Synthesis and Characterization of Calcium Sulfoaluminate Hydrates—Ettringite (AFt) and Monosulfate (AFm)

The goal of the presented work was to find the most favorable conditions for the synthesis and stabilization of chemically pure ettringite and monosulfate. The reaction was carried out by mixing pure tricalcium aluminate (C₃A) and gypsum (CS¯H₂) in an excess amount of water. The impact of hydration time (2-7 days), C₃A:CS¯ molar ratio (1:1-1:3) and water vapor pressure of the selected drying agents (anhydrite-III and supersaturated CaCl₂ solution) on the phase composition of the products was evaluated. After 7 days of hydration, either ettringite or monosulfate was obtained as the main product, depending on the C₃A:CS¯ molar ratio. The synthesis carried out at a C₃A:CS¯ molar ratio of 1:3 produced pure ettringite. In the case of the sample characterized by the ratio of 1:1 (typical of monosulfate), a considerable portion of ettringite (27.9%) was present in the final products along the AFm phase. Therefore, a different synthesis method has to be selected in order to obtain pure monosulfate. The results showed that thermal analysis, X-ray diffractometry and FTIR spectroscopy can be used to distinguish the characteristic features of ettringite and monosulfate.