Calcium sulphate hemihydrate hydration leading to gypsum crystallization

The hydration of calcium sulphate hemihydrate (CaSO4 center dot 0.5H(2)O) leading to the crystallization of gypsum (calcium sulphate dihydrate - CaSO4 center dot 2H(2)O) has been the subject of several investigations over a long period and a vast amount of data is widely distributed throughout in the literature. In this review article an overall picture of the subject is presented. The properties of the two hemihydrates (alpha and beta-), their hydration characteristics, the mechanism of their hydration and the crystal growth of gypsum are discussed. Additives modify the microstructures of the hardened gypsum and reduce its strength. A probable mechanism is discussed. (c) 2007 Elsevier Ltd. All rights reserved.