Powder XRD and 31P and 27Al solid state MAS NMR investigations of phase transformations in aluminium phosphate bonded Al2O3–MgAl2O4 refractories

Abstract

This study elucidates the phase development of an aluminium phosphate bonded Al2O3–MgAl2O4 ceramic body by means of powder X-ray diffraction (PXRD) as well as 31P and 27Al solid state MAS NMR spectroscopy. The binder used for this study was crystalline Al(H2PO4)3 in P2O5 concentrations of 3 wt% and 5 wt% of the total ceramic body. Analyses of the binder itself as well as within the refractory system after thermal treatment of up to T = 1500 °C, reveal how Al(H2PO4)3 acts as a network forming component: Within the refractory body, the binder creates a branched three-dimensional network of Q0, Q1 and Q2 structures by dehydration, chemical reactions, polymerisation and polycondensation. The bonding effect is based on surface reactions of acidic phosphoric components and crystalline phosphate structures on ceramic particles. The degree of cross-linking increases, via P–O–Al and P–O–P bonds, with augmenting temperature (T ≤ 600 °C). This network development is crucial for the formation of a property-determining phosphate bond. At temperatures of T ≥ 600 °C the aluminium phosphate phases react with MgAl2O4 to form Mg3(PO4)2.