Crystal Growth of α-Hemihydrate Gypsum by Dehydration of Dihydrate Gypsum

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A microscopic study has been made on the crystal growth of a-calcium sulfate hemihydrate produced by heating gypsum with water in a microautoclave. The α-hemihydrate crystals were shown to be crystallized from aqueous solution of the gypsum. The growth rate of the hemihydrate determined from the microscopic measurement can be expressed by the following equation over the temperature range of 120-140°C. V=Aexp (-B/RT) The values A and B depend upon the crystal size of gypsum, slurry density and heating temperature. The growth rate of the hemihydrate crystals in aqueous solution of glycerine was depressed with an decrease of the solute diffusion. On the other hand, gypsum was considered to be converted topo-chemically to α-hemihydrate in pure glycerine. The formation rate of the α-hemihydrate measured from its combined water content corresponded to the crystal growth rate measured by microscope.
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