Fe-modified FA-OPC geopolymers for energy-dependent X-/gamma-ray shielding: Limitations and future research

Hybrid fly ash (FA)-ordinary Portland cement (OPC) geopolymers were modified with metallic iron (Fe) to develop multifunctional, lead-free materials for structural radiation-shielding applications. FA and OPC were blended at a 1:1 mass ratio with Fe additions of up to 15 wt%. The composites were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), compressive strength testing, and radiation attenuation measurements under diagnostic X-rays (50-90 kVp) and ¹³⁷Cs gamma rays (0.662 MeV). Fe incorporation progressively densified the geopolymer matrix, resulting in an approximately 184% increase in 28-day compressive strength, from 45 MPa for the Fe-free geopolymer to 128 MPa at 15 wt% Fe. Radiation-shielding performance also improved consistently with increasing Fe content; at 50 kVp, the linear attenuation coefficient increased from 0.192 to 0.230 mm^-1, while the corresponding half-value layer decreased from 3.61 to 3.01 mm. Improved X-ray attenuation was attributed to the higher effective atomic number and greater matrix compactness, which enhanced photon interactions and increased the contribution of photoelectric absorption at lower photon energies. At 0.662 MeV, the enhanced attenuation was governed primarily by Compton scattering and the increased bulk density of the Fe-modified matrix. Overall, these findings demonstrate that Fe incorporation is an effective strategy for simultaneously enhancing the mechanical performance and energy-dependent radiation-shielding capability of FA-OPC geopolymers, highlighting their potential as resource-efficient, lead-free structural shielding materials for medical, industrial, and nuclear applications.