Mechanical strength and cationic dye adsorption ability of metakaolin-based geopolymer spheres

Metakaolin-based geopolymer spheres shaped by three different dripping techniques were investigated in terms of compressive strength and dye adsorption ability. Spheres were produced by injection and solidification in polyethylene glycol (PEG) or liquid nitrogen (ice-templating) and ionotropic gelation (alginate in calcium chloride). Mechanical tests by ISO 18591 evidenced that compressive strength was strictly linked to porosity and composition of the spheres, being 12.9 MPa with 29% of porosity for spheres produced in PEG, 2.1 MPa with a porosity of 64% for spheres shaped in liquid nitrogen and up to 19.6 MPa with a porosity of 59% for hybrid spheres with a skeleton of Ca-alginate. The influence of contact time and initial concentration on a cationic dye (methylene blue) adsorption was studied. Removal efficiency was related to morphology, porosity and specific surface area: after 24 h the percentage of adsorption was 94–98% for spheres shaped in liquid nitrogen while below 75% for hybrid spheres and spheres produced in PEG. By adding TiO2 P25 as photocatalytic phase to the hybrid spheres, an increase of removal efficiency of 33% was observed after 90 min under UV irradiation.