Hazardous wastes used as hybrid precursors for geopolymers: Cosolidification/stabilization of MSWI fly ash and Bayer red mud

Municipal solid waste incineration fly ash (MFA) and Bayer red mud (RM) were coprocessed with geopolymer technology. The two wastes were used as precursors to synthesize a geopolymer. The ratio of the two wastes and the amount of alkali used, and their effects on the properties of the geopolymer, were investigated. The me-chanical properties, mineral compositions, and IR spectra indicated that a 56 d compressive strength of 13.11 MPa was achieved for samples made with 40% MFA and 60% RM and the addition of 7% Na2O as the alkali. The alkaline conditions enabled dissolution of the raw materials, a phase change of the aluminosilicate, and a higher degree of polymerization for the gel products. The SEM-EDS and leaching tests showed that gel networks consisting of N-A-S-H and C-(A)-S-H gels immobilized heavy metals by physical wrapping and chemical bonding. A dense matrix limited the release of the toxic elements. The levels of Zn, Ni, Pb, Cu, Cr, and Cd leaching were within the acceptable limits of Chinese and US norms.