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

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 ?- or ?-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 ?-CaSO₄·0.5H₂O than ?-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 ?-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 synchrotron study of the conversion of CaSO₄·0.5H₂O to CaSO₄·2H₂O, and will be of importance to gypsum producers globally.

SUBMITTER: Gurgul SJ 

PROVIDER: S-EPMC6510200 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature