29Si land 27Al MAS NMR study of the thermal transformations of kaolinite from North China

The thermal transform at ions of kaolinite have been studied using Al-27 and Si-29 magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TG). The experimental results show that this sample is a pure kaolinite which contains similar to3% carbonaceous material as impurity. Al-27 and Si-29 MAS NMR spectra show that the microstructural evolution of the thermal transformation of kaolinite at 450-1050degreesC is similar to that of the kaolinite-mullite reaction series previously published by many authors. Si-29 MAS NMR spectra of mullite at 1190 and 1250degreesC display two resonances at similar to-87 and -92 ppm, corresponding to sillimanite-type geometry around Si and the typical Si environment of mullite, respectively. At 1350degreesC, the splitting of (hk0) and (kh0) of mullite occurs, indicating that the primary mullite transforms into orthorhombic mullite. Simultaneously, the resonance at -92 ppm splits into two signals at similar to-90 and -94 ppni. Al-27 MAS NMR spectra of the mullite consist of three signals centred at similar to-4, 45 and 60 ppm, corresponding to octahedral, distorted tetrahedral and tetrahedral Al sites, respectively.