Effect of Calcite Addition on the Mechanical Strength of Corundum-Mullite Ceramics

Efforts to reduce the usage of imported raw materials in the ceramic industry have encouraged usage of local calcite minerals for production of refractory ceramic products. Corundum-mullite ceramics as refractory products were prepared by using clay, alumina and calcite mineral (CaO) via powder pressing method. The effect of CaO addition from 2.9 to 6.5 wt.% (R1, R2 & R3) on the mechanical strengths such as bulk density, apparent porosity, firing shrinkage and compressive strength were systematically studied as well as the reheat shrinkage and crystallinity of mullite ceramic bodies. The results indicated that the sintering process was prevented by the formation of anorthite phase at higher CaO content. The bulk density increased continuously as the CaO content was increased from 2.9 to 6.5 wt.%, while the apparent porosity and water absorption decreased with the increment of CaO content especially at temperature 1400 °C. On the other hand, addition of small amount of CaO have improved compressive strength of the ceramic body due to reduction in porosity and increase of crystalline phase. Addition of CaO content higher than 4.8 wt.%, would result in increment of crystallinity of the body due to formation of anorthite phase. It was observed that the R1 body (2.9 wt.% CaO) has the lowest reheat shrinkage percentage compared to other bodies and has complied with the Japanese Industrial Standard of mullite refractory product which is lower than 0.2% shrinkage percentage at 1300°C in the furnace for 8 hours.