Reaction mechanisms and microstructural development of MSWI fly ash in geopolymers enhanced by mechanochemical...

Municipal solid waste incineration (MSWI) fly ash utilization in geopolymers is hindered by its low reactivity despite its high calcium content. This study elucidated the reaction mechanisms of MSWI fly ash under the mechanochemical activation by ball milling with various leaching agents (H₂O, HCl, HNO₃, and NH₄NO₃), focusing on how the combined mechanical and chemical forces alter the mineralogical and microstructural characteristics of MSWI fly ash and govern the geopolymerization reaction. The results showed that HCl-assisted milling (1.0 mol/L, liquid-to-solid ratio 1.0) proved most effective, with a Ca leaching efficiency of 61.57 %. This enhancement was attributed to Cl^- penetration into mineral lattices and H⁺-induced dissolution, which reduced the particle sizes of D₅₀ from 31.7 μm to 5.68 μm and generated abundant microcracks, promoting sustained Ca release. The modified fly ash, when incorporated into 30 % blast furnace slag-based geopolymers, yielded a dense and interconnected fibrillar C-(A)-S-H gel network, contrasting with the porous and discontinuous microstructure of untreated controls. Optimal mechanical performance, characterized by a 28 d compressive strength of 36.56 MPa, was attained with an alkali equivalent of 2 and a water-to-binder ratio of 0.4. Microstructural analyses (XRD, FTIR, NMR, TGA, SEM) revealed that mechanochemical treatment accelerated the depolymerization of silicate or aluminate species, enhanced polycondensation kinetics, and promoted the formation of hydration products such as C-(A)-S-H, Friedel's salt, and Kuzel's salt. This work provides insights into valorizing high-calcium industrial wastes through mechanochemical activation, advancing sustainable construction materials.