Use of Municipal Solid Waste Incinerator Fly Ash in Coal Fly Ash-Based Geopolymer Matrix: Mechanical Performance and Heavy Metals Immobilization

Municipal solid waste incineration fly ash (MSWIFA) poses environmental risks due to high chloride and heavy metal contents. This study solidified MSWIFA via geopolymerization with coal fly ash (CFA), using low-temperature calcination and water washing to remove dioxins and chlorides, respectively. All formulations effectively immobilized heavy metals below regulatory limits. Increasing addition of calcined-washed fly ash (CWFA) from 0% to 25% reduced the 28 d compressive strength from 32.5 MPa to 4.8 MPa and fluidity from 185 mm to 128 mm, while increasing the content and modulus of alkali activator both enhanced these properties. The incorporation of moderate Ca(OH)2 promoted the substitution of Na+ by Ca2+ within the gel framework under the combined action of the alkaline activator, thereby favoring the formation of gel network. When the dosage of Ca(OH)2 increased to 7.5%, the 28 d compressive strength increased from 45.75 MPa to 53.33 MPa. However, excessive Ca(OH)2 resulted in more unreacted residues and accelerated carbonation, which occupied active sites and disrupted gel continuity, leading to a slight decrease in strength to 49.85 MPa. This work provided a viable pathway for the utilization of MSWIFA into geopolymer, especially for optimizing Ca regulation to modify compressive strength, fluidity, and heavy metals toxicity.