Effect of P/Al ratio on an acid aluminum phosphate solution: Thermal evolution and hygroscopic behavior

Hygroscopicity is an obstacle to the widespread use of aluminum phosphate binders, but there is still a lack of relevant fundamental research on this topic. Here, the curing behavior, thermal evolution, phase development and hygroscopic behavior of acid aluminum phosphate (AAP) solutions with different P/Al ratios (2.50-4.00) have been systematically investigated. We comprehensively elucidate for the first time the hygroscopic mech-anism of AAP and its derived aluminum phosphate, and establish a dynamic relationship between hygroscopic mechanism, moisture absorption rate, microstructure and phase composition. AAP and its derived aluminum phosphate, namely Al(H2PO4)3, AlH2P3O10, residual H3PO4, [HPO3]n and hydroxyl groups are all hygroscopic. The hygroscopic properties of AAP depend on the phase composition, which is determined by the P/Al ratio and the calcination temperature. Due to the formation of Berlinite and Al8H12(P2O7)9, the reduced formation of AlH2P3O10 and the absence of H3PO4, [HPO3]n and hydroxyl groups, the AAP solution with a P/Al< 3.00 and its derived aluminum phosphate exhibit relatively lower hygroscopicity. Meanwhile, increasing the calcination temperature can also improve the hygroscopicity resistance characteristics. This work complements the basic knowledge of AAP systems, especially those with a P/Al< 3.00. This sets the stage for the fabrication of aluminum phosphate binders and aluminum phosphate-based composites with excellent hygroscopicity resis-tance properties.