The European Union is supporting the transition towards a more circular economy. Waste management is one of the main points in the circular economy. In this way, the steelmaking industry generates huge amounts of waste. In Spain and Italy most of the produced steel is manufactured in electric arc furnaces (EAF), so the main waste resulting from this industry are EAF slags. Several researchers have studied their use in hydraulic mixes, mortar and concrete, showing that their mechanical properties and durability are similar to mixes manufactured with natural aggregates, or even better. In spite of these promising results, still few producers allow this by-product to be applied in real construction works, because of a lack of reliable standards. For the standardization of slag use in civil engineering applications, it is necessary to gain further knowledge about the physical and chemical properties of EAF slag, and its influence in the properties of hydraulic mixes. Accordingly, this research aims to study in detail chemical and physical properties of black (oxidizing) slag, coming from different sites both in Spain and Italy. Additionally, the performances of concretes manufactured with these slags are compared, showing the influence of slag properties on concrete behavior.