The relation between the chemical composition of calcium–silicate–hydrate (C–S–H) phases and its influence on the compressive strength of cementitious materials is investigated by a new approach where prisms of C–S–H phases have been synthesized directly by the reaction of calcium hydroxide, amorphous SiO₂ and water. The synthesized samples employ molar Ca/Si ratios of 0.83, 1.0, 1.25 and 1.50, and the compressive strengths and age-depending changes of the pastes have been followed for up to three months of hydration, after which the microstructures were characterized. The model pastes feature most characteristics of hydrated cements, including zones of different density but similar chemical composition. The experimental data demonstrates that the compressive strengths of the C–S–H pastes increase for decreasing Ca/Si ratio for all synthesized samples and testing ages. The molar volumes of the C–S–H phases decrease with decreasing Ca/Si ratio, which along with the related higher surface areas may partly explain the differences in strength development.