The thermal dehydration of synthetic gypsum

• Published in: Thermochimica acta Vol. 269; pp. 631 - 638
• Main Authors: Strydom, C.A., Hudson-Lamb, D.L., Potgieter, J.H., Dagg, E.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-12-1995
• Subjects: Dehydration; Synthetic gypsum; Dehydration; Synthetic gypsum
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/0040-6031(95)02521-9

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.