Examination of the Release of Heavy Metals from Self-Hardening Slurries with Fly Ash from Municipal Sewage Sludge Incineration, Considering the Character of Its Operation in a Cut-Off Wall

This study aimed to determine the level of immobilization and the mechanism of leaching of heavy metals (Cd, Cr, Cu, Pb and Zn) from self-hardening slurries (SHSs) based on ash from municipal sewage sludge incineration. To mimic the character of the SHS operation in the cut-off wall, the author’s leaching method—a filtration leaching test based on the percolation test—was used. In addition, this paper briefly described the factors affecting the level of heavy metal release from cement composites, as well as the chemical mechanisms and physical processes responsible for this phenomenon. This study showed that SHSs immobilized heavy metals at high levels (≥98.18%), and the aqueous extracts from the samples had a high pH (≥12.9%). Statistically significant correlations were obtained between the concentration of some metals and pH (positive correlation for Cr and Zn), test duration (positive correlation for Cd) and the liquid-to-solid ratio (negative correlation for Cd and Cr). For some of the metals tested, it was possible to determine the release mechanisms: dissolution and wash-out for Cd and Pb, and dissolution for Cr and Zn.