Integrating Copper Slag Into Thermally Active Building Foundations: A Pathway to Sustainable Underground Energy Storage Systems

Underground thermal energy storage (UTES) integrated with building foundations is an emerging pathway to decarbonize space conditioning by shifting low-carbon heat across seasons. This review evaluates copper slag, a high-density, thermally stable byproduct of smelting, as a dual-function medium for thermally active foundations. We synthesize evidence on physicochemical, thermal, mechanical, and environmental performance, emphasizing properties most relevant to foundation-integrated sensible heat storage. Reported specific heat capacities of approximately 0.8–1.5 kJ/kg K combined with densities >3000 kg/m³ yield volumetric energy storage that can exceed typical aquifer-based systems, while cycling studies indicate stable round-trip efficiencies (≈80% over ≥100 cycles) and structural tests show that partial slag substitution in concrete (≈50%) can satisfy strength requirements. A comparative life-cycle perspective suggests that meaningful benefits can be achieved: global warming potential (GWP) reductions of 60%–74% relative to natural-gas baseline systems and 15%–20% embodied-energy savings compared to virgin aggregates, contingent upon design and electricity mix. We also identify the principal constraints to deployment, namely, heavy-metal leaching, thermo-mechanical compatibility under cyclic loads, and the absence of explicit code pathways for foundation-integrated storage, and outline mitigation strategies that span pretreatment, mix design, and containment/barrier engineering. Valorizing an industrial residue in building foundations, copper slag UTES links circular-economy objectives with practical, scalable thermal storage. Targeted research on durability, environmental safety, and standards development is now pivotal for translating this to practice.