Microstructure evolution of porous calcium hexaluminate compacts during first heating

Porous calcium hexaluminate (CaAl12O19 or CA6) structures show long-term stability at high temperatures, high refractoriness, low thermal conductivity, and excellent chemical resistance, thus being Taylor-made for thermal insulation. Previous works attained porous structures from in situ formation of CA6 combining CaO and Al2O3 sources in solid-state reactive sintering. Although the approach is time-and-energy-saving regarding the production of large parts of complex shapes, it faces considerable difficulties related to the expansive formation of calcium aluminates. To overcome such drawbacks, pre-formed porous CA6 aggregates improve the system's dimensional stability before sintering. Despite the straightforward processing, few studies have investigated such materials systemically. This article addresses the combination of pre-formed porous CA6 aggregates and organic and inorganic binders for the production of porous structures by two shaping processes, namely, uniaxial pressing and direct casting of aqueous suspensions. After drying, the samples' microstructure and physical properties evolution were investigated up to sintering (1100-1500 °C) through total porosity, Young's modulus, compression strength, pore diameter, and thermal conductivity measurements, dilatometric analyses, and scanning electron microscopy. Reference samples of coarse calcined alumina were tested under the same conditions to highlight the impacts of CA6 particles' morphology. Compared to them, the CA6-containing samples showed almost no variation in total porosity and average pore size levels during thermal treatments, although their strength and rigidity increased significantly after sintering. Their microstructure remained practically unchanged after drying, showing clusters of large asymmetrical CA6 crystals bonded to each other by their edges, and surrounding a large fraction of 1.8–2.2 μm pores. According to the results, although water content, processing method, and compacting levels are important parameters, particles' microstructure, ratio of intra-particle pores, and asymmetrical shape are key variables for the development of physical properties. Such characteristics strongly contributed to their densification resistance and lower thermal conductivity after exposure to high temperatures.