In this work, dehydration of CaSO4·2H2O and CaSO4·0.5H2O in stagnant air was studied by thermo-Raman spectroscopy from 25 to 300°C. The composition could be identified directly from the Raman spectra by comparing with the known Raman spectra of CaSO4·2H2O, CaSO4·0.5H2O and CaSO4. Thermo-Raman spectra indicated that dehydration of CaSO4·2H2O was a two step consecutive reaction and CaSO4·0.5H2O was the intermediate in the experimental conditions. The band intensities illustrated the variation in the relative amount of each species. The derivatives of the band intensities showed the maximum rate in dehydration of that species. The shifts in band positions also revealed the transformation.
