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Alkalis are important minor constituents of ordinary Portland cement clinker. Na2O, in particular, is incorporated into the aluminate phase and alters its crystal structure. For this study, well defined Portland cement clinkers consisting of alite and aluminate doped with different amounts of Na2O were used. Isothermal conduction calorimetry of cements made from this clinker revealed that doping with Na2O activates the initial heat evolution of the clinker during the initial hours. It also influences the main reaction. In situ X-ray diffraction for monitoring cement hydration demonstrates that the dissolution of C3A and gypsum is accelerated by the incorporation of Na2O. The formation of crystalline ettringite and monosulphate during the first few hours of hydration is delayed. After 90 days of hydration, most of the C3A has reacted to monosulphate, detectable by differential scanning calorimetry, scanning electron microscopy and 27Al nuclear magnetic resonance. An increase of the Na2O content brought about less ettringite in the hydrated samples and changed the morphology of monosulphate to thinner and smaller hexagonal plates.