Abstract: Coal gangue, as a coal−based solid waste, has attracted widespread attention for its resource utilization. In this study, the activity of clayey coal gangue was enhanced through calcination activation and combined with fly ash and slag to study the synergistic enhancement effect of the three materials in cement−based materials. The working properties and mechanical properties of the composite cement mortar were evaluated by using the fluidity test and the compressive/flexural strength test. The activation mechanism of coal gangue, the hydration products and hydration kinetics of composite cement slurry were analyzed by X−ray diffraction (XRD) and heat of hydration. The results showed that with increasing calcination temperature, the activity of coal gangue first increased and then decreased, with the optimal calcination temperature between 700–800 ℃. When fly ash, slag, and coal gangue (calcined at 700 ℃) were combined in a ratio of 5∶2∶3, the fluidity of the cement mortar reached 237 mm, the 28 day compressive strength was 48.96 MPa, and the 28−day activity index reached 104.77%. Specifically, the spherical particles of fly ash mainly played a lubricating role, significantly improving the fluidity of the mortar; coal gangue provided a large amount of active SiO₂, promoting hydration reactions and the formation of hydration products; slag, with high pozzolanic activity and fine particle size, provided more nucleation sites, enhancing the early strength of the mortar. All three materials all play the role of lubricants and nucleation sites to varying degrees, achieving synergistic improvements in fluidity enhancement, hydration reaction promotion, hydration product optimization, and complementary active components, thereby enhancing the performance of cement−based materials. This study provides a new approach and theoretical basis for the application of coal gangue in cement−based materials.