Probing the heterogeneous amorphous network of silicoaluminophosphate binders by advanced multidimensional multinuclear SSNMR and DFT modeling

Phosphate-based geopolymers (PBGs) are silicoaluminophosphate gels with promising binding properties, but their heterogeneous amorphous structure remains poorly understood. Here, we apply for the first time a combination of advanced 1D and 2D multinuclear solid-state NMR (SSNMR) experiments, including 31P{1H} CP-HETCOR, 27Al MQMAS and 27Al{1H/31P} D-HMQC, supported by density Functional Theory (DFT) modeling, to probe the local coordination and atomic spatial proximities in PBGs with different Al/P ratios. Our results suggest that the main phosphorus environments in PBGs correspond to hydrated orthophosphate species coordinated to octahedral AlO6 units, in analogy to mineral variscite. Moreover, 27Al-1H correlation spectra reveal the coexistence of multiple AlO6 environments with variable hydration degrees and different Al–OH populations, further supported by DFT models reproducing 27Al and 1H experimental isotropic chemical shifts. Complementary 29Si MAS and CPMAS spectra demonstrated the presence of an amorphous silica gel predominantly composed of Q4 species, with significant amounts of surface silanol-bearing Q3/Q2 moieties. Although the silica-rich and aluminophosphate-rich networks are likely in close spatial proximity, the present results suggests that their connectivity is limited.