Formation mechanism of high apparent porosity ceramics prepared from fly ash cenosphere

Highlights: • Highly porous ceramics were produced (59.25% apparent porosity). • Sintering temperature decreased the apparent porosity in densification process. • CO₂ emission from CaCO₃ promoted the formation of pores at high temperature. • CaO was transformed into anorthite generating porous shell of fly ash cenosphere. Porous ceramics with high apparent porosity have wide applications in various fields. In this work, high apparent porosity ceramics were fabricated from fly ash cenosphere. Effects of sintering temperature and CaCO₃ on the apparent porosity were investigated. More importantly, the formation mechanism of high apparent porosity was proposed. Results showed that the sintering temperature decreased the apparent porosity in densification process. The addition of CaCO₃ improved the apparent porosity in two ways: one was that CO₂ emission from CaCO₃ decomposition promoted the formation of pores in ceramics; the other one was that CaO from decomposed CaCO₃ was transformed into anorthite with fly ash cenosphere, which generated the porous structure of fly ash cenosphere shell. In this work, the apparent porosity of ceramics with 30 wt% CaCO₃ sintered at 1250 °C reached 59.25%, and the corresponding compressive strength was 70 ± 2.58 MPa. Results also indicated that the skeleton structure of interconnected fly ash cenosphere guaranteed a relatively high mechanical strength of this porous ceramics.