Hydration of Cement

The Chemical reaction that takes place between cement and water is called as hydration of cement. This reacion is exothermic in nature, due to which considerable amount of heat is released during hydration of cement. This is called as ‘heat of hydration’. The hydration of cement is not a sudden process. This reaction is faster in early period and continues indefinitely at a decreasing rate.

What Happens During Hydration of Cement 

During hydration of cement, C3S and C2S react with water and calcium silicate hydrate (C-S-H) is formed along with calcium hydroxide Ca(OH)2.

2 C3S + 6H → C3S2H3 + 3 Ca(OH)2
2 C2S + 4H → C3S2H3 + Ca(OH)2

Calcium silicate hydrate is one of the most important product of hydration process and it determines the good properties of cement. It can be seen from the above reactions that C3S produces more quantity of calcium hydroxide than C2S.

Calcium hydroxide is not a desirable product in concrete mass as it is soluble in water and gets leached out thereby making the concrete porous, particularly in hydraulic structures, thus decreasing the durability of concrete.

Also Read : Different Types of Grades of Concrete

Calcium hydroxide also reacts with sulphates present in water and soils to form calcium sulphate which further reacts with C3A and causes deterioration of concrete. This process is known as Sulphate Attack. The only advantage of calcium hydroxide is that, being alkaline in nature it maintains a high pH value in concrete which resists the corrosion of reinforcement.

It has been estimated that on an average 23% of water by weight of cement is required for chemical reaction with portland cement compounds. As this 23% of water chemically combines with cement, it is called as bound water.

A certain quantity of water is absorbed by the gel pores. This water is known as gel water. The bound water and gel water are complementary to each other. It has been estimated that 15% water by weight of cement is required to fill up the gel pores.

Therefore a total of 38% of water by weight of cement is required for the complete chemical reaction of cement and occupy the space within gel pores. If water equal to 38% by weight of cement is only used then it can be noticed that the resultant paste will undergo full hydration and no extra water will be available for the formation of undesirable capillary cavities.

Also Read : Different Grades of Cement