Geopolymerization of MSWI fly ash and coal fly ash for efficient solidification of heavy metals: Insights into stabilization mechanisms and long-term leaching behavior

High levels of heavy metals (HMs) are present in municipal solid waste incineration fly ash (MSWI-FA), which requires proper management before landfilling. The aim of this study is to investigate the feasibility of codisposal of MSWI-FA and coal fly ash (Coal-FA) using geopolymerization technology to solidify heavy metals and to analyze the long-term leaching behavior of heavy metals (HMs). The active silicon, aluminum, and calcium present in MSWI-FA and Coal-FA are essential for the synthesis of geopolymers. The geopolymer with 20 wt % MSWI-FA (M20) achieved the maximum compressive strength of 25.73 MPa, mainly due to the formation of CA-S-H gels. Heavy metals were effectively immobilized and met the limits of GB18598-2019. The compressive strength of M20 was decreased by 1.6% in sulphuric acid-nitric acid leaching solution for 64 days, but decreased by 57.4% in the acetic acid leaching solution. The geopolymer was more stable in the acid rain environment, resulting in less leaching of heavy metals. The bulk diffusion model (BDM) and Finite Element Method (FEM) could satisfactorily describe the leaching behavior of HMs. The results provide valuable guidance for future waste management practices and the development of solidification processes for MSWI-FA.