Stability of hemicarbonate under cement paste-like conditions

Hemicarbonate is observed in hydrated cement paste even after prolonged hydration and in the presence of calcium carbonate, although monocarbonate is expected to be thermodynamically more stable. In this study, we demonstrate that hemicarbonate is significantly stabilized by the incorporation of foreign anions in its structure, resulting in the formation of solid solutions. We analyse the hemicarbonate-Friedel's salt and the hemicarbonate-monosulfate solid solutions under cement-like conditions. A combination of X-Ray diffraction in wet and dry conditions, Raman spectroscopy, energy dispersive spectroscopy and thermodynamic modelling allows us to differentiate between observed structure and composition. We argue that in modern cement paste, hemicarbonate is stabilized by the sulfates and/or chloride in the pore solution. We also demonstrate that the basal spacing of the hemicarbonate-Friedel's salt solid solution can be used to calculate its stoichiometry only when measured in wet conditions.