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

This paper addresses an evaluation of the effects of variation in the CaCO3-Al2O3 ratio on the physical properties of in situ calcium hexaluminate (CaAl12O19 or CA(6)) porous structures during sintering. Combinations of calcined alumina (alpha-Al2O3) and different amounts of precipitated CaCO3 particles were thermally treated between 500 degrees C and 1500 degrees 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 degrees C, the addition of higher contents of CaCO3 increased total porosity, strength and rigidity levels due to the simultaneous generation of pores (from CaCO3 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 degrees 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 degrees C, for 3 h) and lowest densification rates.