Characterization of porous calcium hexaluminate ceramics produced from calcined alumina and microspheres of Vaterite (μ-CaCO₃)

The evolution in the microstructure of porous calcium hexaluminate (CaAl₁₂O₁₉ or CA₆) formed in situ is significantly affected by the characteristics of precursor raw materials. In this study, microspheres of Vaterite (μ-CaCO₃) of different average sizes (5, 7 and 15 μm) were mixed with fine α-Al₂O₃ and sintered. During heating (590–753 °C), the Vaterite particles collapsed and produced spherical pores surrounded by regions of high CaO concentration. Different quantities of low-density-and-melting-point calcium aluminates (C₁₂A₇, CA and CA₂) were formed above 1100 °C. Small portions of a liquid phase were observed for samples containing thin Vaterite particles, which resulted in a denser structure comprised of CA₆. The samples prepared with coarse Vaterite significantly expanded and the asymmetric shape of the CA₆ crystals prevented densification and generated highly porous structures. Therefore, the tailoring of Vaterite particles’ characteristics enables the design of physical properties of CA₆ porous structures according to the application aimed.