Physicochemical and Thermochemical Study of Ettringite

— The paper presents results of a physicochemical study of ettringite, natural calcium hydrosulfoaluminate Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ⋅26H 2 O, from N’Chwaning, Kalahari, South Africa, by X-ray powder diffraction, IR and Raman spectroscopy, thermal analysis, and calorimetry. The thermal transformation of ettringite has been studied using IR and Raman spectroscopic data. The enthalpies of the stepwise dehydration of the mineral were measured on a NETZSCH DSC 204 F1 (Germany) differential scanning calorimeter. The enthalpy of formation of ettringite from elements Δ f H 0 (298.15 K) = −17 525 ± 40 kJ/mol was determined using a CETARAM (France) high-temperature heat-flux Calvet microcalorimeter by the method of melt solution calorimetry. The enthalpy of formation of Fe-dominant analogue of ettringite Ca 6 Fe 2 (SO 4 ) 3 (OH) 12 ⋅26H 2 O was estimated at −16 649 ± 59 kJ/mol.