Heats of hydration and transition of calcium sulfate

Abstract

The heats of solution at 25 C in 2.09 molal HCI of gypsum, hemihydrate, natural anhydrite, and anhydrous CaS04, prepared by heating gypsum at various temperatures, have been determined. From these values the heats evolved in the hydration to gypsum of hemihydrate, and of anhydrite prepared at 1,000 C, were calculated to be 4,100 30 and 3,990 20 cal/mole, r espectively. The heat evolved in the hydration to gypsum of soluble anhydrite prepared by dehydrating gypsum at 75 C is not less than 6,990 cal/mole . In the course of this work X-ray and microscopic studies gave no indication of the existence of more than two forms of anhydrous calcium sulfate: Soluble anhydrite a nd a modification formed rapidly at high temperatures, the latter being identical with natural anhydrite. The heat evolved in the transition of soluble anhydrit e to natural anhydrite is not less than 3,000 cal/mole. I.