Lightweight, robust hierarchically porous ceramics from cost-effective powders for dye removal

A facile and cost-effective approach for hierarchically porous ceramics has been proposed firstly, using coal fly ash as the main powder while secondary aluminum dross (SAD) as curing and foaming agents. Benefited from the hydrolysis of AlN and Al in SAD, gas and lamellar aluminum hydroxide sol attached to the surface of unreacted particles were produced, meanwhile pH value and viscosity increased, realizing foaming and coagulation casting simultaneously to fabricate porous green body. The increase of SAD amount could contribute to more intensive formation of flaky aluminum hydroxide sol, enhancing mechanical strength of green body, which has high compressive strength of 1.03 MPa at a bulk density of 0.68 g/cm3. Porous mullite-based ceramics could be sintered at a relatively low temperature of 1200–1250 °C owing to alkali metal oxide and amorphous SiO2 in the solid wastes, which also help to form micropores on the pore wall due to partially sintering. Porous mullite-based ceramics possess hierarchical pores, bulk density of 1.00 g/cm3, open porosity of 66.15%, specific surface area of 2.24 m2/g and compressive strength of 9.14 MPa. Their removal rate of malachite green solution reaches 99.1% at a concentration of 100 mg/L and an adsorbent dosage of 100 g/L.