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In this study, mullite-zirconia composites were prepared by reaction sintering of alumina, kaolinite zircon and colemanite powder. Slip casting method was also employed for production of these composites. Two different compositions were prepared with and without colemanite additive. The main purpose of this work was to determine the optimum conditions for mullite-zirconia synthesis in addition to the effects of starting composition, solid concentration of aqueous suspensions and sintering temperature. The resulting sintered materials were characterized in terms of bulk and relative density, firing shrinkage, XRD and SEM. XRD peaks suggested fully developed mullite and zirconia phases by reaction sintering of zircon, kaolinite, alumina and colemanite phases. Zircon usually dissociates at a temperature higher than 1650 °C. However, it has been shown in this study that zircon dissociates at about 1450 °C due to the presence of colemanite. The results of the study indicate that 90% of relative density was achieved as the solid concentration is increased up to 55% in case for slip-casted specimens with colemanite additive (MZC55 with 55% solid concentration). The microstructure of all composites is composed of irregularly shaped mullite grains and round-shaped zirconia grains which are distributed intragranularly and intergranularly.

Purpose: This study deals with the effects of colemanite (Ca2B6O115H2O) on properties ofslip-cast mullite–zirconia composites prepared via reaction–sintering of kaolinite, aluminaand zircon powders.Design/methodology/approach: Colloidal processing (slip casting) is the routetowards preparing these materials using 45 vol.% aqueous suspensions of a mixture offine powders stabilized with polyacrylate solution as a dispersant.Findings: The influence of powder composition on physical, mineralogical properties, andmicrostructure of these composites after firing at 1450, 1500 and 1550°C are observed.The results show that the density of composites tends to increase with the addition of7 wt.% colemanite. XRD analyses reveal that using colemanite during the synthesis ofmullite-zirconia composites lowers the reaction temperature. All of the composites consistof irregularly shaped mullite and round-shaped zirconia grains, which are distributedhomogenously.Practical implications: Mullite–zirconia composites, owing to their chemical inertnessand good resistance against chemical attack-corrosion by siliceous and metallic melts, theyare employed in the glass industry.Originality/value: Reaction sintering process is considered as a promising technology forpreparing mullite-zirconia composites, because it has some advantages such as low costof traditional raw materials, straightforward production technology and low manufacturingcost.