Historically, improving refractory castables' high-temperature performance involved reducing calcium aluminate cement (CAC) content. However, even small amounts of CaO caused issues and CAC-bonded castables require careful drying to avoid damage. Cement-free castables with sol-gel binders, like colloidal silica (CS), emerged as alternatives, but CS-bonded castables still face structural stabilities at high temperatures. Given the limitations of CS-based binders, research has shifted to alternative sols like spinel and mullite. Castables bonded with them offer better high-temperature performance but suffer from extended setting times and low green strength due to lower solid content in initial sols (5-10 wt.% vs. 30 wt.% in commercial CS).
In this context, spinel suspensions with 30 - 50 wt.% solid content were developed. Then, the impact of the solid content on setting and green mechanical properties, as well as thermomechanical properties (elastic modulus and HMOR) was analysed. The findings demonstrate that higher solid content enhances green mechanical properties of spinel-bonded castables, though demoulding time is longer than for CS-bonded castables. However, thermomechanical properties are improved notably during initial heating, surpassing those of CS-bonded castables, especially above 1000 °C, where viscous or liquid phases start to form in CS-bonded materials.