Activated coal gangue supplementary cementitious materials: mineral phase evolution, mechanical response, and microstructural strengthening mechanisms in low-carbon blended cement systems.

Utilizing coal gangue as a supplementary cementitious material (SCM) provides an effective strategy to reduce clinker consumption, mitigate cement-related CO2 emissions, and promote solid waste valorization. However, the unclear mineralogical evolution and structural strengthening mechanisms of these CaO-regulated coal gangue blended cement systems under thermal activation hinder the high-value utilization of low-carbon blended cement systems. This study investigated the phase reconstruction, structural strengthening, and hydration of gangue systems calcined at 600-1000 °C and raw systems blended with 3-15 wt% CaO calcined at 800-1000 °C. Results show that 900 °C is the optimum activation equilibrium, inducing complete dehydroxylation of kaolinite into amorphous metakaolin. Optimizing the CaO dosage to 12 wt% maximized the reactivity of thermally activated coal gangue, enabling its effective use as a reactive SCM in a blended Portland cement system; at a 30% cement replacement level, the 28 d compressive strength reached 36.5 MPa, exceeding that of the neat cement reference. Notably, thermochemical regulation promoted the intergrowth of AFt-like needle-shaped products and C-(A)-S-H gel-like phases, contributing to the formation of a dense three-dimensional hydration-product network and improved mechanical performance. This work establishes a composition-process-mineralogy-performance technical pathway, providing a theoretical foundation for high-performance, low-carbon cement.