Search

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.

Environmental and occupational issues regarding Cr6+ formation urge the replacement of chrome-containing refractories by greener alternatives. For the latest decades, magnesium-containing refractories have been investigated for this purpose, however, few studies evaluated the likelihood of using other chemical elements as inducers of spinel-like phase formation in refractory castables. In this work, the addition of zincite (ZnO) in alumina-based vibrated castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA) was evaluated and compared with its MgO counterpart. The formation of the Zn-based spinel (ZnAl2O4, gahnite) took place at a lower temperature (1100 ºC) than MgAl2O4 (1300 ºC), resulting in the anticipated strengthening of these compositions. Despite the challenges regarding using ZnO – as its volatilization under reduced atmosphere and fast reaction kinetics –, lower linear dimensional changes, comparable cold modulus of rupture and enhanced thermal shock damage resistance was detected.