Researchers have identified water immersion as a practical and effective pretreatment method for recycled concrete aggregates (RCA), significantly improving both strength and durability in recycled aggregate concrete.
Study: Enhancing the performance of recycled aggregate concrete through optimized pretreatment methods: a microstructural perspective. Image Credit: OlegRi/Shutterstock.com
A new article in Scientific Reports evaluates three industrially viable RCA pretreatment methods—sieve-washing, water immersion, and resin impregnation—using a combination of microstructural analysis and durability testing. The goal is to offer a practical framework for selecting treatments based on the measurable properties of the interfacial transition zone (ITZ) and project-specific performance requirements.
Background
Incorporating RCA into new concrete offers a sustainable way to manage construction waste. However, the weak bond at the ITZ between the old and new cement matrices continues to limit performance. To address this, various pretreatment techniques have been developed.
Mechanically, ball milling removes up to 95% of attached mortar through abrasion. Chemically, sulfuric or hydrochloric acid dissolves cementitious residue. Other methods, like microwave-assisted separation or microbial bio-deposition, alter surface properties to improve performance.
While effective in controlled settings, these methods often fall short in practical applications due to high energy demands or environmental concerns. This study, instead, focused on sieve-washing, water immersion, and resin impregnation—methods that offer a balance between performance enhancement and real-world feasibility.
Methods
Researchers prepared concrete mixes with consistent compositions using Portland cement, water, river sand, gravel, RCA, and natural aggregates. The key variation lay in the pretreatment of the RCA: one group was left untreated (air-dried), another was soaked in water for 24 hours, and the third was treated with vinyl-ester resin. A control mix with only natural aggregates served as the benchmark.
Microstructural analysis was conducted using optical and scanning electron microscopy to evaluate the ITZ and overall cement matrix. Elemental composition near the aggregate grains was also analyzed to assess changes in key components across the contact zone.
The team tested mix plasticity, flexural strength (via three rectangular specimens per mix), and compressive strength (using six cubic specimens). Durability was evaluated through a 28-day accelerated carbonation test, using phenolphthalein solution as the indicator.
Results and Discussion
As expected, adding RCA lowered both flexural and compressive strengths compared to natural aggregate concrete. However, water immersion and sieve-washing substantially improved mechanical properties, achieving strength levels close to the control mix after 28 days. Resin impregnation, on the other hand, slightly reduced compressive strength but markedly improved carbonation resistance by limiting CO2 penetration.
This revealed a notable trade-off: while resin impregnation enhanced durability, it did so at the cost of strength. In contrast, water-based treatments provided more balanced improvements in both categories.
One key microstructural insight was the inverse correlation between carbonation depth and the calcium ion gradient near the RCA surface. A steeper gradient indicated denser ITZ and more effective cement hydration—both crucial for reducing CO2 ingress. This gradient proved to be a reliable indicator of ITZ quality and overall durability.
Of the three methods, water immersion stood out as the most balanced option. It supported optimal ITZ formation by controlling moisture release during hydration. Rheological analysis showed that about 50 % of the absorbed water actively contributed to the water-cement ratio, enhancing bonding with the new cement paste.
The results suggest that pretreatment strategies can be tailored based on performance goals. Resin impregnation is well-suited for structures where carbonation resistance is paramount. Water immersion or sieve-washing, on the other hand, are better choices when mechanical strength is the priority.
Conclusion
The study offers a detailed look at how different RCA pretreatments influence strength, microstructure, and durability. Water immersion proved especially effective, enhancing ITZ density through controlled moisture dynamics. Meanwhile, resin impregnation reduced carbonation depth by 38 % compared to untreated RCA, thanks to improved pore structure—but it also weakened the paste-aggregate bond.
After 28 days, concrete made with saturated RCA achieved comparable compressive and flexural strengths to the reference mix, though with slightly greater carbonation depth. Among all variables, saturated and resin-treated aggregates had the most substantial impact on ITZ density and overall concrete performance.
These findings provide valuable insight into the microstructural mechanisms that govern recycled aggregate concrete behavior and offer a data-driven foundation for selecting appropriate pretreatment methods based on specific application needs.
Journal Reference
Kepniak, M., Chylinski, F., & Woyciechowski, P. (2025). Enhancing the performance of recycled aggregate concrete through optimized pretreatment methods: a microstructural perspective. Scientific Reports, 15(1). DOI: 10.1038/s41598-025-14834-y. https://www.nature.com/articles/s41598-025-14834-y
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