The role of crystallinity in the synthesis of hydroxysodalite from natural kaolins.

This study investigates the influence of kaolinite crystallinity on the direct synthesis of hydroxysodalite from kaolin. Three natural samples, codified KM, KE, and DA, were characterized and treated in 6 N sodium hydroxide with or without thermal activation. The mineralogical and structural evolution were analysed using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermo-gravimetric and differential scanning calorimetry (TG-DSC). The crystallinity of the kaolinite within each raw sample was assessed via the Hinckley Index (HI) and P 0 factor. The resulting crystallinity order is as follows: DA > KM > KE. The conversion into hydroxysodalite is assessed via a conversion ratio (CR) of 55%, 45% and <2% respectively for KE, KM and DA. These conversions ratios inversely correlated the crystallinity order, indicating that lower crystallinity favors the conversion. Comparison with synthesis using the metakaolin from each raw clay, revealed that thermally activated samples produced more crystallized hydroxysodalites highlighting the role of structural organization as a key factor for the conversion. Overall, this study demonstrates that lower crystallinity favors the direct conversion of raw kaolinite into hydroxysodalite.