Copper entrapment and immobilization during cement hydration in concrete mixtures containing copper tailings

The use of copper tailings as supplementary cementitious material can reduce the environmental impacts of concrete production and the mining industry. A key concern limiting its application is the potential leaching of toxic metals from the cementitious matrix, especially copper. To analyze and reduce the risk of leaching, the mechanisms by which copper is entrapped in the cementitious matrix were investigated, by combining microscopic and spectroscopic approaches. Decreasing the water-to-binder ratio was statistically relevant to reduce copper leaching. Scanning Electron Microscope micrographs allowed to spatially localize enriched copper clusters within the cementitious hydration products. In the early stages of the cementitious hydration (i.e., 24 h), no spatial correlation between copper and hydration products was found; however, after seven days, copper was spatially associated with calcium silicate hydrates. Cu K-edge X-ray absorption near edge structure spectroscopy provided insights into the chemical speciation of copper in the cementitious matrix. It showed that copper sulfide and oxide phases persisted, whereas the copper sulfate phases were prone to dissolution and reprecipitation as cupric hydroxides induced by the relatively high pH from calcium hydroxides formed during hydration. Promoting the formation of hydration products can further reduce copper leaching from the alkaline cementitious matrix. A better understanding of metal entrapment mechanisms could lead to new strategies that reduce the mobility of toxic elements when using copper tailings, increasing their use as a replacement of cement. With this knowledge, it is expected to answer if it is possible to improve the copper entrapment into the cementitious matrix and if there is a risk of leaching once is entrapped.