Interaction of metakaolin-phosphoric acid and their structural evolution at high temperature

In the present study metakaolin (MK), prepared by calcination of a natural tunisian kaolin at 750 °C, was mixed with different amounts of phosphoric acid (11-48 wt% P2O5) and heated at various temperatures (250, 500, 750 and 1000 °C) for 4 h. The structure of materials was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (MEB), thermal analysis (DTA/TGA). The XRD results showed the formation of an AlPO4 phase for the MK-H3PO4 mixtures containing 11, 17 and 27 wt% P2O5 independently of the heating temperature. While with 48 wt% of P2O5 two major phases have been formed Al(PO3)3 at 500 °C and AlSi2(PO4)3 at 1000 °C. These results were confirmed by 27Al, 31P and 29Si MAS-NMR techniques.