The Influence of Gypsum on the Hydration and Properties of Portland Cement Pastes

The effect of sulfate on the rate of hydration was measured with a conduction calorimeter on neat cement pastes. Mortar prisms were used for physical tests. These studies have shown that a proper regulation of the reaction of early hardening is of much greater importance than merely regulating the time of setting. Such regulation not only influences the rate of hydration, but affects the strength and volume-change characteristics of the hardened cement paste. With some cement compositions the gypsum retards the initial hydration and set, while with others it acts as an accelerator. For each cement there is an optimum gypsum content, and calcium oxide, aluminum oxide content all influence this gypsum requirement. At their optimum gypsum contents the cements all showed essentially equal drying shrinkages.