Spontaneous combustion coal gangue（SC）and non spontaneous combustion coal gangue（NSC）were selected as fine aggregate for cement mortars. The influences of SC and NSC on the phase composition, microstructure of hydration products of cement mortars were observed by SEM and XRD, and the action mechanisms of activity of coal gangue on the microstructure and strength of cement mortar were investigated. Results indicate that the pozzolanic reaction in SC occurs obviously at early age, the new hydration products grossularite, gismondine and andradite will be generated, the early compressive strength of cement mortar is promoted; NSC does not have the influences on the hydration process and microstructure of cement mortar. As fine aggregate, both of the two kinds of coal gangue exist optimal mixture contents. With 20% content（by mass）, the compressive strengths of SC cement mortar and NSC cement mortar are the highest.