Advanced boron-containing refractory castables bonded with calcium-free binders

Abstract

The replacement of calcium aluminate cement (CAC) and/or the addition of specific agents (i.e., mineralizers) have been proposed as alternative routes to develop refractory products suitable for intermediate working temperatures (700–1000 °C). In this sense, the use of calcium-free binders, such as hydratable alumina or silica-based compounds, has gained some attention. Additionally, the incorporation of sintering additives to castables can speed up the expected sintering reactions, due to the generation of liquid phase in the designed microstructure during the first heating treatment of these products. Therefore, this work evaluates self-flowing high-alumina castables bonded with 4 wt.% of calcium-free binders: hydratable alumina, a novel powdered microsilica-alumina-based one or their blend. Besides that, it was also analyzed the influence of adding 0 or 1 wt.% of boron carbide to the castables, as this compound can act as a transient liquid sintering agent. The designed compositions were characterized via flowability, thermogravimetric tests, hot elastic modulus, erosion resistance, cold and hot mechanical strength and other techniques. The castable bonded with the blend of the selected binders showed shorter setting time and lower drying profile performance, as the samples’ explosion for these compositions could be detected even for small samples (50 × 50 mm) when using a heating rate of 5 °C/min. On the other hand, the refractory containing the microsilica-alumina-based binder presented enhanced thermo-mechanical behavior. Furthermore, boron addition favored faster sintering of the castables, resulting in optimized performance around 815 °C or even at 1100 °C for the mixture containing only the silica-based additive.