Designing Colloidal Silica-Bonded Porous Structures of In-situ Mullite for Thermal Insulation

Abstract: Colloidal silica (CS) is a promising raw material for refractory castable ceramics. It consists of stable suspensions of synthetic amorphous silica nanoparticles that behave simultaneously as liquid medium and binder for ceramic particles and as a porogenic agent and highly reactive source of silica to promote in-situ reactions. The setting mechanism of CS balances two opposite effects. Adding more CS to a suspension increases the bonding potential for gelling reactions and strengthening; on the other hand, it also introduces more water into the system, enhancing pore content. Such effects can be advantageously employed in the preparation of porous structures from aqueous suspensions and applied as high-temperature thermal insulators. The present study addresses the production of porous structures of in-situ mullite attained from aqueous suspensions of highly porous transition alumina particles bonded with colloidal silica. Different grades of CS and transition aluminas were combined to present suitable workability (flowability and gelling time) and to generate stoichiometric mullite or mullite-alumina porous structures after sintering.