Impact of modified aggregate gradation on the workability, mechanical, microstructural and radiation shielding properties of recycled aggregate concrete.

The increasing need for sustainable construction practices has driven research into the utilization of recycled concrete aggregate (RCA) in new concrete mixes. However, the variable properties of RCA, particularly its gradation, can significantly impact the performance of the resulting concrete. This study investigates the impact of modified aggregate gradation on the properties of concrete incorporating 100% RCA. Three concrete mixes were prepared: a control mix with the original RCA gradation (RCA1) and two modified mixes (RCA2 and RCA3). The initial gradation was modified by enhancing the 10% passing the #1″ sieve (S1) and diminishing the combined 10% passing the #1″ sieve and 10% passing the #4″ sieve (S2). Subsequently, the effects of these modified gradations were assessed, encompassing workability, mechanical properties (compressive and flexural strength), microstructural characteristics (XRD, SEM, and EDX), and radiation shielding performance. Results demonstrate that, despite reduced workability, the modified mixes exhibited enhanced compressive and flexural strengths. Microstructural analysis revealed denser, more interconnected cement paste matrices and refined interfacial transition zones (ITZ) in the modified mixes. Furthermore, the radiation shielding effectiveness increased with increasing gradation modification (RCA1 < RCA2 < RCA3). Specifically, RCA3 exhibited the lowest half-value layer (HV), tenth-value layer (TV), and mean free path (MF), along with the highest average macroscopic removal cross-section (FCS). These results suggest that modifying the gradation of RCA can effectively improve the mechanical and radiation shielding properties of concrete made with 100% RCA. This enhanced performance makes RCA concrete a viable option for radiation shielding applications in facilities like hospitals and nuclear power plants, contributing to both sustainable construction and improved safety.