Simplified synthesis of silicophosphate materials using an activated metakaolin as a natural source of active silica

In this study, silicophosphate phase was synthesized from an activated metakaolin as a natural source of silica using a simple and new method. The preparation consists to react the activated metakaolin with phosphoric acid at high temperature. The structure of activated metakaolin-H3PO4 mixtures (A-MKP) with various percentages of P2O5 (15-55%wt) was investigated by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM). The XRD results showed the formation of an AlSi2(PO4)3 phase for the A-MKP mixture containing 15 wt% P2O5 and the Si5O(PO4)6 phase for the A-MKP mixtures containing 25 and 35 wt% P2O5. While for the mixture containing 55 wt% P2O5, two phases have been formed; Si5O(PO4)6 and SiP2O7. These results were confirmed by 31P and 29Si MAS-NMR techniques.