Insights into the CaSO4–H2O system: A Raman-spectroscopic study

Even though being subject of natural scientific research for many decades, the system CaSO4–H2O, consisting of the five crystalline phases gypsum, bassanite, and the anhydrites III, II and I, has left many open questions for research. Raman spectroscopy was used because of its structural sensitivity and in situ measurement capability to obtain further insight, studying phase transitions in both, ex situ and in situ experiments. The findings include significant contributions to the completeness and understanding of the Raman spectra of the system. The dehydration path gypsum–bassanite–anhydrite III, was shown to have strong parallels to a physical drying process, thus, depending on many parameters further than only temperature. Raman band width determination was demonstrated to enable the quantitative discrimination of -bassanite and -bassanite as well as the postulated three sub-forms of anhydrite II (AII), all basing on differences in crystallinity. In the latter case, structural variations over burning temperature were elucidated as a combination of reduction of surface area and healing of crystal lattice defects. Based on own and literature data, we propose an only twofold sub-division of AII into reactive ‘disordered AII’ and much less reactive ‘crystalline AII’ with a transition temperature of 650°C ± 50 K.