Hydrothermally tailored high-basalt geopolymer for strength development and indigo carmine removal

The development of sustainable multifunctional construction materials capable of simultaneously providing structural performance and environmental remediation has attracted considerable research interest. In this study, a low-carbon alkali-activated geopolymer was synthesized using an equal proportion of ground granulated blast-furnace slag (GGBFS) and Egyptian natural basalt as aluminosilicate precursors. The influence of NaOH concentration (5 and 10 wt%) and curing regime (ambient curing and hydrothermal treatment at 150 °C for 3-24 h) on the fresh, mechanical, phase, morphological, and textural characteristics of the developed composites was systematically investigated. Hydrothermal curing markedly enhanced geopolymerization, promoted pore refinement, and induced the formation of crystalline calcium aluminosilicate hydrate and zeolitic phases, resulting in a maximum compressive strength of 53.3 MPa for the Geo-BT5/H composite after 12 h of autoclaving. Owing to its superior mechanical integrity, well-developed meso-porosity, and low production cost, Geo-BT5/H was selected as an adsorbent for Indigo Carmine (IC) removal from aqueous solutions. Adsorption was strongly pH-dependent, exhibiting maximum performance under acidic conditions (pH ≈ 2), while adsorption capacity increased with both initial dye concentration and temperature. The BET analysis confirmed the transformation of the pore structure from a Type III to a Type IV/H3 mesoporous network with a specific surface area of 30.91 m² g^-1, which increased to 39.7 m² g^-1 following acid activation. Thermodynamic analysis demonstrated that IC adsorption is an endothermic, entropy-driven, and spontaneous process at elevated temperatures. Nonlinear equilibrium analysis showed that the Redlich-Peterson model provided the best fit to the experimental data, while kinetic modeling indicated that IC adsorption proceeds through a multistage process involving surface adsorption and intraparticle diffusion. Interference studies demonstrated the influence of coexisting species on IC removal, providing additional insight into the adsorption behavior of Geo-BT5/H under more complex aqueous conditions. Furthermore, the geopolymer maintained excellent structural stability and adsorption performance after repeated adsorption-desorption cycles, as confirmed by FTIR and XRD analyses. These findings demonstrate that hydrothermally cured slag-basalt geopolymers represent promising multifunctional materials that integrate high mechanical performance with efficient and reusable dye adsorption for sustainable wastewater treatment applications.