upon mixing cement powder with water ,part of the tricalcium aluminate and gypsum dissolved and react together forming ettringite . the ettringite crystal on the surface of the cement particals are so fine that they cannot bridge the gap between structure .this is the basis of the delay in solidification upon adding gypsum to cement . if we mix cement with water without adding gypsum the tricalcium aluminate immediately reacts with water to form calcium aluminate hydrate . which fills the gap between the cement particals with large crystals and leads to very rapid solidification of the cement slurry