Silica-based geopolymer spherical beads: Influence of viscosity on porosity architecture

Abstract The facile synthesis of hierarchically porous spherical geopolymer beads via pipetting and low-temperature foaming is reported here. The potassium silica solution and metakaolin-slag-based slurries were modified by amorphous fumed silica in order to regulate the SiO2–Al2O3 ratio and to form a porous structure through chemical foaming technique under alkaline conditions. The addition of fumed silica into 0.01 M KOH resulted in continuous dissolution with bimodal particle size distribution in the range between 158 nm and 3.8 μm. The prepared geopolymer slurries exhibited a shear thinning behavior where the flow properties were intensely affected by the fumed silica concentration. The viscosity of geopolymer slurries was in the range of 0.03–1.6 Pa s at a shear rate of 100 s−1 which strongly influences the formation of the inner porosity and wall thickness. The increased viscosity of the slurries leads to the formation of internally connected struts and spherical-shaped beads with a smoother surface. Silica-based geopolymer matrices exhibited a predominantly amorphous phase of ∼90%. The mechanical strengths of spherical geopolymer beads were evaluated by the means of crushing resistance which reached up to 1.982 N/mm2 at internal porosity of 60 vol%. Moreover, X-ray micro-computed tomography (micro-CT) revealed heterogeneous porous architecture in the range of 60–75 vol% with an internal porosity and the presence of combined isolated and interconnected pores between 50 and 500 μm. Thus, this study systematically investigates the influence of viscosity on the porosity, morphology and mechanical strength of silica-based geopolymer beads.