Abstract: In order to improve the activity of coal gangue as a key cementitious component in grouting materials, this study investigated coal gangue calcined at different durations under 800 °C. The coal gangue-based geopolymers were prepared through alkaline activation, and their compressive strength, X-ray diffraction (XRD) patterns, and Fourier transform infrared (FTIR) spectra were analyzed to systematically elucidate the effect of calcination time on the gelling activity of coal gangue. The results indicated that coal gangue exhibited higher gelling activity after 2 hours of calcination but underwent a notable decline at 4 hours, followed by a gradual increase after 6 hours; calcination facilitated the transformation of kaolinite in coal gangue into reactive amorphous metakaolin, whereas prolonged calcination led to the conversion of metakaolin into less active mullite; additionally, the reduction of residual carbon during calcination contributed to the gradual recovery of gelling activity.