The thermal dehydration of synthetic gypsum

Abstract

Mass losses vary between 13.7 and 16.5% and heat of dehydration values between 377 and 420 J g −1 for the dehydration of ground and mixed inhomogeneous gypsum samples. As for the dehydration of CaSO 4·2H 2O, the dehydration of synthetic gypsum proceeds via multi-step reactions. Using a heating rate of 5°C min −1, the very slow dehydration of CaSO 4·2H 2O and impurities in the gypsum samples start at temperatures lower than 95°C. The main dehydration of the calcium sulphate dehydrate part of synthetic gypsum occurs between 95 and 170°C (heating rate 5°C min −1) and seems to proceed via a process with an activation energy of 392 ± 100 kJ mol −1 for α-values between 0 and 0.1. For α-values between 0.1 and 0.7, the reaction can be described by a first-order process with autocatalysis with an activation energy value of 100.5 ± 1.2 kJ mol −1. The third part of the reaction (0.7 < α < 1), up to temperatures of 180°C, gives an activation energy value of 96 ± 15 kJ mol −1. Even at temperatures above 250°C, some CaSO 4·O.15H 2O was still observed. The dehydration of calcium hemihydrate seems to proceed via the formation of CaSO 4·0.15H 2O.