# Green Mild Acid Treatment of Recycled Concrete Aggregates: Concentration Thresholds for Mortar Removal While Avoiding Degradation of Original Limestone Aggregate and Concrete

## TL;DR

This study finds that using acetic acid to clean recycled concrete aggregates can damage the original stone if the concentration is too high.

## Contribution

The study identifies optimal acetic acid concentration thresholds for mortar removal without damaging limestone aggregates in recycled concrete.

## Key findings

- Acetic acid concentrations above 0.4 M cause significant erosion of limestone aggregate surfaces.
- Using 0.1 to 0.3 M acetic acid avoids aggregate degradation and maintains concrete strength.
- High acid concentrations increase water absorption and reduce concrete workability and compressive strength.

## Abstract

While acetic acid has proven effective as a mild acidic treatment for removing adhered mortar from recycled concrete aggregate (RCA) surfaces, its potential for dissolving damage to the surface of the original natural coarse aggregate (NCA) within the RCA and its impact on the resultant concrete properties require careful consideration. This investigation systematically evaluates the effects of varying concentrations of dilute acetic acid solutions, commonly used in RCA treatment protocols, through a multi-methodological approach that includes comprehensive physical characterization, stylus and 3D optical profilometry, scanning electron microscopy (SEM), and nanoindentation analysis. The results show that even dilute acid solutions have an upper concentration limit, as excessive acid concentration, specifically 0.4 M, induces significant textural dislocations on NCA surfaces, creating millimeter-scale erosion pits that increase aggregate water absorption by 18.5%. These morphological changes significantly impair concrete workability and reduce compressive strength performance. Furthermore, microstructural analysis reveals a 45.24% expansion in interfacial transition zone (ITZ) thickness, accompanied by notable reductions in elastic modulus and microhardness characteristics. In practical RCA treatment applications, for RCA containing limestone-based NCA, it is recommended to use acetic acid concentrations between 0.1 and 0.3 M to avoid substantial physical and microstructural degradation of aggregates and concrete.