Textural and structural transformations of kaolinites in aqueous solutions at 200°C

We have studied the processes affecting disordered kaolinites during hydrothermal treatment at 200°C, pH=4 and increasing time of reaction. The starting materials include untreated (natural), poorly ordered Georgia kaolinite (KGa-2), and this sample after intense grinding. Chemical analyses of equilibrium solutions show a higher dissolution rate in the case of the ground kaolinite. In both cases, Si contents are higher than Al contents, indicating a non-stoichiometric dissolution. The solid products of the reactions with natural kaolinite show scarce differences with the starting sample: The XRD patterns reveal a slight sharpening of the basal reflections and a moderate increase of the Hinckley index. The solid products of the reactions with ground kaolinite show, on the contrary, remarkable textural and structural differences with the starting material: Most of the observed particles are spherical and the granulometric analyses indicate a notable increase of the mean particle size. The XRD patterns show an increase in both intensity and resolution of the non-basal reflections, which reflect the decrease of translation defects at the same time that the 13̄1 reflection indicates an increase of the W C-type defects. The evolution observed on the IR spectra, at increasing run times, also indicate a notable increase in crystallinity. The formation of spherical aggregates of kaolinite crystals and the increase in structural order indicate that the hydrothermal reactions caused the recrystallization of the ground kaolinite. These results suggest a dissolution–precipitation process, which is notably less developed in the case of natural kaolinite.