Calcium sulphate dihydrate (CaSO4·2H2O or gypsum) is used widely as building
material because of its excellent fire resistance, aesthetics, and low price. Hemihydrate
occurs in two formations of α- and β-type. Among them β-hemihydrate is mainly used
to produce gypsum plasterboard since the hydration product of the α-hemihydrate is too
brittle to be used as building material /10/. This article addresses the hydration of
hemihydrate since it determines the properties of gypsum and it is influenced strongly
by water and the properties of hemihydrate. The microstructure development of gypsum
during hydration is investigated. The influence of water is studied from its effect on
fresh behavior and void fraction of the gypsum.