Enhancing the performance of recycled aggregate concrete through optimized pretreatment methods: a microstructural perspective

Using recycled concrete aggregates (RCA) in new concrete production presents a sustainable solution to construction waste challenges, yet the inherent weaknesses of the interfacial transition zone (ITZ) between old and new cement matrices remain a critical limitation. This study evaluates three industrially feasible pretreatment methods-sieve-washing, water immersion, and resin impregnation - through a novel combination of semi-quantitative microstructural analysis and durability testing. Our findings demonstrate that resin treatment reduces carbonation depth by 38% compared to untreated RCA, primarily through pore structure modification. At the same time, water immersion enhances ITZ density by optimizing moisture release during hydration. However, resin-treated aggregates exhibited significantly lower compressive strength due to poor adhesion of the new paste. After 28 days, concrete with saturated recycled aggregate showed similar compressive (49.1 ± 0.7 MPa) and flexural strength (5.8 ± 0.3 MPa), but slightly higher carbonation depth than the concrete with only natural aggregate. The calcium concentration at a distance from the recycled grain indicates that saturated and resin pre-treatment influence the ITZ's density best. The research establishes a practical framework for selecting pretreatment methods based on measurable ITZ characteristics and project-specific durability requirements, advancing academic understanding and practical implementation of sustainable concrete technologies.