Foaming mechanism and performance of closed–pore foamed ceramics prepared using calcium carbonate as a foaming agent

The technical feasibility of preparing closed–pore foamed ceramics (FCs) at low sintering temperatures was studied, and FCs with high closed porosity were prepared by a high–temperature foaming method using calcium carbonate as the foaming agent. Rapid heating is crucial for preparing FCs. The accumulation of decomposition–derived CO2 from calcium carbonate by molten green bodies with appropriate viscosity and good continuity during sintering constitutes the foaming mechanism of FCs. During rapid heating, the surface of the green bodies rapidly melted into a whole with appropriate viscosity and good continuity. Thereafter, the increasing amount of decomposition–derived CO2 inside the green bodies was sealed by the molten surface when the temperature within the green bodies increased. The foaming performance of the FCs was improved by optimising the sintering temperature and composition of the green bodies. The FCs sintered from green bodies with 10–20 wt% calcium carbonate and 40 wt% sodium tetraborate at 750–800 °C exhibited a high total porosity of 80–82%, a high closed porosity of 76–78%, a low density of 0.48–0.53 g/cm3, a compressive strength of 2 MPa, and a uniform internal foam structure.