Abstract

The thermal transformations of kaolinite have been studied using 27Al and 29Si 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 ~3% carbonaceous material as impurity. 27Al and 29Si MAS NMR spectra show that the microstructural evolution of the thermal transformation of kaolinite at 450–1050°C is similar to that of the kaolinite–mullite reaction series previously published by many authors. 29Si MAS NMR spectra of mullite at 1190 and 1250°C display two resonances at ~–87 and −92 ppm, corresponding to sillimanite-type geometry around Si and the typical Si environment of mullite, respectively. At 1350°C, 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 ~–90 and −94 ppm. 27Al MAS NMR spectra of the mullite consist of three signals centred at ~–4, 45 and 60 ppm, corresponding to octahedral, distorted tetrahedral and tetrahedral Al sites, respectively.

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