Comparative physicochemical and thermal stability screening of steelmaking slags for high-temperature thermal energy storage applications

The intermittency of renewable energy sources has intensified the demand for cost-effective, high-temperature thermal energy storage (TES) materials. Industrial steel slags — abundant by-products of steelmaking — represent a compelling sustainable alternative to conventional storage media such as molten salts. This study presents a systematic comparative physicochemical and thermal stability screening of three industrially sourced steel slags — Ladle Metallurgy Furnace (LMF), Basic Oxygen Furnace (BOF), and Electric Arc Furnace (EAF) — to evaluate their preliminary suitability as candidate materials for high-temperature TES applications. A multi-technique analytical framework was employed, integrating X-ray fluorescence (XRF), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), thermogravimetric analysis with derivative thermogravimetry (TGA/DTG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). All three slags are predominantly composed of Fe₂O₃, CaO, and SiO₂, collectively accounting for approximately 70–88 wt% of the oxide content. This oxide profile suggests potential relevance to high-temperature TES screening, as Fe-, Ca-, and Si-bearing phases can contribute to phase stability, heat-storage potential, and potential thermal transport pathways. EAF slag exhibited the lowest total mass loss, approximately 0.9–1.0 wt% up to 800°C, together with a comparatively stable crystalline framework characterized by minimal detectable free lime, a belite–magnetite–RO phase assemblage, and a dense Fe-rich microstructure. These features indicate that EAF slag is a promising candidate for further evaluation as a high-temperature TES medium. LMF slag showed intermediate thermal stability, with a total mass loss of approximately 1.8–1.9 wt%, while BOF slag exhibited the highest mass loss, approximately 3.4–3.5 wt%, associated with portlandite, carbonate phases, and free-lime-related instability risks. Based on the indirect physicochemical and thermal stability indicators evaluated in this work, the preliminary TES suitability ranking is EAF ' LMF ' BOF. However, direct measurements of thermal conductivity, specific heat capacity, density, and long-term cyclic stability are required before definitive conclusions can be drawn regarding practical TES performance.