CO2 mineralization with fly ash for underground carbon-negative backfill: Carbon sequestration and heavy metal immobilization

High-calcium fly ash slurry, characterized by elevated pH and heavy metal ions, poses significant groundwater contamination risks when directly used for underground mine backfilling. The carbon-negative backfilling technology was proposed to synergistically integrate fly ash with CO₂ for underground disposal, aiming to simultaneously achieve alkalinity reduction, carbon sequestration, and environmentally safe treatment. The carbon fixation performance and heavy metal leaching behavior of mineralized fly ash slurry were experimentally investigated in this study, with a focus on identifying the key controlling factors and elucidating the heavy metal ions stabilization mechanisms. Fly ash was confirmed from the experiment to have substantial carbon sequestration potential, and temperature predominantly influenced the carbon sequestration performance among three key parameters of temperature, pressure and reaction time. Optimal performances under high-purity CO₂ were achieved at 75 °C, with a carbon sequestration efficiency of 45.36 % and capacity of 46.26 g/kg. Mineralized fly ash effectively mitigated the alkalinity, with pressure as the primary controlling factor. Under simulated flue gas conditions, pH of fly ash slurry was reduced from 12.2 to 10.3, while leaching of heavy metal ions (Zn, Ni, Cd) was significantly suppressed, minimizing environmental risks. These findings provide a theoretical foundation for advancing carbon-negative backfilling technologies and a feasible solution for fly ash disposal in remote coal-electricity integrated enterprises. • Carbon-negative backfilling couples fly ash disposal with CO₂ mineralization. • FA achieves 46.26 g/kg CO₂ fixation and 45.36 % efficiency at 75 °C. • Mineralized FA slurry showed up to 4.9 pH reduction vs. untreated. • Carbonation effectively suppressed FA heavy metal leaching to meet safety standards.