Fly ash coated with alumina sol for improving strength and thermal insulation of mullite porous ceramics

The manufacturing of mullite porous ceramics with high strength and low thermal conductivity was achieved through foam gel-casting processes using fly ash coated with alumina sol layers. This research aimed to investigate the effect of alumina sol concentration on foaming slurry rheology, as well as the influence of alumina sol coating layers on the microstructure, phase compositions and properties of the resulting mullite porous ceramics. Increasing the alumina sol concentration from 5 to 20 wt% improved both the viscosity and thixotropy of the foaming slurries while enhanced the shear thinning behavior. Porous ceramics prepared with fly ash coated with alumina sol exhibited smaller pore size compared to the untreated fly ash porous ceramics. Moreover, the distribution of pores gradually became more homogenous in the porous ceramics with treated fly ash. Meanwhile, the weight-reduction, compressive strength and thermal insulation properties of the porous ceramics were improved significantly. The use of fly ash coated with alumina sol (with a concentration of 20 wt%) in the preparation of porous ceramics resulted in the formation of mullite whiskers within the pore walls. This created micron-size gaps between the whiskers, greatly enhancing the thermal insulation of the porous ceramics. Finally, the porous ceramics that were prepared using fly ash coated with alumina sol (with a concentration of 20 wt%) and sintered at a temperature of 1400 °C had a bulk density of 0.45 g/cm3, a compressive strength of 8 MPa, and a thermal conductivity of 0.15 W/m·k.