A study was carried out to ascertain the effects of two forms of silica, quartz and silicic acid, the methods of mixing, and autoclaving conditions on the formation of the lime–rich crystalline hydrous calcium silicates designated as alpha–, beta–, and gamma–type hydrates. The alpha–type hydrates were synthesized from mixtures of lime slurry and quartz in compositions of 1.8 to 2.4 CaO/SiO2molar ratios (C/S ratios). This series of compounds, free of all contaminants, showed X–ray patterns containing many or nearly all lines of crystalline Ca(OH)2. The beta‐type hydrates, of which synthetic hillebrandite is a member, were prepared in 1.8 to 2.4 C/S ratios from mixes of CaO and silicic acid dispersed in water. This series of phases or closely related phases may extend in composition from 1.5 to 3.0 C/S and includes minerals and other synthetic solids previously believed to be distinctly separate phases. The gamma‐type hydrates having compositions of about 1.5 to 2.25 C/S were prepared from mixes of quartz and CaO dispersed in water and autoclaved at somewhat higher temperatures than the alpha‐type hydrates. The gamma‐type hydrates contained about 0.3 to 0.4 mol. of water per mol, of compound compared with values of 1.00 to about 1.25 for the other tw