Porous Beads of in-situ Calcium Hexaluminate Prepared by Extrusion-Dripping

Abstract: Structures comprised of mixtures of corindon (α-Al₂O₃) and calcium hexaluminate (CaAl₁₂O₁₉ or CA₆) show high refractoriness and chemical resistance. This article addresses the in situ formation of porous alumina-CA₆ beads (approximately 2-3 mm) prepared by an extrusion-dripping process with sodium alginate (SA) as a coagulation agent. The best conditions for a simultaneous dispersion of aqueous mixtures of the precursor raw materials, fine alumina, and calcium carbonate (β-CaCO₃ or calcite), in an SA solution were investigated by Zeta potential and apparent viscosity measurements and turbidity tests. Extrusion-dripped beads of different compositions were consolidated by the ionotropic gelation of SA in a Ca²⁺ solution. A net of thin coagulated-SA filaments restraining solid particles was registered by scanning electron microscopy. Different pore generation mechanisms were observed during heating up to 1500 °C. Variations in the initial Al₂O₃-CaCO₃ ratio yielded a wide range of compositions, microstructures, and physical properties (total porosity, crystalline phase content, and pore size distribution).