Phase composition of bauxite-based refractory castables

Abstract The bauxite-based refractory castables are composed of 90 wt.% well-graded (coarse, medium, and fine) bauxite aggregate, 10 wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) or mullite either preformed or obtained in situ. The phase composition of the prepared castable samples was investigated by using X-ray diffraction (XRD) analysis, scanning electron microscopy, energy dispersive X-ray analysis, differential thermal analysis (DTA) and thermal gravimetric analysis. The XRD patterns indicated the formation of the corundum phase along with mullite phase in all of bauxite-based castable samples. The scanning electron microscopy (SEM) micrographs show densely packed microstructures with an abundant of corundum grains of comparable sizes, rounded and sub-rounded, homogeneously embedded in the matrix. Some needle-shaped mullite crystals are distributed from place to place. The DTA curve of the castable samples showed three endotherms due to the release of free or the physically absorbed water, the dehydroxylation of the chemically bound water and due to some dehydration and dehydroxylation reactions.