Calcium-sodium aluminosilicate hydrate (C-(N)-A-S–H) gels are the primary phases in alkali-activated slag (AAS) systems and govern paste-scale microstructural evolution. Conventional models assume cross-linked and non-cross-linked C-(N)-A-S–H gels with Ca/(Si + Al) > 0.67 and Al/Si < 0.25. However, several studies have reported Al-rich gels (Al/Si > 0.25) in both AAS and in alkali-activated blends with lower calcium content, whose nanostructure and chemistry remain insufficiently resolved. In this study, four amorphous Al-rich C-(N)-A-S–H gels spanning Al/Si = 0.31–0.57 and Ca/Si = 0.56–1.04 were synthesized to enable two orthogonal comparisons: (i) varying Al/Si at constant Ca/Si to isolate Al-for-Si substitution effects, and (ii) varying Ca/Si at constant Al/Si to probe calcium role on network connectivity. Comprehensive characterization revealed the coexistence of local structural environments characteristic of C-(N)-A-S–H/C-A-S–H, (N,C)-A-S–H, and N-A-S–H-like gel domains without the formation of crystalline secondary phases. ²⁹Si NMR revealed a progressive increase in Q⁴(mAl)-rich sites with increasing Al incorporation, while the relative proportion of the C-(N)-A-S–H/C-A-S–H environments decreased with decreasing Ca/Si ratio. ²⁷Al NMR confirmed tetrahedral Al coordination, with chemical shifts varying with Ca/Si ratio, reflecting changes in interlayer charge-balancing. Octahedral Al was detected exclusively in gels with a higher Al/Si at 0.5, indicating the formation of more highly polymerized aluminosilicate environments under high-Al conditions. ²³Na NMR revealed chemical shifts of − 5.81 to − 6.21 ppm, correlating strongly with the Na/Al ratio. Collectively, the findings demonstrate that gels with Al/Si > 0.25 deviate from classical tobermorite-like structures and exhibit increasingly polymerized and structurally heterogeneous aluminosilicate environments.

The growing incineration of municipal solid waste results in hazardous byproducts, particularly municipal solid waste incineration (MSWI) fly ash and air pollution control (APC) residues. The high toxicity of these residues limits their potential for recycling, leading to their direct disposal in landfills. This landfilling poses a significant environmental risk and presents a major challenge in countries with limited availability of land, such as the Netherlands. In this study, the physicochemical properties of Dutch MSWI fly ash and APC residues were evaluated, including, for the first time, an assessment of trace metal concentrations. High concentrations of heavy metals such as Zn, Pb, Cu, and Cd were identified in most MSWI fly ash and APC residues, along with notable concentrations of trace metals like Bi, suggesting new opportunities for resource recovery. The most hazardous residues were characterized by high contents of chloride, sulfate, alkali oxides, or carbonates, along with low calcium content in their chemical composition. These findings provide valuable insights for the targeted treatment and potential recycling of hazardous MSWI fly ash and APC residues currently being landfilled in the Netherlands.