The foaming mechanism and properties of SiO2–Al2O3–CaO-based foamed ceramics with varied foaming agents

To select suitable high-temperature foaming agents, SiO2–Al2O3–CaO-based foam ceramics (SACFC) were successfully prepared with different foaming agents based on phosphorus tailings and coal gangue. The effects of high-temperature foaming agents (CaCO3, carbon black, MnO2 and Fe2O3) on the microstructure and compressive strength of the SACFC were investigated, and the corresponding foaming mechanism was revealed. The results indicated that CaCO3 could increase the porosity of SACFC due to high-temperature decomposition. However, the precipitation of needle-like anorthite reduced the mechanical properties. Because the high-temperature oxidation temperature of carbon black was lower than the molten phase formation temperature, bubbles floated up and gathered on the surface of SACFC, leading to a decrease in strength. The continuous release of O2 from MnO2 during the sintering process promoted the growth and aggregation of large pores, which was detrimental to the strength. The CO2 generated by the redox of Fe2O3 by CO/C was the main factor for the foaming of SACFC. During the high-temperature foaming, the synergy effect of Fe2+ and Fe3+ promoted the formation of a uniformly distributed pore structure. When 2 wt.% Fe2O3 was added, SACFC obtained the optimal comprehensive performance with bulk density and compressive strength of 0.78 g/cm3 and 3.50 MPa.