Calcium sulphate hydrates: a review

The evidence for and against various possible calcium sulphate hydrates is reviewed. The hydrates that exist are calcium sulphate dihydrate CaSO4 . 2H(2)O, calcium sulphate hemihydrate CaSO4 . 1/2H(2)O, and three anhydrous phases: soluble calcium sulphate anhydrite, insoluble calcium sulphate anhydrite, and a high temperature calcium sulphate anhydrite phase. The alpha- and beta-hemihydrates, and the corresponding soluble anhydrites, are not true polymorphs as they differ only in crystallite size. Although the evidence is conflicting, it seems likely that under conditions of water high partial pressures another subhydrate, with the probable formula CaSO4 . 3/5H(2)O, also exists.