The hydration of calcium sulphate hemihydrate was investigated under several conditions of temperature, pH, water/solid ratio and the addition of citric or malic acid. Isothermal conduction calorimetry and scanning electron microscopy showed that introduction of the acids radically changed the mechanism of hemihydrate reactions and the morphology of gypsum crystals, as a result of adsorption processes divided in two stages. The setting times were reduced as the temperature increased from 20 to 40–60°C and the time span of heat liberation was prolonged. The acids acted as setting retarders and as microstructure modifiers, producing compact crystals; such alterations induced higher strength.