Mechanical and microstructural properties of pre-treated incinerator bottom ash as fly ash replacement in alkali-activated materials

Abstract The search for sustainable alternatives to ordinary Portland cement has intensified interest in alkali-activated materials (AAMs), yet the supply of conventional precursors like fly ash is declining. This study addresses this challenge by successfully transforming municipal solid waste incinerator bottom ash (IBA), a problematic residue, into a valuable resource through a simple water immersion treatment. The effect of untreated and treated (10–40% FA replacement) on reaction kinetics, strength development, and microstructural evolution was investigated using isothermal calorimetry, MIP, XRD, TG/DTG, FTIR, and SEM–EDS. The 10-day pretreatment effectively removes soluble chlorides (‑88%) and oxidizes residual metallic aluminum (−96.9 wt.%), mitigating the deleterious expansion and alkali consumption that typically hinder IBA’s use. Results show that untreated IBA has higher total porosity (15–30%), compromised reaction kinetics, and reduced 28-day compressive strength (nearly 50% at 40 wt.% replacement). In contrast, water treated IBA (WTIBA) mixtures exhibit superior microstructure, modified aluminosilicate gel composition (lower Si/Ca and higher Si/Al) and maintain superior compressive strengths at similar replacement level. This work demonstrates a promising processing route for upcycling IBA, enabling up to 20% fly ash replacement without compromising 28-day compressive strength in alkali-activated materials.