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The thermal transformations processes of kaolinite (K) in kaolinitic clays is known to consist of a dehydroxylation into metakaolin (MK) at ≈600°C, followed by the formation of a spinel type aluminosilicate (SAS) at ≈980°C, and finally the development of mullite (M) at higher temperatures (1200-1300°C). The structural characterization of these materials is generally based on X-ray diffraction (XRD) studies, where the XRD features of K and M are well defined due to their crystalline nature, but as consequence of the low crystallinity of MK and SAS, the precise characterization of these phases is not possible using this technique. In this study the nature of aluminum atoms in the different materials obtained by thermal treatments of a kaolinite is investigated using Al K-XANES and compared with other well-known aluminum containing materials. XRD and SEM characterization were also carried out. The results confirm the potentiality of this synchrotron based techniques for the characterization of natural materials and ceramics especially with low crystallinity. Particularly the mullite aluminums Al K-XANES spectra were not reported before.

Aluminum titanate (Al 2 TiO 5) materials and aluminum titanate-mullite-zirconium titanate (Al 2 TiO 5-3Al 2 O 3 .2SiO 2-ZrTiO 4) composite materials were successfully processed from fine commercial powders and characterized. This was achieved by zircon (ZrSiO 4) addition to stoichiometric alumina-titania mixtures. Zircon addition was the principal processing variable explored. This additive stabilizes the unstable aluminum titanate phase, enhances the sintering process, restricts microcrack development and improves the mechanical properties of the bulk material, but has a slight detrimental effect on its thermal expansion behavior (α app from-1.5 to 2.5 x 10-6 °C-1 in the RT-800 °C range). With a clear microstructure configuration change, all the technological properties are directly (linearly) correlated with zircon proportion in the initial formulation in the range between 5 and 30 wt%. Developed phases were established, relatively dense ceramics were produced, and complex microstructures with multiphasic interlocked grains were identified. Also, an interconnected microcrack matrix was observed with no material integrity loss which explained the low or even negative thermal expansion behaviors observed in the developed materials. This, together with the mechanical behavior detected, encourages structural applications with high thermomechanical solicitations. The triplex composite material presented an excellent thermomechanical behavior and low porosity, 48 MPa flexural strength, low stiffness and high sintering grade with low thermal expansion.

In this work, a simple milling sintering route (pressure less) to process dense zircon ceramics from fine (D 50 0.8 µm) zircon powders is explored. Particularly, the milling time effect (0-120 min) and the maximum sintering temperature (1400-1600 °C) were studied. The sintering grade developed microstructure and Vickers hardness (Hv) were evaluated and correlated. The dissociation of silicate into (monoclinic and tetragonal) zirconia and silica was evaluated by X-ray diffraction followed by the Rietveld method; it was found to be below 10 wt% in all the studied ranges. The sintering was enhanced by the milling pretreatment. No sintering additives were incorporated. Dense zircon (porosity below 1 %) ceramics were obtained by a simple millingsintering route of this high refractory powder at 100-200 °C below the sintering temperature used with conventional processing routes to obtain equivalent final properties. The Vickers hardness reached: 9.0 GPa.

The firing transformations of traditional (clay based) ceramics are of technological and archeological interest, and are usually reported qualitatively or semiquantitatively. These kinds of systems present an important complexity, especially for X-ray diffraction techniques, due to the presence of fully crystalline, low crystalline and amorphous phases. In this article we present the results of a qualitative and quantitative X-ray diffraction Rietveld analysis of the fully crystalline (kaolinite, quartz, cristobalite, feldspars and/or mullite), the low crystalline (metakaolinite and/or spinel type pre-mullite) and glassy phases evolution of a triaxial (clay-quartz-feldspar) ceramic fired in a wide temperature range between 900 and 1300 ºC. The employed methodology to determine low crystalline and glassy phase abundances is based in a combination of the internal standard method and the use of a nanocrystalline model where the long-range order is lost, respectively. A preliminary sintering characterization was carried out by contraction, density and porosity evolution with the firing temperature. Simultaneous thermo-gravimetric and differential thermal analysis was carried out to elucidate the actual temperature at which the chemical changes occur. Finally, the quantitative analysis based on the Rietveld refinement of the X-ray diffraction patterns was performed. The kaolinite decomposition into metakaolinite was determined quantitatively; the intermediate (980 ºC) spinel type alumino-silicate formation was also quantified; the incongruent fusion of the potash feldspar was observed and quantified together with the final mullitization and the amorphous (glassy) phase formation.The methodology used to analyze the X-ray diffraction patterns proved to be suitable to evaluate quantitatively the thermal transformations that occur in a complex system like the triaxial ceramics. The evaluated phases can be easily correlated with the processing variables and materials properties. These correlations can be employed for materials characterization, design and processing control.