Variations in gypsum and the reactivity of its product of calcination

Five samples of Australian gypsum and one from Thailand were analysed chemically and subjected to comparable degrees of grinding and calcination. During hydration of the products considerable differences were found in the periods of induction and other kinetic parameters. Both differences in nucleation, and in the transport of calcium sulphate from the grains of hemihydrate to the growing crystals of dihydrate, seemed to contribute to the variations in reactivity. The variations did not seem to be related to the content of sodium sulphate, but a connection with that of chloride was indicated. The samples of gypsum varied in their susceptibility to transformation to orthorhombic calcium sulphate.