A series of cement samples was prepared in the laboratory by grinding Portland cement clinker and granulated blastfurnace cement slag to finenesses of 300 m2/kg and 500m2/kg. Slag contents of 0%, 25%, 50% and 70% were used. The hydration kinetics of the cements were studied by measuring the amount of unreacted slag and alite, portlandite and combined water. The hydration of both slag and alite was found to be independent of their relative proportions in cement. Of the two components, slag was markedly less reactive than alite after any given hydration period. In line with this the portlandite and combined water content decreased as the proportion of slag increased. In the presence of slag, unlike in slag-free cements, the free-lime content started to decline after reaching a maximum, indicating that it was reacting with the slag.