Study of the system CaO---SiO2---CO2---H2O in relation to scawtite under hydrothermal conditions

Hydrothermal studies showed the location of scawtite in the narrow internal tetrahedronfoshagite-xonotlite-CO₂---H₂O of the system CaO---SiO₂---CO₂---H₂O and its compatibility with the calcium silicate hydrates in the 4-phase assemlages. The phase assemblages in this region involve scawtite, foshagite, xonotlite, tobermorite, hillebrandite, calcite and silica. The carbonation series of the 4-phase assemblages were determined in the sections C₄S₃---CO₂---H₂O (C₄S₃ = 4CaO · 3SiO₂), C₇S₆---CO₂---H₂O and C₆S₆---CO₂---H₂O at 473 K and saturated steam pressure in dependence on the CO₂ content. The carbonating influence of CO₂ takes place in the volume of the CaO---SiO₂---CO₂---H₂O system up to the CO₂ concentration limited by the calcite-silica-H₂O triangle. Above this CO₂ content, carbon dioxide appears in a gaseous form together with gaseous H₂O in the 4-phase assemblage calcite-silica-CO₂H₂O. The instability of scawtite under the CO₂ excess (CO₂/C₇S₆ > 1) is influenced by its structural arrangement. The present channels enable penetration of CO₂ molecules into the structure and formation of carbonated phases.