The hydration heat evolution rate and cumulative hydration heat of composite binder containing slag, fly ash or quartz powder and the reaction degrees of slag, fly ash and cement were determined to investigate the hydration heat of slag or fly ash in the composite binder at 25 degrees C, 45 degrees C and 60 degrees C. Results show that a certain amount of hydration heat of slag is observed at 25 degrees C, but the amount of hydration heat of fly ash is very small at 25 degrees C. Elevated temperature promotes the hydration of composite binder and increases the reaction degrees of slag and fly ash. Much more amount of hydration heat of slag or fly ash is determined at elevated temperature. The elevated temperature has greater promoting effect on the reaction of slag than fly ash. There is a good linear relationship between the reaction degree of slag (or fly ash) and the hydration heat of slag (or fly ash). The total hydration heats of per gram of slag and per gram of fly ash are 530 J and 285 J, respectively. The result is acceptable due to the slight deviation between the calculated value and experimental value of cumulative hydration heat of composite binder.