Radiation shielding and microstructural characteristics of nano-silica and nano-alumina modified cement composites.

The increasing use of nuclear technology in medicine, industry, and energy requires effective durable radiation shielding. This study aimed to develop and characterize nano-modified cementitious composites for enhanced gamma-ray shielding with quantitative structure-property relationships. Two-year-aged cementitious composites were prepared by partially replacing ordinary Portland cement with 5 wt% nano-silica or nano-alumina, labeled Si-C and Al-C, respectively, alongside unmodified cement (B-C) as a reference. Comprehensive multi-scale characterization included density measurement, surface area and porosity analysis (BET), particle size and colloidal stability assessment (DLS and zeta-potential), phase analysis (XRD), morphological observation (SEM-EDX), and chemical bonding analysis (FTIR). Density values were 2.78, 2.81, and 2.92 ±0.01 g·cm[Formula: see text] for Si-C, B-C, and Al-C, respectively. BET analysis showed that Si-C and Al-C have 3.1-4.4-fold higher surface areas (290.11-416.07 m[Formula: see text]/g) and smaller pores (6-9 nm) than B-C (94.64 m[Formula: see text]/g, 28.82 nm). DLS/zeta-potential measurements showed particle sizes of 48.4 ±2.1 nm (Si-C) and 78.8 ±3.5 nm (Al-C) with -15.6 to -17.8 mV zeta-potentials versus B-C (1718 ±35.2 nm, -2.0 mV), confirming enhanced electrostatic stabilization and nano-modifiers dispersion. Quantitative XRD phase analysis revealed that Si-C exhibited significantly higher tobermorite content (53.6%) compared to B-C (37.7%) and Al-C (35.5%), indicating enhanced pozzolanic reactivity and C-S-H gel formation. Gamma-ray shielding parameters-including linear and mass attenuation coefficients (LAC, MAC), half- and tenth-value layers (HVL, TVL), effective atomic number (Z[Formula: see text]), and exposure and energy absorption buildup factors (EBF, EABF) were evaluated over an energy range of 1 keV to 100 GeV. Calculations were performed using the Py-MLBUF (Python Machine Learning Buildup Factor) code, and the Py-AMA.Seidy model, validated against NIST XCOM data (differences <0.4%). Al-C showed the highest LAC (53.37 cm[Formula: see text] at 0.015 MeV) and the lowest HVL and TVL, consistent with its highest density and increased Al/Fe content. The Z[Formula: see text] values ranged from ∼ 11.8 to ∼ 17.3, with Al-C exhibiting the highest values. Buildup factors (EBF/EABF) at 1 mean free path (mfp) were lowest for Al-C, indicating reduced secondary photon contribution. Double-layer shielding analysis revealed that placing B-C as the first layer, followed by Si-C or Al-C reduced double-layer buildup factors by 15-25% compared to the reverse order. Microstructural characterization confirmed that nano-silica promoted a dense, homogeneous C-S-H-rich matrix with high tobermorite content, while nano-alumina increased density and promoted C-A-S-H formation. The established structure-density-shielding relationships demonstrate that Al-C is a promising candidate for advanced radiation shielding in nuclear, medical, and industrial facilities.