₃S₂H₃ as the final product of hydration, their equations of hydration can be written as follows:

(i)         For C₃S

2C₃S             +             6H                                 C₃S₂H₃              +                3Ca(OH)₂

(100)                            (24)                                   (75)                                         (49)

(ii)        For C₂S

2C₂S             +             4H                                 C₃S₂H₃              +                3Ca(OH)₂

(100)                            (21)                                   (99)                                         (22)

Figure 1. Development of strength of pure compounds

It can be seen that C₃S produces comparatively lesser quantity of calcium silicate hydrate and more quantity of Ca(OH)₂, than that formed in the hydration of C₂S. Ca(OH)₂ is not a desirable product in the concrete mass, as it is soluble in water and gets leached out making the concrete porous, particularly in hydraulic structures. Under such conditions it is desirable to use cement with higher percentage of C₂S content. C₂S rather hydrates and hardens slowly. It is responsible for the later strength of concrete. It provides less heat of hydration and greater resistance to chemical attack. Figure 1 shows the development of strength of pure compounds. It is found that the ultimate strength for both C₃S and C₂S are nearly the same. Thus, a higher percentage of C₃S results in rapid hardening, higher heat of hydration and an early gain in strength. On the other hand, a higher percentage of C₂S results in slow hardening, less heat of hydration and greater resistance to chemical attack.

Calcium Aluminate Hydrate: The hydration of C₃A leads to the formation of a calcium aluminate system CaO-Al₂O₃-H₂O_. The amount of C₃A in most cement is comparatively small, but its behaviour is very important. The reaction of C₃A with water is very violent and leads to immediate stiffening of paste. The immediate stiffening of paste is called flash-set. To prevent flash-set, 2 to 3% of gypsum is added at the time of clinker grinding. The hydrated C₃A do not contribute to the strength of concrete. As it hydrates very fast, it may contribute a little to the early strength of concrete. On the other hand, their presence is harmful to the durability of concrete particularly where the concrete is likely to be attacked by sulphates. On hydration, C₄AF is believed to form a system of the form CaO-Fe₂O₃-H₂O. This hydrated product also does not contribute anything to the strength. It acts as a flux and accelerates the rate of reaction in the kiln. The hydrates of C₄AF show a comparatively higher resistance to the attack of sulphates than the hydrates of C₃A as shown in figure 2.

Figure 2. Rate of hydration of pure compounds

Calcium Hydroxide: Calcium hydroxide, Ca(OH)₂ is produced during the hydration of C₃S and C₂S. It constitutes about 20 to 25% of the volume of solids in the hydrated phase. The presence of Ca(OH)₂ makes the concrete porous, weak and undurable. Ca(OH)₂ also reacts with sulphates present in water or soil to form calcium sulphate which further reacts with C₃A and causes deterioration of concrete. This is known as sulphate attack.

The effect of Ca(OH)₂ can be reduced by converting it into cementitious product by the use of bending materials like fly ash, silica fume and other such pozzolanic materials.

The only advantage is that Ca(OH)₂ being alkaline in nature maintain pH value around 13 in concrete which resists the corrosion of reinforcements.