Effect of MgO, Y2O3 and boehmite additives on the sintering behaviour of mullite formed from kaolinite-reactive alumina

Reaction sintering of clay and reactive alumina is an innovative as well as cost effective method of mullite formation. In the present investigation, clay obtained from Neyveli (South India) and reactive alumina (mean particle size of 0.7 μm) were used as starting materials. To improve the densification of mullite, magnesia and yttria (1–5 wt.%) were used as sinter additives to the above materials. Boehmite was partly substituted for reactive alumina (1–7 wt.%). As per the stoichiometric composition the raw materials were mixed, uniaxially pressed and sintered at 1500, 1550 and 1600 °C for 3 h. Among the sintered samples, the samples sintered at 1600 °C exhibited better properties. The 3 wt.% MgO addition showed improved density up to 2.91 g cm−3 with the maximum strength of 125 MPa. Y2O3 addition favoured the development of equiaxed mullite, but did not help to increase the density and strength. The boehmite substitution (5 wt.%) yielded maximum density of 96% theoretical density (3.01 g cm−3) and strength of 120 MPa. Microstructural studies showed that the addition of boehmite resulted in a blend of small to bigger crystals throughout the structure.