Recycling municipal solid waste incineration fly ash as a functional foaming agent in slag-based geopolymer

Municipal solid waste incineration fly ash (MSWIFA), a hazardous byproduct containing heavy metals, chlorides, and metallic aluminum, presents significant challenges for reuse in construction materials. This study utilizes the reaction between metallic Al in MSWIFA and alkaline activators to develop self-expanding foamed geopolymers. The in-situ generation of hydrogen gas induces pore formation, while simultaneous polymerization of dissolved precursors into C–(A)–S–H and N–A–S–H gels increases paste viscosity, which plays a key role in foam stabilization. Low-viscosity pastes allow gas to escape or pores to coalesce, while high viscosity facilitates bubble merging and collapse, leading to foam instability. By varying the MSWIFA replacement level, NaOH concentration, and alkali activator ratio (NaOH:Na₂SiO₃), the interaction between foaming, polymerization, and pore morphology was systematically investigated. Key properties such as expansion, porosity, compressive strength, and thermal conductivity were evaluated. Microstructural analyses confirmed the formation of Friedel’s salt, calcite, halite, and aluminosilicate gels. All mixtures achieved over 80 % heavy metal immobilization, complying with USEPA leaching limits. These findings demonstrate the feasibility of using MSWIFA as both a precursor and foaming agent in low-carbon geopolymer systems and offer insights into the coupled mechanisms that govern pore formation and structural development.