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Factors Influencing the Stability of AFm and AFt in the Ca-Al-S-O-H System at 25 C



Pan Feng, Pan Feng, Jeffrey W. Bullard


The equilibrium phase assemblages that have been predicted in previous studies for the Ca-Al-S-O-H system at 25 C are revisited by calculating boundaries between equilibrium solubility surfaces among gibbsite, hydrogarnet, portlandite, calcium monosulfoaluminate, ettringite, and gypsum. Phase diagrams for equilibrium in aqueous solutions are constructed using strict Gibbs free energy minimization as implemented in GEM-Selektor software coupled to the Nagra-PSI thermodynamic database. The results are compared to previous modeling and experimental studies. The calculations indicate regions of stability of the often-observed calcium monosulfoaluminate phase that have not been predicted by previous models. The influences of alkali ions on the stability of ettringite are also examined and found to significantly shift the regions of phase stability in composition space. At low and moderate concentrations, sodium and potassium have almost the same effect on the stability of ettringite, while higher concentrations of potassium stabilize syngenite.
Journal of the American Ceramic Society


ettringite, cement, sulfate attack, thermodynamics


Feng, P. , Feng, P. and Bullard, J. (2016), Factors Influencing the Stability of AFm and AFt in the Ca-Al-S-O-H System at 25 C, Journal of the American Ceramic Society, [online], (Accessed April 17, 2024)
Created March 31, 2016, Updated October 12, 2021