Expansion of compacted incineration bottom ash (IBA) in water: one-year soaking measurement, mechanism analysis, and mitigation methods

Incineration bottom ash (IBA), accounting for ∼80% of municipal solid waste incineration residues, has attracted increasing interest for reuse as construction aggregate. While previous studies have focused on heavy metal leaching, mechanical performance, and hydrogen-induced expansion related to metallic aluminum, the progressive volumetric instability of IBA associated with sulfate-bearing mineral reactions remains insufficiently understood, despite its enrichment in sulfur, aluminum, and calcium similar to expansive stabilized sulfate-bearing soils. This study therefore systematically investigates the progressive expansion behavior of IBA with different particle size fractions. Experiments were performed on cumulative-size IBA, separate-size IBA, and carbonated cumulative-size IBA before and after 1-year soaking. Results showed that cumulative-size IBA exhibited significant expansion during soaking, particularly for fine fractions, with the 0-0.3 mm group displaying up to 18% uniaxial expansion; mass increase and porosity reduction were more prominent in finer particles. Expansion persisted for 7-10 months, far exceeding geotechnical acceptability. In contrast, coarse separate-size IBA (4.75-10 mm) showed limited expansion (∼1%) due to the absence of finer reactive fractions, while accelerated carbonation nearly eliminated expansion (<0.1%). Microanalyses quantitatively confirmed that formation and growth of ettringite and thaumasite were primarily responsible for the expansion, particularly in the long term. For 0-0.3 mm fraction after soaking, the relative percentages of ettringite rose from 2.9% to 21.8% and thaumasite from 0 to 9.6%. The leaching concentrations of heavy metals decreased after soaking, partly due to adsorption by the formed sulfate hydrates. Accelerated carbonation and particle size control are promising strategies to reduce the expansion risks of IBA when used as construction materials.