Ceramic foams is a kind of inorganic non-metallic material with a three-dimensional porous network structure after calcination at high temperatures. The mechanical properties are one of the important factor to evaluate the application of ceramic foams, which affected by the porosity, pore size, pore distribution and skeleton structure, etc. High porosity would reduce the bearing capacity of ceramic foams and limit its application fields. Therefore, this research is mainly focused on how to balance porosity and strength of ceramic foams, as compressive strength could be improved effectively by reducing the pore size, narrowing the pore size distribution or forming a closed pore structure.

When coal gangue is used as raw materials to prepare ceramic foams, it still exists some problems, such as the low utilization, high energy consumption and unclear foaming principle. The key to producing a ceramic foam is to form a melt matrix with appropriate viscosity and to generate a certain amount of foaming gas in the matrix. Due to the complex composition of coal gangue, it is difficult to control the viscosity of the melt matrix at high temperatures, resulting in the pore structure distribution uneven and reducing the mechanical properties of ceramic foams.

In this work, coal gangue was used as the main raw materials, and diatomite powder, talc powder, a small amount of silicon carbide (SiC) foaming agent were added to prepare the ceramic foams with high compressive strength. The effects of raw materials composition, SiC content (mass fraction) and particle size of feed mixture on the pore structure, apparent density and compressive strength and other properties of ceramic foams were investigated.

The coal gangue was crushed and sieved to less than 74 μm. Based on the SiO₂-Al₂O₃-MgO ternary system, the coal gangue, diatomite powder and talc powder were mixed at different mass ratios, and then SiC powders were added as the foaming agent, the mixture and agate balls in a mass ratio of 1∶1 were milled with a speed of 300 r/min. After ball milling, the powders were placed in a corundum boat and heated at 1 190 ℃ for 25 min, then cooled to room temperature in furnace. The chemical compositions of the raw materials were determined by X-ray fluorescence spectrometer (XRF). The crystalline structure of raw materials and ceramic foams was determined by X-ray diffraction (XRD). The thermogravimetric analyzer (TG-DSC) was used to analyze the mass change and thermal analysis of raw materials. The particle size of mixed materials was analyzed via a laser particle size analyzer. The compressive strength of ceramic foams was measured via a pressure test machine. The pore size distribution of the sample was measured by Nano Measurer 1.2, and the water absorption and porosity of ceramic foams were measured by Archimedean method.

When amount of coal gangue reached 60% and 70%, the main phases of ceramic foams were cordierite and quartz, and when content of coal gangue exceeded 75%, sekaninaite phase appeared. When content of coal gangue reached 85%, the average pore size of the ceramic foams decreased from 2.01 mm to 0.80 mm, and the distribution of pore size became uneven as the increase of coal gangue could reduce the SiO₂/Al₂O₃ mass ratio, which would lead to an increase of melting temperature of the matrix, and thus the generated silicate molten material at high temperature decreased and the viscosity increased. As a result, the pore structure with uneven distribution would decrease the bearing capacity. Compressive strength reached the maximum value of 2.2 MPa at the coal gangue content of 75%. When content of SiC foaming agent increased, more small pores were generated in the matrix, these pores could connect to form large pores during sintered process, resulting in a gradual increase in pore size. When content of SiC increased from 0.4% to 1.4%, the compressive strength of ceramic foams decreased from 2.9 MPa to 0.8 MPa, the reduction of particle size of the feed materials could be benefit to the agglomeration and sintering behavior of SiO₂ at low temperature. When the average particle size decreased from 21.1 μm to 5.4 μm, the apparent density and porosity of ceramic foams changed slightly, while the compressive strength gradually increased from 2.6 MPa to 6.1 MPa, being due that the small particle size of SiO₂ in feed mixtures could improve the densification of the pore wall and finally improve the bearing capacity of the ceramic foams.

The main conclusions of this work can be summarized as follows: With the increase of coal gangue content in the feed materials, SiO₂/Al₂O₃ mass ratio decreased from 2.66 to 2.05, the pore structure of the sample deteriorated, and the compressive strength first increased and then decreased. When the content of SiC increased, more gases were released during calcination process, and thus increasing the porosity. When it exceeded 0.8%, the excess SiC should increase the open porosity and thus reduces the compressive strength of ceramic foams. Reducing the particle size of the feed mixture could improve the reaction activity of powders during the high temperature foaming process, the small pores and uniform pore structure could obviously improve the compressive strength of ceramic foams. When mass ratio of coal gangue: diatomaceous earth powder: talc was 7∶2∶1, content of SiC was 0.6%, the average particle size of the mixed materials was 5.4 μm, the apparent density, porosity and compressive strength of the prepared samples were 0.45 g/cm³, 80.6%, and 6.1 MPa, respectively, expected to be applied in the field of heat preservation wall.