A study of the system CaO-SiO2-H2O is important for an understanding of the chemistry of Portland cement. The literature dealing with compounds in this system is reviewed and the stability conditions for individual hydrates discussed. Tentative phase diagrams for the systems CaO · SiO2-H2O and 2CaO · SiO2-H2O are given from data recorded in the literature and from laboratory experiments carried out at the Geological Museum, Oslo.
Some of the hydrates of the second system gradually give off water with increasing temperature and in this respect resemble certain clay minerals e. g. montmorillonite. Hydrates of the first system, however, show a tendency to lose water in stages.