Incinerator Bottom Ash as Aggregate: Leaching Against Washing Cycles and Carbonation Ageing, the Water and Salt Burden, and the Uses That Never Qualify

Municipal incinerator bottom ash (IBA) is produced in large, steady quantities and resembles a coarse aggregate, which makes its use in construction attractive. The obstacle is not appearance or grading but leaching: water passing through the ash mobilises metals, metalloids, chloride, and sulfate. This paper models leaching as a function of two treatments available at modest cost — repeated washing, and carbonation ageing in stockpile — for nine stated elements and salts, and compares the results with stated regulatory-style limits for three uses. SIMULATED, NOT MEASURED: all values are computed from the stated model, not measured on a specific ash. Two treatments behave very differently by element. Chloride and sulfate respond strongly to washing; lead, zinc, copper, chromium, and cadmium respond to both washing and carbonation; molybdenum and antimony respond weakly to both and retain high residual floors of 0.95 and 0.68 mg/kg respectively. Antimony is the controlling element in every use considered. A practical treatment of 3 washes and 12 weeks of ageing qualifies the ash for bound road base, and 5 washes and 12 weeks qualifies it for bound non-structural use, in each case with antimony as the nearest limit. The washing that achieves this uses 7.5 m³ of water per tonne over 3 washes (4.2 m³ fresh with counter-current reuse), removes about 12.94 kg of salt per tonne into the wash water, and produces a first-wash liquor of about 2,927 mg/L dissolved salt that requires treatment — the contamination is transferred, not destroyed. Unbound use in residential settings never qualifies at any wash count or ageing period in this model, because the residual floors for molybdenum and antimony lie above the residential limits before any treatment is applied. That negative result is a property of the floors, not of insufficient washing, and it is printed at full size.