In the metallurgical industry, large amounts of waste called slag are accumulated besides the production of iron and steel. One part, considered as by-product, is recycled but the other part, considered as waste, is still landfilled with potential bad consequences for environment. In this paper, two electric arc furnace slag samples (most common steel production technology in Europe), have been considered and characterized to be used in medium and high temperature thermal energy storage systems. These slags have demonstrated relevant properties to store thermal energy by sensible heat from ambient temperature up to 1000 °C, and their representativeness of the worldwide produced slag in a wide EAF (electric arc furnace) steelmaking process range. The objective is to develop sustainable and low-cost thermal energy storage systems for industry waste heat recovery and in renewable energy applications. At the same time, this new valuable market for slag in the energy field could solve a big part of the waste management problem in the iron and steel sector. © 2015 Elsevier Ltd.