Decomposition and phase transformation mechanism of kaolinite calcined with sodium carbonate

Abstract Thermal decomposition, phase transformation and microstructure change for the mixture of kaolinite and sodium carbonate (Na 2 CO 3 ) calcined at different temperatures were studied through experimental investigation and theoretical analysis. Thermo-gravimetric and differential scanning calorimetric coupled with mass spectrometer (TG-DSC/MS) were utilized to determine the reaction temperature of kaolinite and Na 2 CO 3 . The calcinated products at different temperatures were characterized by X-ray diffraction (XRD), magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FT-IR). The results showed that the solid-state reaction between kaolinite and Na 2 CO 3 mainly occurred within the temperature of 300–850 °C. During the calcining process, kaolinite was decomposed and transformed into meta-kaolin (MK), low-temperature carnegieite (L-phase), nepheline and zeolite in turn, which can be interpreted in terms of the combination change of different oxygen coordination. With the increase of temperature, the [AlO 6 ] octahedron in kaolinite gradually transformed into the [AlO 4 ] tetrahedron in MK by dehydroxylation, and the bridging oxygen between [AlO 4 ] and [SiO 4 ] layers together with the bridging oxygen in [AlO 4 ] layer and [SiO 4 ] layer were broken with the participation of Na 2 CO 3 . Subsequently, the obtained individual [AlO 4 ] and [SiO 4 ] tetrahedrons alternately recombined and formed an anion frame-like structure by sharing the oxygen atom owing to the limitation of Pauling's electrostatic valence rule. In order to further maintain the charge neutrality and structural stability, Na + ions were filled in the anion frame-like structure of L-phase, nepheline and zeolite. This research is of some theoretical significance for the Na 2 CO 3 -assisted thermal activation of kaolinite and kaolinite-based solid wastes.