Observations on the reaction calcium sulphate hemihydrate + water ↔ gypsum

A study by microscopy was made of the dehydration and rehydration of grains and single crystals of gypsum. The results confirm the view that, although dehydration of gypsum may be topotactic, hydration of calcium sulphate hemihydrate is merely a matter of dissolution of calcium sulphate hemihydrate, diffusion, nucleation (heterogeneous), and growth of crystals of gypsum.    Processes occurring during the dehydration of gypsum are discussed, as are also theories for the hydration of calcium sulphate hemihydrate. An equation accounting for the rate for the entire course of hydration of calcium sulphate hemihydrate is discussed.