Abstract Manager

As efforts towards sustainable construction intensify, recycled concrete aggregate (RCA) offers a viable and sustainable alternative to natural aggregates in construction. However, its mechanical properties, primarily high-water absorption and low specific gravity, impede widespread adoption. Scholars attribute these property deficiencies to the presence of adherent mortar and multiple interfacial transition zones (ITZ) within RCA particles. Chemical treatments have been investigated to eliminate adherent mortar and improve RCA properties. Strong acids such as HCl and H₂SO₄ effectively remove mortar but introduce harmful chloride and sulfate ions that compromise concrete durability. As an alternative, studies suggest adopting weak acids, such as acetic acid, to effectively remove the attached mortar at optimized concentration. However, critical gaps remain regarding the treatment selectivity of acetic acid treatment and whether it preferentially attacks the adhered mortar or its effects on the underlying natural aggregates. There remains limited quantitative microstructural evidence distinguishing treatment effects across RCA constituents. This ultimately affects the mechanistic optimization of treatment protocols. This study adopts the scanning electron microscopy (SEM), paired with energy-dispersive X-ray spectroscopy (EDAX), to investigate the microstructural alterations in RCA following treatment with a 0.2 mol acetic acid solution for 24 hours. Three RCA samples from different sources were analyzed at magnifications between 50x and 1300x, with special focus on the mortar (M), old aggregate (OA), and ITZ areas. Elemental mapping measured changes in atomic ratios, particularly Ca/Si and (Al+Fe)/Ca, to evaluate phase transformations and damage patterns. The study’s findings challenge conventional assumptions about weak acid selectivity. Although acetic acid effectively degraded the adhering mortar, depleting Ca/Si ratios to below 2.5 and (Al+Fe)/Ca ratios above 0.4, extensive deterioration occurred in the old aggregate zones. SEM imaging reveals extensive crack patterns in both aggregate and mortar zones, with patterns depending on the chemical composition of aggregate constituents. While some natural aggregates showed extensive cracking comparable to mortar deterioration, others remained relatively intact. The ITZ also consistently exhibited substantial damage across all samples. This study concludes that acetic acid treatment is less selective than previously assumed, as both treatment efficacy and aggregate damage strongly depend on the mineralogical composition of the original aggregate within the RCA. The study provides conclusive microscopic evidence to optimize chemical pretreatment protocols and suggests that purely mechanical treatments may offer more predictable outcomes for RCA improvement.