The hydration characteristics of solid solutions of composition 3CaO·xGeO2·(1 −x)SiO2were investigated at 25°C by isothermal calorimetry. The compositions hydrated were forx= 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Full hydration was achieved for compositions in whichx<0.2. Both the rate of hydration and the total heat evolved for complete hydration vary with composition. Hydrate compositions were determined, and these also show a compositional dependence. The hydration product for tricalcium germanate has a Ca/Ge ratio near 1.5. Although the hydration products of the solid solution are far more fibrous, their morphologies are reminiscent of that of calcium silicate hyd