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

Area of Science:

• Geotechnical Engineering
• Environmental Science
• Materials Science

Background:

• Traditional cement-based backfill for coal waste faces environmental challenges like carbon emissions and heavy metal leaching.
• Alkali-activated systems (AAS) present a promising green alternative, but their comparative environmental performance and immobilization mechanisms require detailed investigation.

Purpose of the Study:

• To systematically compare the environmental characteristics and micro-immobilization mechanisms of cement-based and AAS backfill materials.
• To evaluate the impact of heavy metals (Pb, Cd) on the mechanical properties, leaching behavior, and long-term stability of both backfill types.

Main Methods:

• Preparation of cement-based and AAS backfill materials using coal gangue, fly ash, and desulfurization gypsum.
• Introduction of exogenous heavy metals (Pb2+, Cd2+) to assess micro-mechanism amplification.
• Systematic investigation of mechanical evolution, leaching toxicity, chemical speciation (Tessier extraction), and pH fluctuations over 120 days.

Main Results:

• AAS demonstrated superior mechanical robustness and higher 28-day immobilization efficiency (>95% for Pb, Cd) compared to cement systems, even with heavy metal contamination.
• Both systems converted Pb and Cd to stable residual fractions, but AAS also reduced exchangeable fractions of coexisting metals (Cr, Cu, Zn).
• AAS maintained a stable, less alkaline pH (10.25) promoting thermodynamic stability, unlike the cement system's high pH ( >11.68) which risks Pb re-dissolution.

Conclusions:

• AAS provides a more environmentally sound and stable solution for coal solid waste backfill compared to traditional cement.
• AAS achieves superior heavy metal immobilization through physical encapsulation, chemical precipitation, and isomorphic substitution within its K-A-S-H gel network.
• This study offers a theoretical foundation for selecting green backfill materials and managing heavy metal risks in mining operations.