Sequential triple treatment of recycled coarse aggregates: mechanical, durability, and microstructural performance of concrete with mechanistic insights

The weak interfacial transition zone (ITZ) and high porosity of recycled coarse aggregates (RCA) limit their use in structural concrete. A sequential triple-treatment: HCl chemical treatment, mechanical abrasion, and cement slurry coating, was developed to densify the ITZ. Testing 198 specimens across six mix designs (0–100% RCA), TTRCA at 25% and 50% retained 92.1% and 87.3% of compressive strength, 93.6% and 87.7% of flexural strength, and 95.6% and 89.9% of splitting tensile strength versus the NCA control. Resistivity reached 81–91% of control; shrinkage remained 8–18% above control. XRD confirmed reduced Ca(OH)2 content with enhanced C–S–H formation; SEM revealed reduced ITZ porosity. ITZ densification was identified as the primary governing mechanism, supported by microstructural, mechanical, and durability evidence. Statistical analysis confirmed significant treatment effects (ANOVA, p < 0.001). A critical 50% replacement threshold was established, supporting regulatory RCA limit revision and circular economy goals.