  Citation: Papa, E.; Landi, E.; Miccio, F.; Medri, V. K 2 O-Metakaolin-Based Geopolymer Foams: Production, Porosity Characterization and Permeability Test. Materials 2022, 15, 1008. https://doi.org/10.3390/ ma15031008 Academic Editor: George Wardeh Received: 22 December 2021 Accepted: 25 January 2022 Published: 27 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article K 2 O-Metakaolin-Based Geopolymer Foams: Production, Porosity Characterization and Permeability Test Elettra Papa * , Elena Landi , Francesco Miccio and Valentina Medri * National Research Council of Italy, Institute of Science and Technology for Ceramics (CNR-ISTEC), Via Granarolo 64, 48018 Faenza, Italy; elena.landi@istec.cnr.it (E.L.); francesco.miccio@cnr.it (F.M.) * Correspondence: elettra.papa@istec.cnr.it (E.P.); valentina.medri@istec.cnr.it (V.M.) Abstract: 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 m 2 g −1 . Gas permeability tests were performed, showing a correlation between the pore features and the processing methods applied. The permeability coefficients k 1 (Darcian) and k 2 (non-Darcian), calculated applying Forchheimer’s equation, were higher by a few orders of magnitude for the foams made using H 2 O 2 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. Keywords: geopolymer foam; direct foaming; porosity; permeability 1. Introduction Ceramic foams have been receiving increasing interest due to their exceptional combi- nation of properties that include temperature and corrosion resistance, their low weight, low thermal conductivity, the high permeability and tortuosity of flow paths, high specific surface area, and so forth [1]. At present, macro-porous ceramics have been widely applied in different technological areas, for instance, in thermal and acoustic insulation, pre-cast building materials, sound adsorption and noise reduction, as catalyst carriers and in wastewater treatment, high- temperature exhaust gas filtration and corrosive gas filtration, among others [2–4]. In line with net-zero emissions strategies, the development of foams from sustainable raw materials and using low energy production processes is of particular interest. In this context, geopolymer foams have some advantages in comparison with ceramic foams. Indeed, geopolymers are synthetic alkaline alumino-silicate inorganic polymers obtained at a temperature below 100 ◦ C through a chemical reaction between a highly alkaline aqueous solution and a powder containing silicon and aluminum elements [5,6], as well as from waste materials. Furthermore, foaming methods often used for ceramics can also be adopted for geopolymers. For this reason, geopolymer foams have been the focus of attention in the field of eco- friendly porous materials, also thanks to their low shrinkage after forming, advantageous thermomechanical resistance and chemical similarity to ceramics, together with their zeolite-like properties, and so on [7]. They have been employed as thermal and acoustic insulators, membranes and catalyst supports, for photocatalytic degradation applications and as heavy metal and dye adsorbents, to give just a few examples [8]. Materials 2022, 15, 1008. https://doi.org/10.3390/ma15031008 https://www.mdpi.com/journal/materials