Low-carbon cementitious material from municipal solid waste incineration bottom ash for street furniture and outdoor paving: A circular economy perspective

This study investigates the feasibility of employing municipal solid waste incineration bottom ash (WBA) as the sole precursor for producing alkali-activated binders (AA-WBA), with the aim of developing low-carbon mortars and concretes for non-structural urban applications within a circular-economy framework. The precursor, originally in the 8-30 mm particle-size fraction reported in previous studies, was milled to obtain material below 125 µm. A series of activation conditions was examined by varying the NaOH concentration, the NaOH-to-sodium-silicate ratio, and the liquid-to-solid ratio, together with three precursor particle-size ranges (≤63 µm, 90-100 µm, and 100-125 µm). The optimal formulation (1:4/0.6/4 M; 90-100 µm) achieved satisfactory mechanical performance in paste form and developed a dense microstructure characterised by the formation of C-(A)-S-H/N-A-S-H gels, as evidenced by TGA, FT-IR, and SEM analyses. As a proof of concept, this binder was used to manufacture a full-scale concrete pedestrian paving element, which exhibited adequate mechanical performance for outdoor pedestrian use at 28 days. Leaching and ecotoxicity tests indicated low metal release and no significant toxic effects, thereby demonstrating the environmental safety of the material and its potential contribution to more sustainable construction systems.