Insight into the physicochemical aspects of kaolins with different morphologies

The physicochemical aspects of four soft kaolins were investigated using X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared spectroscopy (FTIR), differential scanning calorimetry-thermogravimetry (DSC-TG) specific surface area and surface zeta potential measurements. The correlation between degree of crystal order, thermal stability, morphology and surface characteristics of the kaolinites were examined and discussed. The endothermic peak temperatures, in particular, showed a clear positive correlation with the calculated crystallinity index of the kaolinites. The physicochemical characteristics of a natural kaolinite rod were considered in greater detail. Rod-like kaolinite had lower degree of order and zeta potential (pH < 3) than the platy kaolinites but was more ordered than expected for halloysite, and did not show a distinct central void. A schematic morphology for the proposed arrangement of curved kaolinite layers making up the rods is presented.

► Obvious correlation exists between crystallinity and morphology of kaolinites.
► Rod-like kaolinite has lower order degree and zeta potential than flake-like one.
► Kaolinite rods possibly originate from the layers wrapping and tube stacking.