Tailoring the incineration bottom ash surface through alkaline-treatment for high-volume upcycling in sustainable mortars

High-volume municipal solid waste incinerator bottom ash (IBA) use in construction is hindered by high porosity and reactive aluminum, which degrade performance. This study evaluates alkaline pre-surface treatments (NaOH, Ca(OH)2, and carbide slag) for 100% sand replacement in alkali-activated slag-fly ash mortars. Results showed that alkali-treatment triggers a dissolution–reprecipitation mechanism (pH 12–14) to transform the IBA surface, forming a continuous C–(N)–A–S–H gel coating. Unlike untreated IBA, which suffers from hydrogen-induced poor bonding, treated IBA creates a dense, seamless interfacial transition zone. NaOH-treated IBA mortars (pH 14) achieved a 28-day compressive strength of 38.4 MPa (35.5% increase) and 40% lower capillary absorption, though it poses higher costs and carbon emission. At equivalent treatment pH of 12, Ca(OH)2 outperformed carbide slag and NaOH, achieving a 21% increase in mortar compressive strength relative to untreated IBA. All treated mortars passed US EPA TCLP limits through dual encapsulation.