AbstractThe hydrothermal reactions in lime‐quartz pastes of varying lime/silica ratios have been studied at temperatures from 90 to 200° in periods not normally exceeding 5 days. The compounds present were identified byX‐ray powder photography. Reaction is seldom complete and mixtures of two or more hydrated calcium silicates are frequent. The use of different samples of lime and quartz gives differing results. A hitherto unreported compound, provisionally called phase F, of possible formula C5S3H2Cement chemical nomenclature will be employed where applicable; C=CaO, S=SiO2, H=H2O. However, the abbreviations CSH(I) and CSH(II) are used to mean ‘calcium silicate hydrate (I)’ and ‘calcium silicate hydrate (II)’, and do not imply monocalcium silicate monohydrate compositions.has been prepared at temperatures near 165°. Thermobalance runs have shown that synthetic xonotlite and tobermorite differ in water content from the naturally‐occu