Synthesis of porous hierarchical geopolymer monoliths by ice-templating

• Ice-templating was successfully applied to a water-based reactive sol–gel system.
• Unidirectional lamellar geopolymers can be synthesized by ice-templating.
• Geopolymer intrinsic mesoporosity is combined with ice-induced porosity.
• Characterization of hierarchical porous lamellar monoliths.
• Water addition affects porosity and micro- and macro-structures.

Hierarchical geopolymers with mesoporous matrices and lamellar macro-porosities were produced for the first time by ice-templating (freeze-casting) technique in a water-based sol–gel system.Starting from an aqueous solution of metakaolin and potassium silicate, geopolymerization was triggered through a maturation step without reaching complete consolidation. Different amounts of water (20, 50, 70 vol.%) were then mixed with the geopolymer paste to induce lamellar ice growth by unidirectional freezing. The consolidation was completed during freeze-casting and drying.Ice-templated hierarchical geopolymers had 53–83% total porosity and N2 BET specific surface area values from 4 to 46 m2 g−1, depending on the amount of water added. A broad mesopore Barret Joyner Halenda distribution was detected between 4 and 100 nm with maxima from 5 to 7 nm, while macropores in the 1-to-100 μm range were identified by Hg intrusion porosimetry. Lamellar monoliths (height 25 mm) were produced with a selected composition (50 vol.% of additional water) in order to assess the reproducibility of the ice-templating technique combined with geopolymerization.

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