Highly porous cordierite ceramics from engineered basic activation of metakaolin/talc aqueous suspensions

The cellular structure, in alkali activated metakaolin-based suspensions, foamed by intensive mechanical stirring, is stabilised by the viscosity increase caused by gelation, in a condition of 'inorganic gel casting'. The approach is so flexibile that it may be applied to mixtures embedding fillers, such as reactive gamma-Al2O3 powders, playing a fundamental role upon ceramic conversion. The present study is dedicated to highly porous cordierite foams, obtained including talc as further component, and applying a heat treatment at 1200-1250 degrees C, in air. A key intermediate step is represented by the removal of Na+ ions from 'green' foams, by ion exchange in ammonium nitrate solution (24 h), before ceramization. Direct ceramization is also feasible, if the gelation is achieved by reaction with tetra-methyl-ammonium hydroxide, instead of NaOH. The new gels are effective in yielding phase-pure cordierite, otherwise feasible, with NaOH activation, by means of much longer ion exchange treatment (120 h).