Phase and Microstructural Evolution, and Densification Behaviour of Kaolin Powder Compacts

ABSTRACT

Two types of kaolin clay samples of different origins were studied using X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy and thermogravimetric/differential thermal analysis. Mineralogical and morphological characteristics of die-pressed kaolin samples were investigated. Kaolinite was identified as the major mineral phase; however, other minerals, e.g. illite and quartz were also present as subordinates. The transformation heats during heating were quantified from DTA measurements and phase changes were followed by XRD analyses. The present results indicated a series of phase transformations as the kaolin was fired at elevated temperatures. The formation of mullite was detected by XRD at a temperature as low as 1100°C. Microstructural evolution investigations showed that mullite needles were formed within the powder compacts during sintering. Size and aspect ratio of the mullite grains increased with the increase of firing temperature. The powder compacts showed anisotropic shrinkage after sintering, mainly due to preferred orientation of the kaolin flakes within the powder compacts. The density was found to be increasing with increase of firing temperature until 1300°C.

GRAPHICAL ABSTRACT

Keywords:

• Differential thermal analysis
• Kaolinite
• Microstructure
• Mullite
• Sintering temperature