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A kinetic and mechanistic study into the transformation of calcium sulfate hemihydrate to dihydrate

Gurgul, S; Seng, G; Williams, G; (2019) A kinetic and mechanistic study into the transformation of calcium sulfate hemihydrate to dihydrate. Journal of Synchrotron Radiation , 26 pp. 744-784. 10.1107/S1600577519001929. Green open access

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Abstract

The conversion of CaSO4·0.5H2O to CaSO4·2H2O is of great importance industrially, being the reaction behind plasterboard production and the setting of medical plasters. A detailed kinetic and mechanistic study of this process was conducted using time-resolved synchrotron X-ray diffraction in this work. The CaSO4·2H2O product is very similar regardless of whether the α- or β-form of CaSO4·0.5H2O is used as the starting material, but the reaction process is very different. The induction time is usually shorter for α-CaSO4·0.5H2O than β-CaSO4·0.5H2O, and a greater conversion percentage is observed with the former (although in neither case does the reaction proceed to 100 % completion). The temperature of the system, widely used in industry as an indirect measure of the extent of the hydration process, is found to be a poor proxy for this, with the maximum temperature reached well before the reaction is complete. The Avrami-Erofe’ev and Gualtieri models could both be fitted to the experimental data, with the fits being substantially closer in the case of α-CaSO4·0.5H2O. The rate of reaction in the Avrami model tends to increase with the amount of gypsum seeds added to accelerate the process, and the importance of nucleation declines. The Gualtieri analysis suggested that the rate of nucleation increases substantially with the amount of seeds added, while there are less distinct changes in the rate of crystal growth. At low seed concentrations (< 0.5 % w/w) the rate of crystal growth is greater than the rate of nucleation, but at concentrations above 0.5 % w/w nucleation is faster. These findings represent the first non-invasive study of the conversion of CaSO4·0.5H2O to CaSO4·2H2O, and will be of importance to gypsum producers globally.

Type: Article
Title: A kinetic and mechanistic study into the transformation of calcium sulfate hemihydrate to dihydrate
Open access status: An open access version is available from UCL Discovery
DOI: 10.1107/S1600577519001929
Publisher version: https://doi.org/10.1107/S1600577519001929
Language: English
Additional information: This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Keywords: gypsum; calcium sulfate, hemihydrate, hydration, conversion
UCL classification: UCL
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > UCL School of Pharmacy
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > UCL School of Pharmacy > Pharmaceutics
URI: https://discovery.ucl.ac.uk/id/eprint/10069539
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