Explain Hydration of cement.

The chemical reaction between the cement and water which turns cement into the binding material with strong adhesive property is called hydration of cement.The following are the approximate equations showing the reactions of `C_3S` and `C_2S `with water.

The reaction of pure `C_3A` with water is very fast and this may lead to flash set (Rapid and early loss of workability which cannot be restored). To prevent this flash set, gypsum is added at the time of grinding the cement clinker.

`2C_3A+6H = 2C_3AH_6`

Hydration of `C_3A` in presence of gypsum:
`C_3A+32H + 3CaSO_4 = C_6AS_3H_(32)` (tri-sulphate hydrate - ?ettringite?)
`C_3A+ 18H + CaSO_4 = C_4ASH_(18)` (mono-sulphate hydrate)

Addition of gypsum in cement control the flash set reacti....

The chemical reaction between the cement and water which turns cement into the binding material with strong adhesive property is called hydration of cement.The following are the approximate equations showing the reactions of `C_3S` and `C_2S `with water.

alt="Explain the Hydration of cement."

The reaction of pure `C_3A` with water is very fast and this may lead to flash set (Rapid and early loss of workability which cannot be restored). To prevent this flash set, gypsum is added at the time of grinding the cement clinker.

`2C_3A+6H = 2C_3AH_6`

Hydration of `C_3A` in presence of gypsum:
`C_3A+32H + 3CaSO_4 = C_6AS_3H_(32)` (tri-sulphate hydrate - ?ettringite?)
`C_3A+ 18H + CaSO_4 = C_4ASH_(18)` (mono-sulphate hydrate)

Addition of gypsum in cement control the flash set reaction of C3A. In presence of gypsum it
reacts with 32 molecule of water resulting large volume change. As this reaction takes place
in green stage of concrete, volume change will not be the problem. But if this takes place in
harden stage of concrete due to external sulphate environment or presence of excess of
gypsum in cement, the large volume change cause crack & detoriation in concrete called
sulphate attack.

`C_4AF + 2CH + 10H = C_3AH_6 + C_3FH_6`

The hydration products of `C_4AF` are similar to those of `C_3A`. However, the reaction rate of `C_4AF` is slower than that of `C_3A`.

The quantity of heat developed upon the complete hydration at the given temperature is called heat of hydration. The heat of hydration follows following pattern:

`C_3A > C_3S > C_4AF > C_2S`