Reuse of copper slag as a supplementary cementitious material: Reactivity and safety

Copper slag (CS) is a potential supplementary cementitious material (SCM). However, unified reactivity indices of different types of CS do not yet exist. In addition, little is known about the safety of reusing CS as a SCM. Here, we collected 10 CS samples, including 5 air-cooled and 5 water-cooled samples, to establish reactivity indices and assess the safety (i.e., the soundness, radioactivity and leaching of heavy metals). Geochemical modeling was subsequently used to reveal the leaching mechanisms. The results show that the 28-day reactivity indices range from 66.6-68.2% for the air-cooled samples and 67.3-74.3% for the water-cooled samples; by reducing the water content, they can be increased to approximately 90% and above 95%, respectively. Neither the air-cooled nor the water-cooled CS causes soundness problems, but at a 50% replacement level, they both cause radiological hazards. Moreover, reuse of CS as a partial cement replacement poses leaching risks of Cu, Pb, Zn, Ni and As. The geochemical simulation indicates the important role of the Fe element in stabilizing the heavy metals, since Cu, Pb, As, Cr and Mn heavy metals are all controlled by Fe-related phases (specific adsorption and co-precipitation). In addition, some heavy metals can also be controlled by oxides, silicates and chromate precipitates. Our results extend the knowledge of using CS as a SCM and call attention to its potential hazards.