Innovative approaches in the Portland cement industry, aligned with circular economy principles, offer a promising solution to reduce the environmental impacts. These methods can initially target the architectural elements with lower structural demands, such as urban furniture and paving, before being applied to areas with higher cement usage. Alkali-activated binders (AABs) made from secondary resources present a sustainable alternative to Portland cement (PC), promoting resource recovery, conservation, and a low-carbon economy. Incinerator bottom ash (IBA), traditionally landfilled, has shown potential as a precursor for AABs due to its aluminosilicate content. Repurposing IBA for urban furniture and paving transforms it into a valuable secondary resource. Accordingly, this is the first study to utilize IBA as the sole precursor for urban furniture or paving applications. Research, including state-of-the-art studies and proof of concept developed in this work, demonstrates that IBA-based AABs can produce cast concrete suitable for non-structural urban elements, meeting the technical, environmental, and ecotoxicological standards. Using IBA in AAB formulations not only reduces the reliance on primary raw materials but also contributes to significant energy savings in binder production and lowers greenhouse gas (GHG) emissions, resulting in a reduced carbon footprint. Furthermore, producing concrete from local residual resources, such as IBA, facilitates the reintegration of municipal waste into the production cycle at its point of origin, fostering a sustainable approach to urban development and supporting the circular economy.

Incinerator bottom ash (IBA) is a heterogeneous by-product mainly composed of mineral phases. Despite its variable composition, it is generally classified as a silica-rich secondary raw material, containing significant amounts of aluminium, calcium, and amorphous phases. These characteristics make IBA a potential candidate for use as a supplementary cementitious material (SCM) in blended cement formulations. However, due to its heterogeneity, evaluating the behaviour of different particle size fractions is essential to ensure consistent performance. In this study, the pozzolanic activity of various IBA fractions was assessed using three standard reactivity tests: the modified Chapelle test, the modified R ³ test, and the strength activity index (SAI). The results varied depending on the test applied, but a general trend was observed in which coarser fractions exhibited greater pozzolanic activity than finer ones. The fraction larger than 8 mm consistently showed the best performance across all tests. Additionally, the environmental impact of each fraction was analysed through batch leaching tests to determine the potential release of metal(loid)s. Regarding environmental safety, leaching results confirmed that all particle size fractions met the requirements to be classified as non-hazardous at the end of their life cycle. These findings support the use of IBA, particularly its coarser fractions, as a sustainable and safe SCM in the production of blended cement.

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.