Thermal analysis of calcium sulfate dihydrate and supposed a and forms of calcium sulfate hemihydrate from 25 to 500 C

Abstract
Thermal studies were carried out on CaSO4 · 2H2O and the supposed α and β forms of CaSO4 · 1/2H2O in the region of 25 to 500 ° C. using differential thermal analysis, thermogravimetry, and differential scanning calorimetry methods. Two large endothermic effects and a smaller exothermic effect were found in the differential thermogram of CaSO4 · 2H2O while a single endothermic as well as an exothermic effect were found in the thermograms of the supposed α and β forms of CaSO4 · 1/2H2O. However, the exotherm of α-CaSO4 · 1/2H2O had lower peak and first-break temperatures than that of the β-form. The peak and first-break temperatures of the endothermie effects of CaSO4 · 2H2O) and α and B-CaSO4 · 1/2H2O and the exothermic effect of α-CaSO)4 · 1/2H2O were pressure dependent, shifting to lower temperatures as the gaseous pressure within the DTA cell was decreased. The noted differences in the thennogrms possibly are due to kinetic factors because of the differences in crystallinity of the supposed α- and β-CaSO4 · 1/2H2O. No differences in the supposed a and β forms of CaSO4 · 1/2H2O were detected by thermogravimetry and differential scanning calorimetry studies. The formulation of a two-step dissociative process in the complete dehydration of CaSO4 · 2H2O was supported by the results of the thermogravimetry study.