Raman spectroscopy of the dehydration process of gypsums

In the present research, dehydration mechanisms of gypsums from different sources and grades are studied by Raman spectroscopy. Two temperature profiles, one aggressive and another at moderate temperatures, are used. It is found that Terra Alba gypsum dehydrates to form hemihydrate (CaSO4·0·5H2O) and an interstitial gypsum phase at 128°C. The interstitial phase formed contains less water than hemihydrate. Anhydrite (CaSO4) is produced at a temperature of 140°C or higher. The commercial rock gypsum studied generates both hemihydrate and anhydrite at 128°C, and its impurities do not dehydrate until the temperature is increased to 500°C. The dehydration process of a synthetic gypsum indicates that it has an interstitial gypsum phase dehydrating in a similar way to the interstitial phase found in the Terra Alba gypsum. For all these gypsums, anhydrite is the only dehydration product at 500°C. These findings are further confirmed by X-ray diffraction and thermogravimetric analyses.