Metakaolin-based geopolymer beads: Production methods and characterization

The production of porous metakaolin-based geopolymer beads was investigated using three different dripping based techniques. The processes were based on the formulation of geopolymer slurries produced using metakaolin and a potassium-disilicate activating solution, and exploiting injection and solidification method in polyethylene glycol (PEG) or liquid nitrogen and ionotropic gelation of alginate in calcium chloride. Geopolymer slurries and maturation steps were deeply investigated in order to optimize the consolidation process and to design the meso-macro-porosity in the final beads. The geopolymer beads were characterized in terms of shape and dimensions, macro- and micro-structure, composition, porosity distribution and specific surface area (SSA). In detail, the three processes allowed to obtain beads from 2 to 3 mm in diameter. Injection and solidification in PEG and in liquid nitrogen produced beads with 45% and 64% of porosity, a pore modal diameter of 0.41 ?m and 0.72 ?m, and SSA equal to 19 and 76 m g respectively. Ionotropic gelation yielded beads with a final porosity around 59%, a modal pore diameter of 0.27 ?m and SSA around 16 m g.