Phase and microstructural evolution during sintering of mixture of 75:25 Nigerian kaolin and calcined alumina powder compacts

The microstructure progression of a uniformly mixed 75 wt% of Nigerian sources of kaolinite clay powder and 25 wt% finer grade of calcined alumina powder (d50) of 6 - 8 mμ was investigated in this present work. The hydraulically compact samples of the mixture were heated in the temperature range of 1400 – 1600 oC and their physical mechanical properties were evaluated. It was observed that 1400 and 1500 oC heated samples did not show any major densification and resulted in higher apparent porosity in the range of 26 - 28%. The samples heated at 1600 oC shows improved densification with significant reduction in apparent porosity (< 5.0%). The phase and microstructural evolution during heating of the samples were studied using XRD and SEM/EDAX technique. The results revealed the presence of quartz (SiO2) and mullite (3Al2O3.2SiO2) as major phases and corundum (Al2O3) as minor phase in the 1400 oC heated samples while mullite, corundum as major and quartz as minor phases were identified in the 1600 oC heated samples. The Fe2O3 present in the clay promoted the growth of the size of mullite needles. Due to the development of a largely dense compact microstructure at 1600 oC, the flexural strength improved significantly to 45 MPa from 15 MPa obtained at 1400 – 1500 oC. The aspect ratio of mullite crystals is higher in 1600 oC heated samples compared to 1500 oC heated specim