Thermal transformations metakaolin – spinel type aluminosilicate: Al and Si k-XANES characterization

Full text: Since the pioneering work of G.W. Brindley and M. Nakahira it is known that one of the open problems concerning the metakaolin phase, the manner in which it transforms into a spinel-type phase and mullite, and the relation of this spinel-type phase to mullite, because the series of reactions by which kaolinite transforms to mullite is perhaps the most important in the entire field of ceramic technology. The thermal transformations of kaolinite (K) in metakaolin (MK) and the metakaolin in a spinel type aluminosilicate (SAS) were studied by Al and Si K-XANES to correlate the information obtained with this technique with that obtained by conventional X-ray Diffraction, and clarify and assign the structural changes. Simultaneously with XANES technique is possible to observe changes in the electronic structure of the system. For K à MK transformation, by Al K-XANES it was obtained results consistently with reported Aluminium coordination changes occurred in the kaolinite dehidroxylation process. In general, Al K-XANES spectra of aluminum-containing oxides showed three distinguishable peaks: one at 1566.0 eV assignable to tetrahedral AlO₄ and two assigned to AlO₆ characteristic at 1568.0 and 1572.0 eV. In the case of SAS there are several features, particularly there is a shoulder at 1566 eV and two AlO₆-peaks one at 1567.8 and other at 1571.4 eV. No major changes in the Si K-XANES were observed, i.e. the environments of Si it is not primarily affected for thermal transformations, except for a slight variation in the 3p-holes density. This leads to the conclusion that the structural changes are attributable to changes in the Al environment. (author)