MECHANISM OF POLYPHOSPHATE GEL FORMATION IN THE NA2O-AL2O3-P2O5 SYSTEM

Aluminum-sodium polyphosphate gels were prepared using aluminum nitrate and sodium polyphosphate solutions. Gel formation occurs when P/Al molar ratio is larger than 2.8. Al-27 and P-31 nuclear magnetic resonance showed that octahedral aluminum complexes with PO3- ligands are formed in the solutions. Complexes with two metaphosphate ligands, Al3+(H2O)4(PO3-)2, are predominant in the gel. The presence of P-OH groups and of strongly bonded water was determined from infrared spectrometry. The presence of these complexes led to the conclusion that the gelification of aluminum-sodium polyphosphate solutions is conditioned by the formation of an Al3+ complexes network which is consolidated by weak bonds such as hydrogen bonds.