Influence of features and firing temperature on the ceramic properties and phase evolution of raw kaolins

The influence of the characteristic features and firing temperature on the ceramic properties of raw kaolin samples were examined studying a wide range of firing temperatures (1000–1500 ºC). The techniques of investigation have been particle size analysis, Transmission Electron Microscopy (TEM), X-ray powder Diffraction (XRD), X-ray Fluorescence analysis (XRF), and Thermal Analysis using Termodilatometry (TD), Thermalgravimetric analysis (TGA) and Differential Thermal Analysis (DTA). Uniaxial pressed cylindric bodies were obtained and fired from 1000 to 1500 °C/2 h. TEM allowed investigate morphological differences and identification of kaolinite and halloysite. The mineralogical analysis indicated that the kaolinite content is high (80–90 wt%). The contents of oxide impurities are relatively low although in a sample is 7.6 wt% on a calcined basis. The characteristic sharp DTA exothermic effect of kaolinite was observed in the range 900–1000 °C. The ceramic properties of the group of kaolin samples has been determined: linear firing shrinkage, water absorption capacity, apparent density and open porosity. Sintering diagrams allowed investigate the progressive decrease of water absorption and the increase of firing shrinkage. In some kaolin samples the water absorption reached zero at 1450–1500 ᵒ C. High sintering temperatures have been observed when kaolinite is present in high contents and the fluxes content is low. The maximum values of apparent density were determined, with a sample with the highest value (2.75 ± 0.10 g/cm 3 ). The open porosity changes from ∼ 34–38 % at 1000 ᵒC up to zero or minimum values (< 3 %) at 1500 ᵒ C. This behaviour is associated to the progressive sintering of the particles and filling of pores by glassy phase originated by the presence of fluxes and the influence of a low particle size. The formation of mullite and cristobalite by firing have been studied by XRD. Mullite has been detected from 1000 to 1100 ᵒC and the crystals developed as increasing firing temperatures. Cristobalite (α-cristobalite) has been identified at 1200–1300 ºC. The presence of an alkaline melt could impede the crystallization of cristobalite. This study presents a comparative research because all these commercial kaolin samples have been examined under the same experimental conditions. Consequently, the results have allowed to provide new data about raw kaolin powders with high kaolinite content in the range 80–90 wt%. • Crystalline phase’s formation and their evolution have been studied. • Glassy phase formation could impede the crystallization of cristobalite. • A comparative research of commercial kaolin samples have been examined.