Phase equilibria in the high-temperature portion of the system CaO-SiO2-H2O were determined by the use of hydrothermal quenching techniques. Three previously unidentified phases, designated X, Y, and Z, having the probable compositions 8CaO · 3SiO2 · 3H2O, 6CaO · 3SiO2 · H2O and 9CaO · 6SiO2 · H2O were found to be stable to temperatures above 800° C. at moderate water pressures. X-ray diffraction patterns, optical properties and infra-red absorption spectra were obtained, and are diagnostic for the various phases.

The equilibrium temperature for the reaction C8S3H3** = α′ Ca2SiO4 (bredigite)+CaO+H2O increases from 820° C. at 2000 psi to 870° C. at 15,000 psi. The curve for C6S3H = α′Ca2SiO4+H2O ranges from 790° C. at 7,500 psi to 810° C. at 15,000 psi; and the reaction C9S6H = rankmite+H2O varies from 807° C. at 2000 psi to 820° C. at 15,000 psi. C3SH2 is in equilibrium with X(C8S3H3)+Ca(OH)2 at 505° C. at 10,000 psi and 520° C. at 15,000 psi. Afwillite decomposes at about 215° C. at 15,000 psi.

X is apparently a calcium analogue of chondrodite (Taylor, personal communication), and a related phase X* formed from compositions near the C2S ratio at low pressures may be another member of the chondrodite-type series. Attempts made to synthesize members of the chondrodite series in the MgO-SiO2-H2O system for comparison were largely unsuccessful. Possible applications of the results to the study of cement materials and mineral associations are discussed.

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