Process of Hydration

The process of hydration is mainly the formation of minute crystals of calcium and gels fom the solution of cement and water.The hydration process of portland cement is as follows-

Tricalcium silicate hydrates as follows
3(3CaO.SiO2)+6H2O=3CaO.2SiO2.3H2O  + 3Ca(OH)2
                                          Calcium Silicate          LIme

This can be written as -
3C3S+6H2O= C3S2H3+3Ca(OH)2

Dicalcium silicate hydrates as follows
2(2CaO.SiO2)+4H2O=3CaO.2SiO2.3H2O+Ca(OH)2

This can be written as
2C2S+4H2O=C3S2H3+Ca(OH)2

The reaction of triclcium aluminate (C3A) with water very fast and immediately cause stiffening of cement paste which is termed as flash setting.This can be reduced by mixing gypsum(CaSo4.2H2O)
with cement clinker during grinding.Gypsum and C3A reacts to form calcium sulphoaluminate which eventually form calcium sulphoaluminate which eventually form calcium aluminate(hydrate).
3C3A+6H2O+3CA.3Ca3SO4.3H4O=C3A.H2O
 This retards the action of the starting of initial setting.     

Gypsum not only reacts with C3A.These accelerate the hydration of calcium silicate.Gypsum is added very carefully to cement because excess will cause expansion and disintegration of  cement paste and also slower the initial setting.