Mechanical performance of structural concrete incorporating mild acetic acid-washed coralline limestone aggregate

Coralline limestone aggregate (CLA) from Timor Island is locally abundant, but its porous biogenic structure and weakly bonded surface deposits restrict its use in structural concrete. This study evaluated mild acetic acid washing as a pre-treatment process using concentrations of 1%, 3%, and 5% and soaking durations of 30, 60, and 90 min. Aggregate responses were assessed through mass loss, specific gravity, water absorption, aggregate crushing value (ACV), XRD, XRF, and SEM-EDS. Concrete containing treated CLA as 100% of the coarse aggregate was tested for slump and 28-day compressive and splitting tensile strengths. Acid concentration was the dominant factor, and 94.3–96.5% of the total 90-min mass loss occurred within the first 30 min. The optimum treatment was C1-T30 (1% for 30 min), which best balanced surface-deposit removal, limited pore development, and preservation of crushing resistance. All mixtures achieved slumps of 90–110 mm. C1-T30 concrete reached compressive and splitting tensile strengths of 41.1 and 2.52 MPa, respectively, compared with 25.9 and 1.99 MPa for the water-washed control, representing increases of 58.7% and 26.6%. XRD confirmed that the CLA remained calcite-dominated, while XRF indicated a relative reduction in sediment impurities containing Fe–Al–Si-rich constituents, supporting selective cleaning and improved aggregate–matrix interaction. SEM-EDS showed a more clearly exposed Ca-rich carbonate surface for C1-T30, whereas C5-T90 displayed more extensive open cavities and locally eroded regions. Using the cradle-to-gate assumptions, locally treated CLA reduced embodied carbon by an estimated 11.78 kg CO₂e/t, or 49.4%, relative to the use of conventional aggregate.