New insights into the thermal evolution of aluminophosphate solutions: A complementary XRD and solid state NMR study

The thermal evolution of aluminium phosphate solutions is investigated in this contribution. The study is based on complementary results obtained from standard X-ray diffraction (XRD) analyses accompanied by solid state nuclear magnetic resonance (NMR) investigations. Binders are prepared between 150 and 1400 °C from two aluminium phosphate solutions differing by their P/Al ratio (3 and 3.5). New insights about the presence of amorphous phases, minor compounds and residual phosphoric acid in the binders, not provided by previous XRD study, are afforded by XRD/NMR investigations, leading to a better understanding of the structure of the binders. At high temperature (>600 °C), both solutions (P/Al = 3 and 3.5) lead to the formation of binders composed of cubic aluminium metaphosphate, Al(PO3)3 [A]. However, the presence of residual phosphoric acid in the P/Al = 3.5 solution entails the formation of Al(PO3)3 [A] at lower temperature. Advanced solid state NMR sequences (31P{1H} cross polarisation, 2D 31P RFDR and 2D 31P{27Al} CP-HETCOR) allow for a complete 31P and 27Al NMR signals assignments.