DEHYDRATION OF GYPSUM

Studies of the dehydration of gypsum in dry air atmospheres have shown that the hemihydrate of calcium sulfate is not a stable phase in the temperature range 90-140 deg C. The final product of dehydration was always anhydrous. Discontinuities in the weight loss versus time curves for powdered gypsum at a point corresponding to the loss of about 75 percent of the original sample water is evidence that dehydration involves consecutive reactions and not simply the transformation of gypsum to the anhydrous salt. These rate studies also indicated that the activation energy for dehydration of gypsum is considerably less than that associated with the dehydration of the hemihydrate. The polyscrystalline phase that formed when monocrystals of gypsum were dehydrated had hexagonal symmetry and showed preferred orientation relative to the monoclinic parent crystal. the c sub O axis of the dehydration product was aligned approximately perpendicular to the direction in the original single crystal. /Author/