Nanostructure and chemistry of amorphous Al-rich C-(N)-A-S–H-type gels via aqueous precipitation: insights for alkali-activated materials

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. 29 Si NMR revealed a progressive increase in Q 4 (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. 27 Al 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. 23 Na 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.