Geopolymer solidification of medical-municipal co-incineration fly ash and the selective heavy metal stabilization mechanism

Medical–municipal co-incineration fly ash poses a heavy metal pollution challenge owing to its high CaO and Cl contents and very low Si+Al levels, yet it remains poorly addressed. Here, a water-washing pretreatment combined with geopolymer solidification was developed using natural tuff and bottom ash as Si–Al precursors. Raw materials, solidified products, and process parameters were characterized by XRF, XRD, SEM–EDS, FTIR, Tessier sequential extraction, and response surface methodology, with long-term durability assessed under acid/alkali immersion, high temperature, and freeze–thaw cycling. At the optimal formulation (bottom ash 14.2 wt.%, alkali activator 10.1% as Na₂O, silica modulus 1.65, liquid-to-solid ratio 0.35, curing at 65 °C for 24 h), the fly-ash-free geopolymer reached a 28-day compressive strength of 26.3 MPa, and at fly ash incorporation up to 40 wt.% the 28-day strength decreased to 22.4 MPa with Pb, Zn, Cd, Cr, and Cu leaching remaining below GB 5085.3-2007 limits. The results indicate a selective dual-stabilization mechanism: Pb was immobilized mainly by chemical bonding (residual fraction increase >60%), whereas Cd and Zn were primarily physically encapsulated in gel mesopores. The solidified matrix retained 53.2% of its strength after pH 2 immersion and 79.5% after 60 freeze–thaw cycles. This work extends the selective dual-stabilization mechanism to co-incineration fly ash and tailors a formulation to its high-Cl and high-Pb features. Pore-structure data support the 40 wt.% limit and provide a basis for safe treatment and reuse.