Direct formation of the -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 H 2 O and SO 4 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 H 2 O and the replacement of four SO -2 4 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 γ-CaSO 4 upon heating. Upon cooling water re-enters into the γ-CaSO 4 structure at around 363 K to form bassanite. This observation, that the dehydration of gypsum directly yields γ-CaSO 4 (anhydrite) without the intermediate formation of hemi-hydrate (bassanite), is further corroborated by the dehydration behavior of bassanite. The second-order SO 4 modes of bassanite observed around 2218, 2136, and 2096 cm -1 were replaced with the bands of γ-CaSO 4 at about 378 K upon heating.