Solid solution limit of Fe2O3 in mullite crystals, produced from kaolin by solid state reactions

Porcelain, wall/floor tiles, and roofing tiles are classified as traditional ceramics and are produced from natural raw materials such as clay, quartz, and feldspar. The raw materials are selected by the oxide content which provides the desired properties of the final product. However, impurities such as Fe2O3 and TiO2 may have a significant influence on both physical/mechanical properties and aesthetic appearance. Fe2O3, which is of particular importance, causes black maculation formation in tablewares, in which whiteness is essential. The only way to overcome the black maculation is to corrupt the Fe2O3 crystal structure and to employ Fe2+/3+ cation, making a solid solution with a host crystal (i.e. mullite-3Al2O3·2SiO2). The aim of this study is to identify the solubility limit of Fe2O3 in mullite crystals. With the scope of the study, an illite/kaolinite was utilized by adding an increasing amount of Fe2O3 (e.g. 1-2-4-6-8-10 wt%) into the recipes fired at 1300 °C for 3 h after homogenization. According to the mullite phase content, it is concluded that ~ 5 wt% of Fe2O3 could be dissolved in mullite crystal structure on the basis of the Rietveld refinement of the XRD patterns.