Mitigating heavy metal leaching and ASR expansion in copper heap leach residue concrete using cement and cement-fly ash-silica fume blends: Experimental and microstructural insights

Copper heap leach residue (CHLR) is generated after copper is recovered from low‑grade copper ores. The CHLR leachate indicated a very high concentration of heavy metals above the threshold requirement of US EPA 2009 and Australian drinking water guideline. In this study, 25–75 % CHLR was repurposed as a partial replacement of natural aggregates in concrete, and heavy metal concentrations in the leachate of concrete met the drinking quality standards. However, concrete containing 50 % CHLR coarse and fine aggregates were 52.9 MPa and 54 MPa after six months, indicating 10.5 % and 8.6 % reduction in compressive strength compared to the control with corresponding alkali-silica reaction (ASR) expansion of 0.072 % and 0.094 %, respectively. The cement content of these concrete mixes was replaced with 30 % fly ash and 5 % silica fume, and the compressive strength of 50 % CHLR coarse and fine aggregates was 48.7 MPa and 48.4 MPa after six months, representing 2.4 % and 3 % less compared to control, while the corresponding ASR expansion was reduced to 0.035 % and 0.058 %, respectively. The BSE-EDS and nanoindentation on the interfacial transition zone of the samples containing 50 % CHLR and 35 % pozzolans produced a higher volume of high-density C-S-H to resist the ASR expansion, improved strength development, and lowered embodied carbon and energy of these concretes. • 100 % OPC concrete containing 25-75 % CHLR showed low heavy metal leaching, which was below US EPA and ADWG limits. • 30 % FA and 5 % SF blended cement mixes reduced ASR expansion in CHLR aggregate concretes. • FA and SF modified concretes produced more HD and UHD C-S-H compared to OPC mixes, improving microstructure and ASR expansion.