Study on synergistic solidification method for municipal solid waste incineration fly ash: Core chemical stabilization and shell physical encapsulation

To address the challenges of low utilization rates, complex hazardous substances (such as heavy metals and dioxins), and safety concerns in the resource utilization of municipal solid waste incineration fly ash (MSWIFA), this study developed a core-shell structure by combining an alkali-activated MSWIFA core with an ultra-high-performance concrete (UHPC) shell. This system establishes a synergistic solidification mechanism through the chemical immobilization and physical encapsulation. The results indicate that the alkali activation forms C-(A)-S-H and N-A-S-H gels, achieving chemical immobilization efficiencies of 89.4 %, 97.4 %, and 81.2 % for Zn, Cr, and Cu, respectively, while failing to fully immobilize Pb and Cd. Encapsulation with UHPC significantly reduced the leaching concentrations of heavy metals, with an inhibition effect on the leaching of heavy metals that can reach up to 99 %, with all elements meeting the prescribed limits. Furthermore, the dioxin content decreased substantially to 13 ng TEQ/kg, representing a 70 % reduction compared to the raw MSWIFA. Additionally, the encapsulated core-shell structure exhibited a single-particle strength of 4.17 MPa and a water absorption rate of only 0.19 %, demonstrating excellent mechanical properties and compactness. The optimized pore structure and dense interfacial transition zone (ITZ) further enhanced the environmental safety of the core-shell structure and its potential for use as an aggregate. This synergistic solidification technology effectively improves the performance and stability of MSWIFA-based aggregates, providing a promising solution for the resource utilization and pollution control of MSWIFA. • The core-shell synergistic solidification method for municipal solid waste incineration fly ash (MSWIFA) is presented. • The synergistic chemical immobilization and physical encapsulation are successfully achieved. • Encapsulation method significantly reduced heavy metal leaching and dioxin levels, achieving a 70 % reduction in total toxic equivalent of dioxin.