Ice-templating of geopolymer architectures

Geopolymers are produced by reacting an aluminosilicate powder (metakaolins, blast furnace slags, fly ashes, pozzolana etc.) with an aqueous alkali hydroxide and/or alkali silicate activating solution. The water based production process of geopolymers allows the tailoring of the intrinsic mesoporosity of the material and the possibility to apply different process techniques, commonly used for the production of ceramics, to induce ultra-macroporosity. In particular, geopolymers with controlled lamellar macroporosity and total porosity ranging from 60% to 70% were obtained through the ice-templating (water based freeze casting) of partially geopolymerized slurries. Ice-templating is an environmentally friendly process that implies the use of ice as sacrificial pore template. The unidirectional freezing of the aqueous geopolymer suspension determines the lamellar growth of ice crystals. Then, ice sublimation finally produces porous architectures with unidirectional channel-like (lamellar) pores that are the replicas of the ice crystals. The study on the production process highlighted as the macrostructure (lamellar pores morphology) was strongly affected by several process parameters, such as: viscosity of the starting slurry, maturation treatment, amount of water targeted for icetemplating, mould geometry. Lamellar monoliths, boards and tubes were realized by varying these parameters, showing as the process can be adopted for the realization of samples with different size and geometry suitable for novel tailor-made catalyst supports.