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/m 3 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.