Tailoring and study of the porosity in geopolymer based materials

Geopolymers are produced by reacting an alumino-silicate powder with an aqueous alkali hydroxide and/or alkali silicate solution. The production process in aqueous medium allows the tailoring of the porosity from the nanometric to the millimetric range. Water affects the intrinsic mesoporosity of the geopolymer matrix, since it acts as a pore former during the polycondensation stage, and ultra-macroporosity can be induced in the material employing different methods, to obtain hierarchical porous systems in which mesopores are directly connected to macro- and finally to ultra-macropores. Geopolymers are often compared with ceramics for their similar final properties arising from the inorganic structure. The main difference, referring to the process formation of these porous materials, is that ceramic foams are usually treated at high temperature for the burnout of additives or templates and the final consolidation. On the other hand, geopolymers have the advantage to be consolidated through a chemical reaction that occurs at low temperature. Methods used in the process formation of porous ceramics can be adapted to obtain geopolymers with different architectures, pore size distribution, interconnectivity and so on; direct foaming techniques can be used to obtain foams with rounded ultra-macroporosity. Freeze-casting, belonging to the sacrificial template method, can be used to obtain unidirectional anisotropic macropores with the formation of unique geopolymer lamellar porous structures. Lastly, the use of inert or partially reactive fillers results in a further functionalization of the geopolymer, with effective production of highly macroporous composites (as the use of vermiculite). Tailoring the porosity in geopolymers is of paramount importance for their potential applications in thermal insulation, filtration, catalysis, etc., and a deep characterization of the final materials, in order to understand how the porosity may be developed or modified during the preparation, must be performed combining different techniques as N2 adsorption/desorption, Hg intrusion and µ-Computed Tomography (µ-CT).