<p>This study investigated the effect of partially replacing cement with varying percentages (10% and 20%) of rice husk ash (RHA) and sugarcane bagasse ash (SCBA) on the physical properties (setting times, consistency, and workability), mechanical properties (compressive and tensile strength), microstructure (SEM, XRD, and EDX), and radiation shielding characteristics of high-strength concrete (HSC). Results demonstrated that the pozzolanic activity of the agricultural waste materials contributed to reduced setting times and enhanced cement hydration, positively impacting mechanical properties, particularly at the optimal 10% replacement level for both RHA and SCBA. At this replacement level, compressive strength was improved by 12.42% and 8.66%, respectively, while tensile strength increased by 7.84% and 3.92%, respectively. The microstructure at the optimal replacement levels also exhibited improvements compared to the control mix, as confirmed by SEM analysis. Additionally, XRD results for concrete mixes incorporating agricultural ashes revealed enhanced calcium silicate hydrate (CSH) peaks, supporting the hypothesis of their pozzolanic activity. EDX analysis results, combined with the different sample densities, served as input data for developing simulation models using Monte Carlo method and Phy-X software to evaluate the radiation shielding efficiency of the designed HSC mixes. Radiation shielding simulation demonstrated that concrete samples incorporating SCBA exhibited superior γ-ray attenuation compared to the RHA samples. Furthermore, the concrete sample with 20% SCBA admixture displayed a higher fast neutron removal cross-section than all other samples. Notably, the radiation shielding performance of HSC containing RHA and SCBA was evaluated for the first time in this study, providing novel insights into the dual mechanical-shielding functionality of sustainable concrete.</p>