Effects of the initial CaO-Al2O3 ratio on the microstructure development and mechanical properties of porous calcium hexaluminate

Abstract This paper addresses an evaluation of the effects of variation in the CaCO 3 -Al 2 O 3 ratio on the physical properties of in situ calcium hexaluminate (CaAl 12 O 19 or CA 6 ) porous structures during sintering. Combinations of calcined alumina (α-Al 2 O 3 ) and different amounts of precipitated CaCO 3 particles were thermally treated between 500 °C and 1500 °C. Their physical properties (Total Porosity, Flexural Strength, Flexural Elastic Modulus and Thermal Linear Variation), microstructure (Scanning Electron Microscopy), and phase composition (X-Ray Diffraction) were assessed. For thermal treatments conducted at 500–1100 °C, the addition of higher contents of CaCO 3 increased total porosity, strength and rigidity levels due to the simultaneous generation of pores (from CaCO 3 decomposition) and low-melting-point compounds (such as C 12 A 7 ) that helped the sintering of particles. A competition between the maintenance of pores resulting from the expansive formation of CA 6 and the densification of the structure was observed at 1300–1500 °C. Compositions whose CaO fraction was equal to or larger than the stoichiometric content for the production of CA 6 showed the highest porosity levels (up to 58% after sintering at 1500 °C, for 3 h) and lowest densification rates.