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Structural evolution during the dehydration of gypsum materials

¹ Department of Chemistry, University College London, 20 Gordon Street, London WC1H OAJ, UK and School of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, UK
² Department of Materials, Imperial College, Prince Consort Road, South Kensington, London SW7 2BP, UK

^* E-mail: s.jacques{at}mail.cryst.bbk.ac.uk

The dehydration of pure and waste gypsums has been examined using in situ synchrotron angle-dispersive X-ray diffraction. Pure gypsum was studied under a number of defined environments; various industrial waste gypsums were also studied under a common standard environment. It is found that the dehydration of gypsum to anhydrite proceeds via the hemihydrate and -anhydrite phases and the interplay and behaviour of these phases has been determined by full structural `Rietveld' refinement. In the study of the pure gypsum system, the hemihydrate structure is shown to be preserved as water is lost. A `zero-water hemihydrate' is observed before refinement in the higher symmetry -anhydrite cell is possible. The waste gypsum materials studied showed significant differences in the temperatures at which key transformation events occurred; these observations raise implications concerning the re-use of by-product gypsum materials. Finally, high temperature data are re-examined in the search for a variation of the anhydrite structure.

KEYWORDS: gypsum, dehydration