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Kinetics of gypsum dehydration at reduced pressure: an energy dispersive X-ray diffraction study

¹ Università di Roma Tor Vergata, Dipartimento di Scienze e Tecnologie Chimiche, Via della Ricerca Scientifica 1, 00133 Roma, Italy
² Sapienza Università di Roma, Dipartimento di Scienze della Terra, P.le A. Moro 5, 00185 Roma, Italy
³ Sapienza Università di Roma, Dipartimento di Chimica, P.le A. Moro 5, 00185 Roma, Italy

^* Corresponding author, e-mail: Marilena.Carbone{at}roma2.infn.it

Gypsum (CaSO₄·2H₂O) dehydration kinetics were investigated through energy dispersive X-ray diffraction (EDXD), under reduced pressure (100 Pa), in the temperature range 313–353 K. The process follows the JMAK (Johnson-Mehl-Avrami Kinetic) model. The fitting procedure of the Arrhenius expression provides an activation barrier of 18(2) kcal/mol. Under these experimental conditions, dehydration proceeds via a single-step conversion path gypsum -anhydrite. Separate experiments of bassanite (CaSO₄·0.5H₂O) dehydration, carried out at similar conditions, indicate, as expected, a faster process -anhydrite being the final product. According to the structural relationships between bassanite and -anhydrite, dehydration should occur via the escape of water molecules along the axis of the channel (c axis) following a one-dimensional behaviour. Therefore, no Avrami model (which implies nucleation and growth of a new phase) can be applied.

Key-words: gypsum, dehydration kinetics, bassanite, -anhydrite.

This article has been cited by other articles:

				P. Ballirano and E. Melis Thermal behaviour and kinetics of dehydration in air of bassanite, calcium sulphate hemihydrate (CaSO4{middle dot}0.5H2O), from X-ray powder diffraction European Journal of Mineralogy, October 1, 2009; 21(5): 985 - 993. [Abstract] [Full Text] [PDF]