Cross-Scale Investigation of Grinding-Speed-Regulated Mechanical Activation, Energy Efficiency, and Hydration of Spontaneous Combustion Coal Gangue

Mechanical activation is widely used to enhance the reactivity of spontaneous combustion coal gangue (SCG), but its effectiveness is usually evaluated by grinding time or final fineness, while the role of grinding speed remains less understood. This study examined grinding speed as an independent activation variable by integrating particle-size distribution (PSD), specific surface area (SSA), Rosin–Rammler analysis, fractal characterization, grinding-response analysis and energy evaluation. Hydration reactivity was assessed by TG–DTG, XRD–Rietveld analysis and compressive strength. Increasing the speed from 200 to 400 rpm continuously refined SCG particles and increased SSA, whereas the grinding responses of particle-size reduction and surface generation followed different power-law relationships with speed. PSD, RRB and fractal results suggested a transition from impact-dominated breakage at low speed to mixed impact–abrasion behaviour at 300 rpm, followed by increased fine-particle agglomeration at 400 rpm. Although particle refinement continued at 400 rpm, both energy-efficiency indices calculated from the mean measurements were highest at 300 rpm among the three tested speeds. Increasing the speed from 300 to 400 rpm raised the specific grinding energy from 0.088 to 0.175 kWh/kg, while the 28-day compressive strength increased only from 34.0 to 34.8 MPa. S300 also showed low residual CH content and Hc/Mc formation, indicating effective coupling between SCG activation and cement hydration. These results indicate that grinding speed influences not only particle refinement but also activation efficiency, with S300 exhibiting the most favourable balance between hydration enhancement and energy demand among the three tested speeds under the investigated conditions.