Development of greener and more effective high-temperature thermal insulators

Aiming at reducing the remarkable shrinkage observed when porous ceramics are fired, the in-situ formation of hibonite (also known as CaO·6Al₂O₃ or CA₆) was evaluated in a direct-foamed alumina-based macroporous thermal insulator with the addition of distinct calcium sources and contents. Among the tested compositions, that containing 12.9 wt% of CaCO₃ showed promising results as high porosity (~ 80 %), no shrinkage after firing at 1600 ºC for 5 h, suitable mechanical strength, and a 200 ºC-lower strengthening temperature. However, the in-situ formation of CA₆ took place just at high temperatures (~ 1600 ºC), implying a high energy consumption. To address this issue, SiO₂, ZnO, and TiO₂ were evaluated as mineralizing agents. The addition of 2.8 mol% of TiO₂ induced CA₆ formation at 1300ºC and low thermal conductivity (0.54 Wm^-1K^-1 at 1200 ºC), without significant changes in the refractoriness.