Assessment of heavy metal immobilization in self-hardening slurries with the addition of ashes from municipal solid waste incineration and coal combustion in the context of hydraulic engineering applications

This study aimed to assess the immobilization of heavy metals (Cd, Co, Cr, Cu, Ni, Pb, Zn) in hardened self-hardening slurries (SHS) made with municipal solid waste fly ash (MSWFA) and conventional coal fly ash (CCFA) intended for cut-off wall construction. Five SHS mixtures were prepared using tap water, Portland cement, sodium bentonite, CCFA, and MSWFA. The microstructure and phase composition were analyzed by SEM and XRD. After 28 and 90 days of curing, samples were leached according to PN-EN 12457-4:2006, and metal concentrations in eluates and digested components were determined by flame AAS. The results were compared with regulatory limits for heavy metal content in leachate and soil. CCFA exhibited the highest Cd, Cu, Pb, and Zn contents, while MSWFA showed the lowest total heavy metal content. The eluates displayed an alkaline pH (≈12.5) and high salinity. Pb and Cd releases exceeded non-hazardous waste limits, while other metals remained below these limits. All tested metals demonstrated high immobilization levels (≥99.99%) regardless of recipe or curing time. Dense C-S-H matrix formation with hydrocalumite contributed to metal immobilization. SHS could potentially be a safe and environmentally friendly method for stabilizing and solidifying fly ash in its raw form. Heavy metals were strongly bound within the slurry structure, however, further research is needed.