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% P 2 O 5 ) 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 AlPO 4 phase for the MK-H 3 PO 4 mixtures containing 11, 17 and 27 wt% P 2 O 5 independently of the heating temperature. While with 48 wt% of P 2 O 5 two major phases have been formed Al(PO 3 ) 3 at 500 °C and AlSi 2 (PO 4 ) 3 at 1000 °C. These results were confirmed by 27 Al, 31 P and 29 Si MAS-NMR techniques. [ABSTRACT FROM AUTHOR]