Iron-Rich Slag-Based Geopolymers for Radioactive Waste Management: Characterization and Performance

Iron-rich metallurgical slag is an underutilized precursor in alkali-activated materials (AAMs), despite its abundance and potential in sustainable construction and waste immobilization. This study evaluates a binary AAM system (Aachen GP), comprising 50 wt.% blast furnace slag (BFS) and 50 wt.% iron-rich slag (Fe2O3≈ 24.6 wt.%), against a BFS-only reference (SCK GP). Characterization included isothermal calorimetry, Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM–EDX), Brunauer–Emmett–Teller (BET) surface area, water permeability, porosity, and compressive strength. Aachen GP showed delayed setting (32.9 h), reduced cumulative heat (∼70 J/g), and lower bound water (4.6% at 28 days), indicating limited gel formation. Compared to SCK GP, it had higher porosity (38.4%), water permeability (1.42×10−10 m/s), and BET surface area (12.4 m2/g), but lower 28-day strength (14.4 MPa vs. 43 MPa). Structural analysis revealed unreacted crystalline phases and limited amorphous gel. While Aachen GP meets regulatory strength thresholds (≥8 MPa) for low- to intermediate-level wasteforms in Belgium and Germany, its elevated porosity may impact long-term containment. Further studies on radionuclide leaching and durability under thermal and radiation stress are recommended.