Sustainable valorization of MSWI residues: Mechanochemical immobilization of heavy metals and use as supplementary cementitious materials

Potentially toxic elements (heavy metals and chlorinated organic compounds) and metallic aluminum (Al⁰) in municipal solid waste incineration (MSWI) residues pose a critical barrier to their utilization as cementitious materials. In this study, the mechanochemical treatment of all types of MSWI residues - individually, together, or with CaO was carried out to immobilize heavy metals, mitigate highly toxic organic compounds, and enhance cementitious properties. The leaching rate of Sb from the original bottom ash (BA), as well as Cr, and Pb from the air pollution control (APC) residues, was reduced by 99.9 %, 57.4 % and 54.4 %, respectively, to a non-hazardous limit after treatment. The highly toxic chlorinated organic compounds in the APC residues were converted to less toxic forms following treatment. Changes in bonds and chemical states can help elucidate the mechanisms of mechanochemical treatment. However, increased metallic Al in the fly ash (FA) after milling deteriorated the mechanical properties of the cement mortar. Contrarily, the treatment of FA with CaO addressed the metallic Al issue and enhanced the strength of the cement mortar up to 47.4 MPa at 28 days that equalled the CEM I reference. The mechanochemical treatment of MSWI residues with CaO enhanced remarkably the compressive strengths of their cement mortars. The mechanochemical treatment of a mixture of BA, FA, and APC in suitable fractions transformed MSWI residues into non-hazardous substances with demonstrated potential as supplementary cementitious materials (SCMs), offering a comprehensive pathway for utilization.