Mechanism of pore generation in calcium hexaluminate (CA6) ceramics formed in situ from calcined alumina and calcium carbonate aggregates

Porous structures of calcium hexaluminate (CaAl12O19 or CA6) are resistant to densification, chemical attack and thermal shock and have been widely employed in thermal insulators for high temperatures (above 1400 °C). Despite its technological importance, little is known corcerning the in situ formation of CA6 from the early calcination temperatures (500-900 °C) up to full sintering (above 1000 °C). This study investigated the mechanisms of pores generation in in situ CA6 obtained from a mixture of calcined alumina (α-Al2O3) and chemically precipitated calcium carbonate (CaCO3) aggregates. The evolution of the microstructure was followed by scanning electron microscopy, X-ray diffraction, dilatometric analysis, and measurements of physical properties, in the 500-1500 °C temperature range. Samples of 50-40% total porosity and different degrees of linear expansion were obtained after sintering at 1100-1500 °C, respectively. The crystallization of liquid phases of eutectic composition played a major role in the formation and maintenance of pores after CaCO3 decomposition.