Reactivity of Calcium Sulphate Hemihydrate

Abstract

CALCIUM sulphate hemihydrate reacts with water to yield calcium sulphate dihydrate. The rate of this reaction differs considerably from sample to sample, the variations in reactivity being due, at least in part, to conditions prevailing during the dehydration of the parent dihydrate. We have investigated the reactivity of calcium sulphate hemihydrate prepared by calcining various samples of finely powdered reagent grade calcium sulphate dihydrate (44 per cent −100 mesh and +200 mesh, 54 per cent −200 mesh) spread as a thin layer (120 gm. per 500 cm.²) on a tray in an air oven. After calcination the material was exposed to the atmosphere at room temperature to allow absorption of moisture until the composition of hemihydrate was attained¹. The reactivity of the product was assessed from the rate of increase in temperature of mixtures of 100 gm. of hemihydrate with 60 gm. of water, held in a Dewar flask². The mixtures were made at 20° C. Two parameters of the reaction were studied, namely, the induction period defined as the time taken for the rate of increase in temperature to exceed 0.1 deg. C./min., and k the slope of the approximately straight part of the plot against time of log₁₀ΔT/ΔT_f, where δT deg. C. is the increase in temperature and ΔT_f is the increase in temperature corresponding to complete hydration. The constant k is a measure of the self-acceleration of the reaction^1–3. The induction period is inversely related to the reactivity.