Abstract: The hydration mechanism and behavior of composite cementitious system with low water-binder ratios (W/B) are different from those of ordinary cementitious system,and the traditional conclusions about cement hydration are no longer appropriate under this condition.The hydration heat of composite cementitious system of cement-fly ash-slag with different low W/B ratios was measured in 7 days,based on which the effects of low W/B ratio and mineral admixture content on its hydration behavior and mechanism were explored by the cement hydration kinetics.TEM and XRD were also conducted to analyze the hydration products difference of composite cementitious system with low W/B ratio.Results show that the early and late hydration exothermic rates of the composite cementitious system do not change significantly when its W/B ratio is 0.2 and the fly ash-slag content is less than 50%,and the compound mineral admixture could slightly promote the cement hydration under this condition.The addition of fly ash and slag inhibits the hydration of composite cementitious system in 10~17h when the W/B ratio is 0.25 or 0.3.The maximum hydration exothermic rate of the cementitious system increases with the decrease of W/B ratio from 0.3 to 0.2,and the hydration process changes from NG-I-D to NG-D,no longer experiencing the phase boundary reaction.With the decrease of W/B ratio,the fibrous hydrated calcium silicate (C-S-H) will transform into spherical structures after hydrating 28 days.