Direct formation of the γ-CaSO4 phase in dehydration process of gypsum:: In situ FTIR study

The dehydration mechanism of natural single crystals of gypsum was investigated in the temperature range 300-430 K by in situ infrared (FTIR) spectroscopy. The thermal evolution of the second-order modes of H2O and SO4 groups in gypsum, in the wavenumber range 4850-5450 cm(-1) and 2050-2300 cm(-1) respectively, were used to probe the dehydration and rehydration sequence. A total disappearance of the combination modes of H2O and the replacement of four SO4-2 bands (2245, 2200, 2133, and 2117 cm(-1)) observed at room temperature by three bands (2236, 2163, and 2131 cm(-1)) observed at 390 K indicates the direct formation of gamma-CaSO4 upon heating. Upon cooling water re-enters into the gamma-CaSO4 structure at around 363 K to form bassanite. This observation, that the dehydration of gypsum directly yields gamma-CaSO4 (anhydrite) without the intermediate formation of hemi-hydrate (bassanite), is further corroborated by the dehydration behavior of bassanite. The second-order SO, modes of bassanite observed around 2218, 2136, and 2096 cm(-1) were replaced with the bands of gamma-CaSO4 at about 378 K upon heating.