The effects of alkaline dosage and Si/Al ratio on the immobilization of heavy metals in municipal solid waste incineration fly ash-based geopolymer

The present research explored the application of geopolymerization for the immobilization and solidification of municipal solid waste incineration (MSWI) fly ash. The influence of alkaline activator dosage and Si/Al molar ratio on the compressive strength and microstructure of MSWI fly ash-based geopolymer was investigated. A geopolymer with the highest strength was identified to occur at an intermediate alkaline activator dosage and Si/Al ratio, and the optimal Na/MSWI fly ash and Si/Al molar ratio was close to 2.8 mol kg−1 and 2.0, respectively. IR spectra showed that higher alkaline activator dosage enhanced the structural disruption of the original aluminosilicate phases and a higher degree of polymerization of the geopolymer networks. At low Si/Al ratio, there was an increasing number of tetrahedral Al incorporating into the silicate backbone. As the Na/MSWI fly ash ratio increased, the microstructure changed from containing large macropores to more mesopores and micropores, indicating that more geopolymers are formed. Furthermore, the pore volume distribution of geopolymers was observed to shift to larger pores as the Si/Al ratio increased, which suggests that the soluble silicon content serves to reduce the amount of geopolymers. Heavy metal leaching was successfully elucidated using the first-order reaction/reaction-diffusion model. Combining the results from the microstructure of samples with the kinetic analysis, the immobilization mechanism of Cr, Cu, and Zn was inferred in this study. The methodologies described could provide a powerful set of tools for the systematic evaluation of element release from geopolymers.