Hydrothermally synthesized aluminium phosphate cements

The phase compOSitIOn, exothermal reaction energy, hydrothermal-induced phase transition and development of microstructure in aluminium phosphate cements (APCs), formed by the hydrothermal-catalyzed acid-base reaction between alumina as the base reactant and NH₂H₂PO₄-based fertilizer (Poly-N) as the acid reactant, were investigated. Although the phase compositions of APC depended mainly on the species of alumina and the hydrothermal temperature, the acid-base reaction at 100°C led to the formation of amorphous ammonium aluminium orthophosphate (AmAOP) salt complexes as the major reaction product. The phase transition of AmAOP → crystalline (NH₄) ₃Al₂(PO₄)₃ and NH₄Al(PO₄) 2 · OH · 2 · 5H₂O occurred at 200°C. Increasing the temperature to 300°C promoted the phase transformations (NH₄) ₃Al₂(PO₄)₃ → Al₂PO₄ (OH)₃, and NH₄Al(PO₄)₂ · OH · 2 · 5H₂O → AlPO₄: conversion of non-reactive alumina reactant into γ-AlOOH also occurred. A moderate combination of amorphous and crystalline phases in the cement bodies was responsible for the development of strength. An excessive amorphous → crystal phase transition resulted in a loss in strength, because of the in situ growth of crystalline compounds formed in the amorphous salt matrices, which bind the non-reactive alumina particles into a coherent mass.