Treatment of fine and medium fractions of MSWI bottom ash for use in concrete: A German case study

Incineration bottom ash (IBA) is the main solid residue from municipal solid waste incineration. IBA mostly contains minerals that can be used as secondary construction materials in unbound applications as well as concrete after appropriate treatment. Major challenges, in particular for its utilization in concrete, are residual metal contents, soluble salts, as well as the high porosity of the material. The goal of this study was to investigate the processing of the fine (0-2 mm) and medium (2-8 mm) IBA fractions with respect to their utilization as a partial substitute for binder and aggregates in concrete. Therefore, the IBA was treated in a two-stage process and the material fractions produced were utilized as secondary aggregates and partial cement replacement in concrete paving stones. The processing led to a reduction in residual metal concentrations, e.g. the metallic aluminum was reduced by 67 % and 60 % in the processed fine and medium fractions, respectively. Soluble metals and salts could be reduced to some extent, but remaining salts (up to 1800 mg/l and 2300 mg/l for chloride and sulfate, respectively) hinder the use of the IBA in reinforced concrete. The results demonstrate that IBA has promising potential for use in concrete paving stones, but further optimization is needed to meet requirements such as tensile splitting strength and weathering resistance (the tensile splitting strength was approx. 20 % lower and the frost resistance was about 4 times higher). Overall, partial replacement of binder and aggregates can effectively contribute to reducing the environmental footprint of concrete products.