Recycled clay brick powder (RCBP) was used to replace partially cement to prepare composite cementitious materials, and the hydration kinetics and microstructure evolution of the composite cementitious materials were investigated by measuring the types of hydration products, hydration heat and microstructure. The results show that RCBP has little effect on the types of hydration products. However, at the later stage of cement hydration, the pozzolanic reaction occurs between RCBP and Ca(OH) 2 . Meanwhile, RCBP promotes the dispersion and dissolution of cement and provides many nucleation sites for C–S–H gel, increases the effective water-binder ratio and accelerates the heat release of cement hydration, however, the cumulative heat release decreases with the dosage of RCBP. In addition, RCBP can increase porosity at early stage of hydration, but the pozzolanic reaction between RCBP and Ca(OH) 2 at the later stage of hydration would produce more hydration products, filling the pores, refining the pore diameter and improving the pore structure. Furthermore, RCBP affects the formation and microstructure of C–(A)–S–H gel, and promotes the formation of spherical C–(A)–S–H gel.