Self-Sustaining Calcination of Coal Gangue Coupled with Cement Production: An Industrial-Scale Evaluation of Material Performance and Energy Balance

The accumulation of coal gangue has caused environmental issues, including spontaneous combustion, dust emissions, and leaching of hazardous elements. Using coal gangue as a supplementary cementitious material offers a promising approach to solid-waste valorization and carbon emission reduction in the cement industry. In this study, a coupled process integrating coal gangue calcination with cement production was established and validated on an industrial demonstration line with a designed annual output of approximately 100,000 tons of calcined coal gangue. At 800 °C and a feed rate of 20 t/h, the system operated without external fuel and relied solely on the inherent calorific value of coal gangue. Waste heat recovery fully covered the total electricity consumption. Calcination temperature exhibited a nonmonotonic effect on product performance: carbon content decreased with increasing temperature, whereas the strength activity index reached its maximum of 100% at 800 °C. Considering carbon content, strength activity index, and energy consumption, 800–825 °C was identified as the optimal temperature window for the industrial suspension reactor, which differs from static calcination conditions. A thermodynamic boundary equation was established, revealing that calcite content exerted ~3.24 times greater influence on the calorific value threshold than kaolinite. The self-sustaining boundary covered a broad compositional range of 40–98% kaolinite and 0–10% calcite, corresponding to calorific value thresholds of 527–1056 kcal/kg. At a 40% replacement level, the prepared low-carbon cementitious material achieved a 28-day compressive strength of 47.2 MPa, meeting the 42.5-grade cement requirement. Compared with P.O 42.5 cement, the clinker content was reduced by ~29 percentage points; this corresponded to an energy saving of 23.35 kgce/t, an energy saving rate of 27.8%, and a carbon emission reduction of 9.9%. The main novelty lies in the industrial-scale process integration, on-site energy balance, self-sustaining operating boundary, and validated material performance. This coupled process provides a viable technical pathway for the low-carbon utilization of coal gangue in the cement industry.