The kinetics of dehydration of synthetic calcium sulphate dihydrate have been studied between 80" and 152", at partial pressures of water vapour between and 45 mm. The products of reaction are controlled by both temperature and partial pressure. At temperatures greater than 11 0' the dehydration is controlted by a diffusion mechanism, but a t lower temperatures the reaction is complex and both nucleation of product and boundary control are important. Activation energies for some of the processes have been calculated.

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Measurements of surface area and pore size distributions have been used to compare the two forms of calcium sulphate hemihydrate. The a-form, prepared from solution has a low surface area and transitional pores. Dehydration causes a reduction in the volume of the smaller transitional pores and subsequent rehydration causes little change. The &form has a higher surface area and larger pores. Dehydration increases the volume of the smaller transitional pores and again the effect of rehydration is negligible.