Acid-Activated Copper Slag as a Cementless Binder: Microstructural Densification and Strength Development

This study provides a proof of concept for the acid-induced hardening of iron-rich copper slag, an iron-silicate-rich industrial by-product of pyrometallurgical copper smelting, to investigate a fundamental pathway for understanding Fe-mediated binder formation without supplementary cementitious materials or alkaline activators. Granulated copper slag was treated with HCl solutions of varying molarities (0.03 M, 0.3 M, and 3 M) and cured at 50 °C, then evaluated over a 56-day period. The 3 M HCl-activated specimen achieved a compressive strength of 100.7 MPa at 56 days, accompanied by significant pore structure refinement as confirmed by mercury intrusion porosimetry (MIP). Comprehensive characterization using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) revealed that acid activation promotes selective dissolution of iron from the iron silicate matrix, followed by reorganization into Fe–O–Si bonding and recrystallization into secondary iron oxide phases including magnetite, hematite, and akaganeite. Strength development was strongly correlated with pore structure densification, consistent with the extended Bhattacharjee model. However, the high free chloride content (up to 3.93wt%) exceeds ACI 318 limits, restricting application to plain concrete or chloride-tolerant environments. Overall, this manuscript serves as an exploratory mechanism study demonstrating that high-strength binders can be produced solely by inducing the dissolution and reprecipitation of metal ions within copper slag.