Crystallization and quantification of crystalline and non-crystalline phases in kaolin-based cordierites

Kaolin is most often used as traditional raw material in ceramic industry. The purpose of the study was to obtain understanding of the structural and chemical variability of cordierite ceramics influenced by chemical and mineralogical properties of six raw kaolins taken from different localities when they are applied in ceramics mixtures with vermiculite and sintered up to 1300 degrees C. The X-ray diffraction and simultaneous thermogravimetric and differential thermal analysis were used to identify and characterize crystalline mineral phases and the course of reactions during the heating. The percentages of the crystalline and non-crystalline phases were newly determined by recalculation of the bulk chemical analyses of kaolins and cordierite ceramics using Chemical Quantitative Mineral Analysis (CQMA) method. Varying amounts of minerals in kaolins: kaolinite from 73.3 to 85.0, muscovite from 4.2 to 9.9, and quartz from 6.0 to 19.5 (mass %) affected amount of cordierite/indialite from 75.2 to 85.1, enstatite from 5.8 to 8.9 (when are calculated as their maximal possible percentages), and non-crystalline phases from 8.8 to 15.1 (mass %) in cordierite ceramics. Regression analysis predicted high relationship between quantity of: (a) kaolinite in kaolins and crystalline cordierite and (b) quartz in kaolins and non-crystalline phases in the ceramics. The migration of potassium from muscovite into the cordierite structure, melting point and crystallization of cordierite/indialite phases and pore size variability in relation to impurity of kaolins are documented and discussed.