K₂O-Metakaolin-Based Geopolymer Foams: Production, Porosity Characterization and Permeability Test

In this paper, four near-net shaped foams were produced via direct foaming, starting from a benchmark metakaolin-based geopolymer formulation. Hydrogen peroxide and metallic silicon were used in different amounts as blowing agents to change the porosity from meso- to ultra-macro-porosity. Foams were characterized by bulk densities ranging from 0.34 to 0.66 g cm^-3, total porosity from 70% to 84%, accessible porosity from 41% to 52% and specific surface area from 47 to 94 m² g^-1. Gas permeability tests were performed, showing a correlation between the pore features and the processing methods applied. The permeability coefficients k₁ (Darcian) and k₂ (non-Darcian), calculated applying Forchheimer's equation, were higher by a few orders of magnitude for the foams made using H₂O₂ than those made with metallic silicon, highlighting the differing flow resistance according to the interconnected porosity. The gas permeability data indicated that the different geopolymer foams, obtained via direct foaming, performed similarly to other porous materials such as granular beds, fibrous filters and gel-cast foams, indicating the possibility of their use in a broad spectrum of applications.