•Highly porous ceramic foams were firstly prepared by combining the direct foaming method and agar consolidation method, using fly ash as the raw material, which is economical, practical, environment friendly.•Meanwhile, the production of ceramic foams is a useful way of recycling large amounts of fly ash, which could efficiently resolve the disposal problem of fly ash.•Additionally, the change of thermal conductivity of highly porous ceramic foams along with different amounts of agar gelation agents was systematically researched.The ceramic foams with low thermal conductivity (0.0579–0.0826 W m−1 K−1) were successfully prepared by using fly ash as main raw material, combining direct foaming method and agar consolidation method. Additionally, the change of thermal conductivity of highly porous ceramic foams along with different amounts of agar gelation agents was systematically researched. The experimental results show that the samples with different amounts of agar gelation agents exhibit similar pore morphology. As the content of agar increased the pore size decreased first and then increased, but the porosity continued to decrease. Meanwhile, lower thermal conductivity was achieved with higher porosity, smaller pore size and more small pores of sintered product, but the porosity dominated.

•Using industrial waste fly ash as main material to reduce its environmental endanger.•Its synthetic process is simple and environmentally friendly and needs no sintering.•Its thermal conductivity is as low as 0.0721 W/m K.•It can be used as low cost insulation material in some conditions.•The properties of samples made at various experimental conditions were discussed.Porous fly ash-based geopolymer material was produced using fly ash and sodium water glass as original material and H2O2 as foaming agent. The changes caused by the geopolymerization and decomposition of H2O2 on the properties of the final products were investigated by applying curing on geopolymer mortars with different amounts of sodium water glass (60, 80 and 100 g) and H2O2 (4, 6 and 8 g) added at various curing temperatures. The samples were cured at two different temperatures (55 and 85 °C). Physical properties such as porosity, density and thermal conductivity and mechanical property were determined from the completely dried samples. As a result, this study confirmed that the amount of sodium water glass, H2O2 and curing temperature had effects on properties of the samples. Given the compressive strength obtained and the environmental and economic effect, the optimal thermal curing temperature and the amount of sodium water glass and H2O2 were 55 °C, 80 g and 6 g, respectively. Sample synthesized at this condition had porosity of 79.9%, thermal conductivity of 0.0744 W/m K and compressive strength of 0.82 MPa. Therefore, it shows promise as thermal insulation material in some situations.

•Fabrication of dense cordierite ceramic through reducing Al2O3 mole ratio from 2.0 to 1.4 was studied, while the others fabricated dense ceramics by adding metal oxide or fine powders.•In this paper, changing the ratio of the starting materials themselves can fabricate dense cordierite ceramics.•At 1.4 Al2O3 mole ratio, the bulk density tends to the maximum value of 2.50 kg/m3.Dense cordierite ceramic was fabricated through reducing Al2O3 mole ratio from 2.0 to 1.4. The effect of the content of Al2O3 on crystalline phases, sintering characteristics and microstructures were studied. The green bodies, made of kaolin, attapulgite and basic magnesium carbonate, were sintered at 1200 °C. The result suggests that reducing the Al2O3 mole ratio can fabricate dense cordierite ceramic. At 1200 °C, cordierite became the main crystallization phase with a little of sapphirine. With the Al2O3 mole ratio decreasing, the shrinkage and bulk density increased but porosity reduced. When the mole ratio of Al2O3 reduced to 1.4 and sintered at 1200 °C, the bulk density tends to the maximum value of 2.50 kg/m3.

•Using fly ash as main material to reduce its environmental endanger.•Combination of foaming and slip casting method.•Effects of some factors on the properties of the material.•Thermal conductivity as low as 0.0511 W/(m K).•Comparison of some traditional and innovative insulating materials.The paper introduces a new kind of thermal insulation material using coal fly ash as main raw material by the method of foaming and slip casting process. The foaming ability of the suspensions with solid loading of 30, 35 and 40 wt% at different foaming agent additive levels were measured. The green bodies were then sintered under different sintering temperatures from 850 °C to 1000 °C for 2 h following the steps of foaming and drying. Effects of solid loading of suspensions and sintering temperature on the porosity, compressive strength and thermal conductivity were investigated. The sintered bodies, with porosity from 86.3 vol.% to 94.5 vol.% and compressive strength from 0.43 MPa to 1.01 MPa, contained spherical pores with no preferred orientation. The thermal conductivity of the sintered thermal insulation material, measured by the transient plane source (TPS) at room temperature, could reach as low as 0.0511 W/(m K). As an environmental friendly material, it is suitable for wall application to save energy.