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
Sanjida Khair, SM Arifur Rahman, Faiz Uddin Ahmed Shaikh, Prabir Kumar Sarker
Abstract
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.