Cementitious systems based on blends of normal portland cement with different materials are used for the treatment and conditioning of toxic and hazardous wastes. The addition of waste material may interfere with the hydration reaction in the cement matrix and consequently with the setting and with hardening process. Some of the waste materials accelerate the hydration process, others act as set retarders, The progress of hydration can be monitored by heat evolution measurements and the calorimetric results are considered as indicators of the degree of hydration. In this study microcalorimetry was used to evaluate the heat output during the hydration of cement and of blends of cement and blast furnace slag treated with solutions of heavy metals.