Surfactant-stabilized foamed geopolymers as sponge-city filter modules: structure-property links for adsorption,...

This study quantifies structure-property relationships in surfactant-foamed porous metakaolin-based geopolymers, linking pore architecture to methylene blue (MB) adsorption, thermal insulation, compressive strength, and filtration. MB is used here as a representative cationic organic probe to elucidate the coupling between pore connectivity and mass-transfer/adsorption kinetics. Sixteen formulations were prepared using H₂O₂ (2 wt%) as the foaming agent and either sodium dodecyl sulfate (SDS, 1.6-3.0 wt%) or vegetable oil (3.0-6.0 wt%) as stabilizers under ambient and 60 °C curing. Surfactant-containing systems exhibited total porosity >70%; in SDS systems, open porosity typically exceeded 85% of the total. Thermal conductivity spanned 0.125-0.54 W/(m·K); unfoamed matrices were 0.5311 W/(m·K) (ambient) and 0.45 W/(m·K) (60 °C), whereas foamed samples predominantly fell within 0.10-0.30 W/(m·K). Bulk density and conductivity followed a strong linear relation. The unfoamed matrix exceeded 50 MPa at 3 d and 60 MPa at 28 d; H₂O₂ foaming reduced strength substantially, consistent with increased porosity. Enhanced pore connectivity accelerated adsorption kinetics: the matrix required >200 min to equilibrate, H₂O₂-only specimens equilibrated within 60 min, and SDS-modified specimens (1.6-2.3 wt%) within 20 min. Under 2 wt% H₂O₂ + 2.3 wt% SDS at 60 °C, maximum MB uptakes reached 0.212/0.455/0.906 mg/g at initial concentrations of 10/20/40 mg/L, respectively. While MB served as the model adsorbate, intended engineering deployments focus on contaminants prevalent in urban stormwater-specifically selected hydrophobic organics-and thus prioritize use as pre-treatment media in inlet vaults, filter layers within permeable pavement bases, and rooftop runoff modules. In such installations, low thermal conductivity can help buffer seasonal temperature swings, while the quantified links between pore connectivity and flux and between density and conductivity provide design guidance for multifunctional geopolymers that integrate adsorption, filtration, and insulation.