Solubilities of CaO–SiO2–H2O phases at 25°, 55° and 85°C

Results of solubility determinations on naturally-occurring calcium silicate hydrates are reported for okenite, gyrolite, tobermorite and xonotlite: also for phase pure laboratory preparations of tobermorite, afwillite, foshagite and hillebrandite. Samples, typically 100–200 mg, were contained in sealed reactors and dispersed in initially pure, CO2-free water and equilibrated for up to 110 days. The pH, calcium and silicon solubilities are reported at 25°, 55° and 85°C. Steady state solubilities are attained in 7 days at 25°C but more rapidly at 55° and 85°C. Recommended Ksp values are tabulated. Sources of error are discussed: shortage of samples required that the same mineral had to be redispersed sequentially in fresh leachant and this may have affected the surface state of incongruently soluble phases. Natural minerals may be unreliable in the context of solubility determination owing to their large content of non-essential ions, for example Na+, K+ and Mg2+.