II⇔I Transition and Eutectic Phenomena of Calcium Sulfate Anhydride

Abstract

The thermal behaviors of calcium sulfate anhydride at high temperatures above 1000°C, such as II CaSO₄4=t I CaSO₄ transition, CaSO₄⇔ CaO SO₃ decomposition and CaO-CaSO₄ eutectic phenomena, were investigated mainly by thermal analysis (micro-TG-DTA) under atmosphere of nitrogen gas.
Although several investigators had suggested that CaSO₄ CaO + SO. decomposition occurs after the termination of I CaSO₄ I CaSO₄ transition (transition temperature: 1180 °C), the decomposition of CaSO₄ really started around 1O^2-0°C. Therefore, CaSO₄ I CaSO₄ transition happened in the process of CaSO₄ decomposition. The cause was explained that the decomposition pressure of I CaSO₄ and that of I CaSO₄ did not differ so much because both have similar crystal structure and the tetragonal lattice of II CaSO₄ make possible to approach the cubic lattice of I CaSO₄ by anisotropic expansion before starting the transition.
The eutectic phenomena of CaO-CaSO₄ were confirmed by a broad endothermic peak around 1350° on the DTA curve. The eutectic composition was given as 18 wt % CaO and 82 wt % CaSO₄ from the variation in height of the eutectic peak by the addition of CaO to II CaSO₄. The eutectic crystal was also observed on scanning electron microscopy.
The activation energy for the decomposition of CaSO₄ was calculated as 90 kcal/mol from TG curve and that of ground li CaSO₄ was 80 kcal/mol. The activation energy was not so influenced by mechanochemical effect because the structural distortion given to the crystal grain of IL CaSO₄ disappeared rapidly at high temperatures above 1000°C.