Leaching behavior and alkali activation potential of copper and black slags: Compliance and pH-dependence test, and development of alkali-activated pastes

This study performed a mineralogical and environmental characterization of copper slag (CS) and black slag (BS) in their untreated powdered forms. The environmental evaluation focused on the leaching behavior of constituents of potential concern (COPC), using pH-dependent and compliance leaching tests. Under acidic conditions, CS showed a high release of Zn, Cu, Pb, and Ba, while BS exhibited elevated levels of Ba, Cu, V, and Cr. At highly alkaline pH, CS showed an approximately 50 % reduction in Zn and Pb release, although Cu release increased by over 35 %. In BS, increasing the pH resulted in reduced leaching of all analyzed elements, with most remaining below inert waste limits, except for Ba, Cr, Hg, Se, and V. Based on the compliance leaching test results, CS was classified as inert waste, and BS as hazardous waste. Following this initial assessment, both slags were alkali-activated (AA) using solutions of sodium hydroxide (NaOH) and sodium silicate (SS) to assess their performance as precursors for alkali-activated materials. In terms of mechanical behavior, CS pastes reached compressive strengths of about 20 MPa at 3 days and more than 33 MPa at 60 days. BS pastes achieved compressive strengths close to 4 MPa at 3 days and exceeded 25 MPa at 60 days. These compressive strengths show potential for use in prefabricated non-structural elements, such as masonry blocks and bricks.