Mechanical anisotropy and microstructural investigation of copper heap leach residue incorporated 3D printed concrete

The integration of industrial by-products into construction materials has emerged as a promising approach to enhance sustainability in the built environment. This study investigates the mechanical, microstructural, environmental, and economic performance of 3D-printed concrete (3DPC) incorporating copper heap leach residue (CHLR) as a partial or full replacement of natural fine aggregate. CHLR was incorporated at 0–100% replacement levels, and specimens were evaluated for compressive and flexural strength across different printing orientations, alongside Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTG), nanoindentation, scanning electron microscopy (SEM), and sustainability metrics. Results showed that compressive, split tensile and flexural strengths increased up to 28, 34 and 17%, respectively, at 75% CHLR replacement, achieving maximum values of 97.71, 4.06 and 7.98 MPa, respectively, before declining at full replacement. Strength anisotropy decreased notably with CHLR incorporation, with the lowest degree of anisotropy at 75% replacement, indicating improved interlayer cohesion. Nanoindentation and SEM analyses confirmed that the C75 mix exhibited the most uniform and compact microstructure, characterized by reduced porosity, improved packing, and stronger aggregate-paste interfaces. FTIR and TGA results indicated that CHLR acted primarily as an inert filler, influencing hydration through physical packing and interlocking rather than pozzolanic reactivity, while also increasing carbonate content at higher replacement levels. Sustainability analysis revealed marginal reductions in embodied carbon (around 0.5%) and embodied energy (around 1.3%) due to consistent binder content, but substantial improvements in eco-strength efficiency (22%) and cost index (24%) at 75% CHLR, driven by higher mechanical performance and lower aggregate costs. Full replacement, however, led to reduced performance despite further cost savings. Overall, a 75% CHLR replacement offers the optimal balance between mechanical performance, microstructural integrity, environmental efficiency, and economic viability, demonstrating its potential as a sustainable fine aggregate alternative for 3DPC.