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1 | INTRODUCTION Cordierite (2MgO•2Al 2 O 3 •5SiO 2) is one of the most important phases of the MgO-SiO 2-Al 2 O 3 system, having a low coefficient of thermal expansion, excellent thermal shock resistance, low dielectric constant, high resistivity, high chemical resistance, high refractoriness, and high mechanical strength. 1-7 Cordierite-based materials are used in various industrial fields in applications such as refractories, heat insulation, filters, membranes, heating elements, microwave and electromagnetic wave absorbers. 8 There are various methods used to synthesize cordierite, among which solid-state reaction, sol-gel, and glass crystallization stand out. 9 Cordierite synthesis by solid-state reaction is considered to be the most widely used in industry due to its simplicity and low cost. 10 Regarding the formation of cordierite by solid-state reaction, some researchers 11 described this reaction as occurring through two well-defined steps. In the first step, cordierite begins to form at 1100°C, between mullite, enstatite, and cristobalite, and reaches a peak at 1275°C. In the second stage, previously formed cordierite reacts through a differential fusion mechanism with enstatite and cristobalite, forming a liquid at around 1330°C to 1355°C. This liquid is made up of eutectic cordierite-enstatite-silica. With increasing temperature, the liquid begins to react with the remaining

Refractory ceramics were produced from clays and alumina waste. Specimens were shaped by uniaxial pressing, subjected to thermal analysis by dilatometry and heat-treated in a conventional furnace at 1300 and 1400ºC, applying a heating rate of 5ºC/min and a dwell time of 2 and 3 hours at the maximum temperature. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and physicomechanical properties. The dilatometric analysis before heat treatment indicated that the formulation containing the largest amount of fluxing oxides presented the highest linear shrinkage. The XRD analysis revealed that mullite was the major phase and needle shaped crystals typical of mullite obtained from clay minerals were observed by SEM. The increase in firing temperature and dwell time at the maximum temperature improved the physicomechanical properties of the specimens. The thermal expansion coefficient (TEC) in the range of 25 to 1000ºC varied from 6.2 to 6.9 x 10 -6°C-1 .

Clays are very important raw materials in the manufacturing of traditional ceramic products. The present study was intended to study the phase transformations of smectites submitted to thermal treatment in conventional stove, applying slow and fast cycles. We used three clays, two national and one imported, which were beneficiated and characterized by mineralogical analysis. Later, they were submitted to a thermal treatment at 1100, 1200 and 1250°C, with heating rate of 5 and 30°C/min, remaining under the maximum temperature for 60 minutes. We observed the presence of mullite peaks starting at 1100°C for all samples. It was verified that the sample with the highest alumina/silica ratio presented more intense mullite peaks, and the sample with the high MgO content, besides the mullite phase, also formed cordierite.