Multiscale Analysis of Low-Concentration Acid Soaking for Adhered Mortar Removal in RCA

Abstract: The utilization of recycled concrete aggregates (RCA) in construction has garnered significant attention as a sustainable alternative to natural aggregates (NA), offering both environmental and economic benefits. However, the presence of adhered mortar on RCA surfaces presents substantial challenges, potentially compromising the mechanical and durability properties of concrete incorporating these materials. Traditional methods for removing adhered mortar, such as mechanical abrasion and thermal treatment, have demonstrated inconsistent results and operational inefficiencies. Among the various methodologies explored, acid soaking has emerged as a promising approach for the removal of adhered mortar. The method is specifically suited in the Indian context, where granite predominantly serves as the parent NA in concrete production. The inherent resistance of granite to acid attack renders it relatively inert to the deleterious effects of mild acidic environments. Consequently, its application to RCA derived from granite-based concrete will selectively target and dissolve the calcium-rich adhered mortar, minimally affecting the underlying granite aggregate. This selective dissolution mechanism potentially allows for the efficient removal of cementitious materials without compromising the fundamental physico-mechanical properties of RCA. However, the optimal parameters for acid soaking—including acid type, concentration, duration, and aggregate size—remain underexplored. Previous studies have primarily focused on high-concentration soaking or extended soaking periods, which may compromise aggregate integrity and pose environmental concerns. This study aims to address these knowledge gaps by evaluating the effectiveness of low-concentration 24-hour acid soaking using a novel multiscale approach for adhered mortar removal quantification (MAAMQ). The study investigated the efficacy of sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and phosphoric acid (H₃PO₄) simultaneously varying the concentrations at 0.1 M, 0.25 M, and 0.5 M on RCA size fractions of 31.5–26.5 mm and 26.5–19 mm. The comprehensive approach allows for a nuanced understanding of the interplay between acid type, concentration, and aggregate size in the treatment process. The methodology employs an image analysis method (IAM) and scanning electron microscopy (SEM) to quantitatively assess the extent of mortar removal and changes in surface morphology, respectively. Tests such as weight reduction, thickness index (Ti), and specific gravity of RCA were performed to identify the changes in physical parameters. Furthermore, a statistical analysis was performed to validate the relationships between fixed variables (RCA size, acid type, and acid concentration) and dependent parameters (percentage mortar removal (IAM), weight reduction, thickness index, and specific gravity). This rigorous approach enhances the reliability and generalizability of the outcome of this investigation. By optimizing the acid soaking parameters for RCA, this study aims to contribute significantly to the development of more effective and environmentally sustainable recycling practices in the construction industry. The insights gained from this research have the potential to improve the quality and performance of RCA in concrete applications, thereby advancing the broader goal of sustainable infrastructure development.