Studies on the Densification Process of Kaolin-Quartz Bodies

Abstract

Densification process of the bodies consisting of kaolin and quartz grains was investigated kinematically by means of isothermal shrinkage measurement, and microstructurally by X-ray diffraction method and observation with polarization and scanning electronmicroscopes.
From the results of linear expansion and shrinkage measurements of the specimens, the bodies showed distinct shrinkage of three stages occurred at about 500°, 900° and 1100°C or higher temperatures.
It was inferred that the shrinkage occurred at about 500°C was caused by decomposition of kaolin minerals, and the shrinkage decreased with increase of quartz content of the body, because of the expansion associated with α-β transformation of quartz at 573°C. The shrinkage appeared at about 900°C seemed to occure with diffusion sintering process at metakaolin grain boundary. Apparent activation energy of the shrinkage process was increased with increase of quartz content of the body. The increase might be caused by interruption to the grain boundary diffusion with presence of quartz grains incorporated in the body. The rate constant of the shrinkage at 900°C was also decrease with increase of the quartz content.
The shrinkage observed at about 1100°C might be associated with densification process occurred by viscous flow of spinel type intermediate phase grains with small amount of melt formed from a little impurities contained in kaolin parts. Apparent activation energy and frequency factors for the shrinkage process were not affected by the presence of quartz grains, but addition of the small amounts of quartz tended to promote densification of the bodies, and further addition of quartz decreased markedly rate constant of the shrinkage at 1060°C.
The bodies shrank more rapidly at a higher temperature than 1100°C. The shrinkage also seemed to be due to viscous flow, but it diminished when almost all the pores in the body were closed in the glassy matrix. Further, changes of the constituent minerals and microstructure of the bodies with the densification process were described.