Low-Concentration Acid Treatment of Adhered Mortar in RCA: A Multiscale Study Using Image Analysis
Pranav Saraswat, Mordhwaj S. Parihar, K.C. Sandeep, Bhupendra Singh
Abstract
This study presents a multiscale approach to assess the effectiveness of a 24-h low-concentration acid treatment (≤0.5 mol/L) for removing adhered mortar from recycled concrete aggregates (RCA). The investigation includes three acids (HCl, H3PO4, and H2SO4) at concentrations of 0.1 M, 0.25 M, and 0.5 M on two RCA size ranges (31.5–26.5 mm and 26.5–19 mm). The efficacy of the treatment was analyzed using image analysis methods (IAM) and scanning electron microscopy (SEM) to provide qualitative insights into mortar removal and surface morphology changes. Physical parameters such as weight reduction, thickness index (Ti), and specific gravity of RCA were also measured. Statistical analysis through MANOVA was conducted to evaluate the influence of RCA size, acid type, and concentration on mortar removal efficacy. Results indicate weight reduction ranged from 10% to 38%, with Ti varying between 0.2 and 0.6. Specific gravity after treatment ranged from 2.17 to 2.69 for RCA sizes 31.5–26.5 mm and from 2.17 to 2.49 for sizes 26.5–19 mm. IAM confirmed that 0.1 M HCl achieved a maximum mortar reduction of 42.6% and 57.3% for 31.5–26.5 mm and 26.5–19 mm, respectively. SEM identified Ca(OH)2 in RCA, explaining HCl’s superior mortar dissociation compared to H3PO4 and H2SO4. Higher acid concentrations (0.5 M) reduced efficiency due to surface saturation. Statistical analysis confirmed that RCA size, acid type, and concentration significantly influenced mortar removal. These findings suggest that 0.1 M HCl offers an effective and improved solution for enhancing the quality of RCA. This treatment will improve RCA’s performance in sustainable construction applications and contribute to more efficient recycling practices in the construction industry.