Physicochemical Characterization and Alkaline Activation of Municipal Solid Waste Incineration ashes: a Multi-Analytical Approach for Material Valorization

The expansion of Waste-to-Energy (WtE) plants generates increasing amounts of solid residues: non-hazardous bottom ash (BA) and hazardous air pollution control fly ash (FA). The upcycling of these side-streams is currently hindered by regulatory, technical and chemical barriers, due to the presence of constituents such as heavy metals, metallic aluminum, and chlorides. This work addresses the above barriers and evaluates the potential of municipal solid waste incineration (MSWI) ashes as low-carbon alkali-activated binders. To this aim, a multi-analytical framework was applied to Norwegian BAs and Italian boiler FA. Quantitative phase analysis (XRPD-QPA) was combined with MATLAB-based mass balance calculations to assess the chemical composition of the amorphous fraction, which constitutes the most reactive part of MSWI ashes. Microstructural and micro-chemical analyses were conducted via SEM-EDS and Digital Image Analysis (ImageJ). Based on the above preliminary analyses, binder mix designs and alkali-activated samples were cast and cured at 20°C, 25°C or 60°C, prior to mechanical testing. Thermodynamic modeling predicted phase assemblages, pH evolution, and carbonation behavior. XRPD-QPA analysis identified the amorphous fraction as the dominant phase, reaching ≈ 83 wt.% in BAs (SiO2 ≈ 58% and Na2O ≈ 17.8%) and ≈ 60 wt.% in FA (CaO ≈ 42.4 % and SiO2 ≈ 23.8 %). Image analysis and EDS mapping classified FA amorphous cenospheres into three compositional classes: Ca-Si, Ca-Al and (Ca-)Fe dominated. Samples formulated with NaAlO2 and NaOH activators suffered from macro-porosity caused by elemental aluminum (Al0) oxidation and H2 gas formation, and efflorescence caused by carbonation of the alkaline pore solution. Incorporating metakaolin and sodium silicate mitigated these issues, producing a dense matrix with a peak compressive strength of 2.47 MPa. Conversely, coarser FA grainsizes limited both workability and strength (≤ 1.0 MPa). Thermodynamic simulations verified that increasing the amount of Na in the alkaline activator used for BA prevents amorphous silica precipitation, while FA modeling revealed the co-precipitation of C-(N-)A-S-H and N-(C-)A-S-H phases. Overall, these findings demonstrate that MSWI residues are viable precursors for sustainable binders, offering a pathway for non-structural applications through further targeted mix optimizations.