Thaumasite is a hydrated calcium silicon carbonate sulfate with formula, CaSiO_3·CaCO_3·CaSO_4·15H_2O; it has been observed to form in hardened Portland cement as a result of the reaction between calcium silicate hydrate, carbonates and sulfates. This reaction lowers the mechanical strength and prompts the deterioration of concrete, even when manufactured with sulfate-resistant Portland cement. The gaps in the present understanding of the thermodynamics, kinetics and mechanisms of this reaction stand in the way of formulation of effective preventive measures. The purpose of this study was to determine the range of thaumasite stability from thermodynamic calculations for closed hydrated cement systems at 298 K. Thaumasite solubility was measured, its dissociation constant was determined and the phase assemblage for the CaO-SiO_2-CaCO_3-CaSO_4-H_2O closed system at 298K was calculated. The study covered the composition of the nine invariant points in which one of the solid phases is thaumasite. The ion composition ranges in which thaumasite is stable were established. It is concluded from these findings that, from a thermodynamic standpoint, thaumasite can form across a wide range of ion concentrations in hydrated cementitious systems. Ettringite / thaumasite compatibility was studied in a limited area of the CaO-SiO_2-CaCO_3-CaSO_4-Al_2O_3-H_2O system. The results show that ettringite is less stable than thaumasite under carbonate action but is a stable phase in areas of the diagram having a carbonate content lower than found in the areas in which thaumasite, by contrast, can be stable.
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