Studies on the Densification Process of Kaolin-Feldspar Bodies

Abstract

Densification processes of the bodies consisting of kaolin and feldspar were investigated kinematically by means of isothermal shrinkage measurement, and microstructurally by X-ray diffraction method and observation with polarization microscope and scanning electron microscope.Results of thermal shrinkage measurement of the specimens showed three distinct shrinkages occurred at about 500°, 900° and 1100°C or higher temperatures. The shrinkage occurred at about 500°C seemed to be caused by the decomposition of kaolin minerals, so that the shrinkage decreased with the increase of feldspar content of the bodies. The shrinkage observed at about 900°C seemed to occur with diffusion sntering process of metakaolin grain boundary. Apparent activation energy of the shrinkage process was increased with the increase of the feldspar content. The increase might be caused by interruption of the grain boundary diffusion with the presence of feldspar particles incorporated into the bodies. The rate constant of the shrinkage process at 900°C decreased with the increase of the feldspar content. The decrese seemed to be interpreted by the decrease of metakaolin content in the bodies.The shrinkage observed at about 1100°C might be associated with densification processes occurred by viscous flow of spinel type intermediate phase grains with small amount of the melt formed from a little impurities contained in the kaolin and feldspar fine particles. Apparent activation energy for the shrinkage process increased appreciably with the increase of feldspar content. The increase seemed to be caused by the increase of fusion products of the feldspar with rising temperature. The rate constant of the shrinkage process at 1060°C was increased with the increase of feldspar content. The increase seemed to be caused by the same reason mentioned just above.It was inferred that the shrinkage occurred at higher temperatures than 1100°C was also caused by the viscous flow mechanism. The shrinkage diminished when most pores in the body were closed in the glassy matrix. When the bodies were fired at 1500°C, all bodies showed bloating, that is, the bodies became an over firing stage. Further, changes of the constituent minerals and microstructure of the bodies with the densification process were described.