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The use of ceramic solid particle technology in TES-integrated CSP plants offers a high solar-to-electricity ratio and enhanced storage densities, thanks to their high operational temperatures and wide temperature ranges. Metallurgical slags with composition similar to that of the state-of-art bauxite particles can be used as a sustainable and economical secondary raw material to prepare solid particles. In this study, the as-received state and the high-temperature phase and microstructural changes of two fayalite slags from copper and lead production were elucidated by XRD and SEM/EDS methods in a comparative manner. Solid particles were prepared from slags by the oil dropping method, with subsequent heat treatment. Solar-thermal-application-related functional properties of slag particles, such as heat capacity, absorptance, and thermophysical properties, were evaluated by differential scanning calorimetry (DSC), spectrophotometer, heating microscope, and high-temperature compressive tests, respectively. Owing to the formation of more stable Fe-rich phase components and less amount of glassy phase, copper slag is found to be a more promising secondary resource than lead slag in terms of material and functional properties.