Highly porous mullite ceramics from engineered alkali activated suspensions

Air may be easily incorporated by vigorous mechanical stirring, with the help of surfactants, of activated geopolymer‐yielding suspensions. The cellular structure is stabilized by the viscosity increase caused by curing reactions, configuring an “inorganic gel casting”. The present paper is aimed at extending this approach to mullite foams, obtained by the thermal treatment of engineered alkali activated suspensions. “Green” foams were first obtained by gel casting of a suspension for Na‐geopolymer enriched with reactive γ‐Al₂O₃ powders. Sodium was later extracted by ionic exchange with ammonium salts. In particular, the removal of Na⁺ ions was achieved by immersion in ammonium nitrate solution overnight, with retention of the cellular structure. Finally, the ion‐exchanged foams were successfully converted into pure mullite foams by application of a firing treatment at 1300°C, for 1 hour. Preliminary results concerning the extension of the concept to mullite three‐dimensional scaffolds are presented as well.