Dehydration Process of Gypsum

概要

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The effect of atmosphere on the dehydration of gypsum was investigated by means of differential thermal analysis and X-ray diffraction. Two endothermic peaks were observed in the differential thermogram of gypsum under relatively high water vapor pressure. However, the two peaks shifted to lower temperature and overlapped completely with a decrease in water vapor pressure. On the other hand, only a peak was observed in that of β-hemihydrate, and the peak temperature became much lower than that of gypsum with a decrease in water vapor pressure. In addition, it was confirmed by X-ray diffraction that β-hemihydrate was formed during the dehydration of gypsum at 93-C in air, and that β-hemihydrate was not detected at 62°C in vacuo, but β-hemihydrate was formed at lower temperature (e. g. 40°C) even in vacuo. From above results, it was concluded that the dehydration of gypsum was essentially the consecutive reaction, CaSO4-2H2O→β-CaSO4-1/2H2O→III-CaSO4, irrespective of the atmosphere. The phenomena of two stages (in air) or one stage (in vacuo) dehydration of gypsum was due to the difference in the rates of both reactions depending on the temperature and the water vapor pressure.
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