Tailoring the porosity in geopolymer based materials and composites

Geopolymers are produced by reacting an alumino-silicate powder (metakaolins, blast furnace slags, fly ashes, pozzolana etc.) with an aqueous alkali hydroxide and/or alkali silicate solution. They constitute a family of materials with variable properties within those characteristic of ceramics, cements, zeolites or refractories, depending on formulation. The production process in aqueous medium of geopolymers allows the tailoring of the porosity from the nanometric to the millimetric range. A hierarchical porous system in which meso-pores are directly connected to macro- and finally to ultra-macro pores can be constructed. Water affects the intrinsic meso-porosity of the geopolymer matrix, since it acts as a pore former during the polycondensation stage. Ultra-macro-porosity can be obtained by direct foaming, exploiting for example the redox reaction of metallic powders (such as Si or Al) in alkaline solution to induce porosity through hydrogen evolution. Porous architectures with main unidirectional anisotropic macro-pores are realized by freeze-casting, where the ice crystals act as pore network template. Finally, the addition of fillers results in a further functionalization, producing a porous composite.