Thermal Transformation Of Kaolinite Research Articles

Crystal‐chemical investigation of the “cubic Al–Si spinel” issued from the thermal transformation of kaolinite and halloysite

AbstractSince its discovery in 1959 as a transient nanocrystalline product of the thermal transformation of aluminum‐rich clays, the so‐called Al–Si spinel has been the object of numerous studies aimed at determining its composition. In this work, samples obtained from fired kaolinite‐ and halloysite‐containing clays were analyzed using X‐ray diffraction, leading to conclude in a single cubic (space group Fd‐3m) form, with an unvarying size of crystallites of 5 nm and a constant cell volume, typical of a transition alumina form. Trial‐and‐error Rietveld fitting procedure revealed a crystal structure consisting in an oxygen close packing and a disordered cation sublattice that differs from the known forms of spinel alumina. Rietveld quantitative phase analysis based on this model was successfully implemented on a series of fired halloysite clays, giving a first quantitative insight on the transformations in the 980–1260°C range.

Fluoride ions as structural probe-ions in 19F MAS NMR studies of cement materials and thermally activated SCMs

The potential of 19F magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy in structural studies of fluoride guest ions in cementitious materials is investigated. This includes detection and structural investigations of fluoride ions incorporated in the model compound boehmite (AlOOH) and in four samples of kaolinite (Al2Si2O5(OH)4) from different sources and experiments, which follow the thermal transformation of kaolinite into metakaolin. Furthermore, the structural environments of fluoride ions incorporated into calcium-silicate-hydrate (C-S-H) phases, either synthesised or originating from hydration of fluoride-mineralised Portland clinkers, are investigated. The favourable NMR properties of 19F provide the basis for a range of double-resonance NMR techniques, such as 27Al{19F} and 29Si{19F} cross-polarisation (CP) and 19F{29Si}19F forth and back CP, which probe internuclear connectivities by way of dipolar couplings. The information gained from these experiments facilitates a reliable assignment of the resonances from the fluoride guest ions in the studied model compounds and calcium-silicate-hydrate phases. It is proposed that fluoride ions may deliberately be introduced into a cement system as a structural probe in model studies that follow the reactivity and structure of specific sites that fluoride substitutes for.

Application of TMA and DTA for study of the crystallization behaviour of SiO2 in the thermal transformation of kaolinite

TMA/DTMA with DTA studies on the thermal changes of kaolinite were performed with special emphasis on the crystallization behaviour of amorphous SiO2. The results of four different physical techniques (semiquantitative XRD, DTA, TMA and DTMA) were clearly in good agreement with the fact that the 1420°C exotherm in DTA is due to silica crystallization.

On the Thermal Transformation of Kaolinite and Halloysite

On the Thermal Transformation of Kaolinite and Halloysite