Alkali-bonded ceramics with hierarchical tailored porosity

Alkali-bonded ceramics and foams with tailored porosity have very interesting applications as thermal insulators, catalysts, filters, etc. Alkali-bonded ceramics were prepared starting from metakaolin and potassium silicate, selecting the process conditions, to change the intrinsic nano-micro-porosity of the geopolymer, and studying their influence on the degree of geopolymerization. Optimum geopolymerization conditions were chosen to develop porous 3D networks by inducing interconnected ultramacroporosity (up to the millimetre range) in the material, exploiting the ability of silicon powder to generate H2 in the reaction environment. The in situ foaming was strongly dependent on the water content of the geopolymeric precursors and the subsequent process of water elimination. The H2 formation is, in fact, a water-consuming process, just as consolidation is, thus increasing the viscosity. The geopolymeric inorganic matrices and the related foams were fully characterized in terms of microstructure, intrinsic and induced pore size distribution, specific surface area, geopolymerization degree, and surface accessibility of prepared materials. The thermal behaviour of the materials was also studied. The experimental findings highlighted the versatility of the foams that may be appropriately designed based on the possible application.

► Different geopolymerization conditions changed the intrinsic nano-micro-porosity.
► Interconnected ultra-macro-porosity was produced by using Si as foaming agent.
► Alkali-bonded ceramics with hierarchical tailored porosity were produced.
► Foams may be properly designed as a function of the potential applications.