Phase stabilities and thermodynamic properties of crystalline phases in CaO–SiO2–H2O above 100 °C

Abstract

The current study revises the phase diagram of the CaO–SiO2–H2O system at saturated steam pressure between 100 and 250 °C by combining experimentally observed phase relations, solubility data and thermodynamic restrictions. A new self-consistent thermodynamic dataset for crystalline C-S-H phases is reported. The dataset revises known thermodynamic properties and derives values for phases for which no data have been published up to now. Chemical compositions and molar volumes of the phases were updated based on recent crystal structure refinements. Additionally, thermodynamic properties are deduced for hydroxylellestadite, scawtite and spurrite as they are relevant for autoclaved concrete and metamorphism/metasomatism of rocks. Portlandite dominates together with hillebrandite, xonotlite and low-quartz the equilibrium phase assemblage up to 250 °C. If the formation of the latter three phases is delayed by kinetic reasons, numerous metastable phases can occur.