Comparative study on typical heavy metal immobilization in cement-based and alkali-activated backfill materials

Backfill mining effectively disposes of coal-based solid waste, but traditional cement-based backfill materials face challenges regarding carbon emissions and long-term heavy metal immobilization stability. Alkali-activated systems (AAS) offer a green alternative, yet a systematic comparison of their environmental characteristics and micro-immobilization mechanisms against cement systems remains lacking. Herein, cement-based and AAS backfill materials were prepared using coal gangue, fly ash, and desulfurization gypsum. Exogenous heavy metals (Pb²⁺, Cd²⁺) were introduced to amplify micro-mechanism signals, systematically investigating mechanical evolution, leaching toxicity, chemical speciation, and pH fluctuations. Results indicate that AAS exhibits superior mechanical robustness against heavy metal interference compared to the non-linear fluctuations of the cement system. Furthermore, AAS demonstrated higher 28-day immobilization efficiency, maintaining over 95% for Pb and Cd even under 0.4 wt% exogenous heavy metal loading, whereas the cement system dropped below 90%. After 120 days of long-term curing, Tessier extraction revealed that both systems convert Pb and Cd into stable residual fractions, with AAS further reducing the exchangeable fractions of coexisting metals (Cr, Cu, Zn). Crucially, the cement system maintained a highly alkaline environment (pH > 11.68) throughout the 120-day monitoring period, risking amphoteric Pb re-dissolution, while the AAS pH decreased to 10.25 within 120 days, ensuring optimal thermodynamic stability. Mechanistically, while the cement system relies on sulfate precipitation for Pb (prone to pore boundary enrichment), AAS achieves deep synergistic immobilization via physical encapsulation, chemical precipitation, and isomorphic substitution within its unique 3D K-A-S-H gel network. This study clarified the differences in the environmental characteristics and immobilization mechanisms of the two backfill materials, providing a theoretical basis for green backfill selection and heavy metal risk control.