Decomposition and phase transformation mechanism of kaolinite calcined with sodium carbonate

Thermal decomposition, phase transformation and microstructure change for the mixture of kaolinite and sodium carbonate (Na2CO3) 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 Na2CO3. 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 Na2CO3 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 [AlO6] octahedron in kaolinite gradually transformed into the [AlO4] tetrahedron in MK by dehydroxylation, and the bridging oxygen between [AlO4] and [SiO4] layers together with the bridging oxygen in [AlO4] layer and [SiO4] layer were broken with the participation of Na2CO3. Subsequently, the obtained individual [AlO4] and [SiO4] 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 Na2CO3-assisted thermal activation of kaolinite and kaolinite-based solid wastes.