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A mechanism to explain the lower onset strengthening temperature induced by CaCO3 in alumina-based macroporous ceramics is proposed, which relies on hydrocalumite-like phase formation during processing. Close to 600 °C, such phases are decomposed to lime and mayenite (12CaO·7Al2O3), where the latter, due to its intrinsic nanoporosity and high thermal reactivity, generates bonds between the ceramic particles at ~700ºC, resulting in microstructure strengthening. Based on this premise, the authors concluded that other Ca 2+ sources could act similarly. Indeed, compositions containing Ca(OH)2 or CaO showed the same effect on the onset strengthening temperature, which reinforces the proposed mechanism. The results attained indicated that macroporous insulators could be thermally treated at lower temperatures, just to acquire enough mechanical strength for installation, finishing in-situ their firing process. Besides that, lower sintering temperatures could be used to produce macroporous ceramics that would be applied in low thermal demand environments, e.g. aluminum industries.

Aiming the in situ formation of CA6 (CaO•6Al2O3) at alumina-based macroporous insulators, distinct Ca 2+ sources and contents were used and their effect on some of the refractories properties were investigated. Adding CaCO3, Ca(OH)2 or CaO resulted in the decrease of the strengthening onset temperature (TS) and also of the linear shrinkage. However, a higher amount of Ca(OH)2 and CaO could not be used because of their effect on reducing the insulator total porosity. The composition prepared with 12.9wt% of CaCO3 was the most promising one, leading to an expansion of 0.81% after firing at 1600ºC for 5h, TS of 680ºC and low thermal conductivity. These results point out the potential reduction of sintering temperatures and to the possibility of in situ firing the ceramic insulator. These features enable the development of a macroporous refractory composition with a higher thermal insulating effectiveness, which can help industries to decrease their energy demand.

Aiming at reducing the temperature of CaO.6Al2O3 (CA6) formation, SiO2, ZnO, and TiO2 were evaluated as mineralizing agents in an Al2O3-CaCO3 based macroporous thermal insulator. For the concentrations tested (0.6mol% to 2.8mol%), SiO2 presented a small effect in the temperature of CA6 formation. However, ZnO and TiO2 favoured full CA6 generation at 1400ºC, 200ºC below the reference composition. All analysed systems, besides 2.8Si, showed high porosity (>80%), suitable mechanical strength and expansion after firing at 1600ºC for 5h. Softening temperature evaluation highlighted that ZnO-and TiO2-containing compositions could be used at higher temperatures than the SiO2 ones. Thus, compositions containing 2.8mol% of ZnO or TiO2 were evaluated according to their thermal conductivity (keff), showing lower keff than the reference composition. Therefore, using these mineralizing agents, which are easily available, may provide reductions in the energy input to produce CA6-containing thermal insulators and benefits to all other required properties.