Isothermal conduction calorimetry has been used to monitor the early hydration of Portland-blastfurnace slag (BFS)-blended cements. Portland:BFS composite cements with ordinary Portland cement replacements from 0 to 90 wt% were studied at curing temperatures from 12 degrees to 90 degrees C. Peak II, principally associated with alite (Ca3SiO5) hydration, was accelerated with increasing temperature for all blends. Peak S, associated with BFS hydration, was particularly noticeable at 40 degrees and 60 degrees C. At higher curing temperatures, peak S merged with peak II, indicating thermal activation of BFS. Novel plots of total heat output against percentage replacement show that BFS contributes to the heat of hydration, even at temperatures below its thermal activation.