Direct synthesis of porous calcium-hexaluminate aggregate for refractory applications

The synthesis of macro-porous calcium-hexaluminate (CaO∙6Al 2 O 3 , CA 6 ) aggregates composed of CA 6 platelets was examined using various Ca- and Al-precursors. The effects of annealing temperature, time, atmosphere, and AlF 3 as a mineralizer on the formation of plate-shaped CA 6 were also studied. The Ca:Al ratio was adjusted for full conversion to CA 6 , which is the most stable phase among the CaO-Al 2 O 3 intermediate compounds. The conditions for the synthesis of plate-shaped CA 6 aggregates with adequate strength were uncompromising, requiring heat treatment at ≥ 1500 °C for 15 h, which was also significantly affected by the type and size of the precursors. CA 6 aggregate synthesized directly using coarse Al 2 O 3 and CaO with 10 wt% CaO∙Al 2 O 3 cement as a binder phase revealed the optimal macro-porous microstructure for castable refractory applications, showing porosity and compressive strength of 65% and 2.9 MPa, respectively.