Development of porous geopolymer spheres

Two simple processes for the production of geopolymer porous spheres are preliminary reported in this study.The processes are based on the formulation of metakaolin-based geopolymer slurries, produced using a potassium-based alkaline activating solution, and exploiting an injection and solidification method in liquid nitrogen or polyethylene glycole (PEG) solutions. When liquid nitrogen was used, the geopolymer slurries were left at room temperature for several hours (maturation step) in order to trigger the geopolymerization without reaching a complete chemical consolidation. After, the mixture was injected as it is or mixed with water or an organic template to modulate the final porosity. The spheres were then freeze dried to remove the solidified water and complete the chemical consolidation of the geopolymer. Conversely, with the PEG solutions, the slurries were directly injected in the solutions heated at different temperature (60-80°C). After the consolidation, the spheres were rinsed with water and left in a heater to complete the geopolymerization. The main goal was to synthetize geopolymer spheres with hierarchical porous structures differing in term of intrinsic mesoporosity and architecture, which means distribution of voids/material within the spheres. The tuning of the water content in the starting mixtures allows to affect and to tailor the intrinsic mesoporosity of the geopolymer matrix, being water a pore former during the geopolymerization [1, 2], moreover, the porosity of the spheres can be further tailored adding organic templates. Geopolymer slurries composition and maturation steps were deeply investigated to optimize the consolidation process and to design the porosity in the final spheres, to find potential applications in fields as catalysis, filtration, adsorption and so on. The selected geopolymer spheres were characterized in term of size, morphology, macro- and microstructure, composition-stoichiometry, porosity distribution and specific surface area.