To exploit Electric Arc Furnace (EAF) slag in civil engineer applications, such as land filling, road constructions and concrete production, chemical and structural stability are fundamental requirements. In fact, steel slag may suffer from weathering processes (hydration, dissolution,...) that induce the leaching of heavy metals (Cr, Ba, V, etc.) that can be dangerous to humans and the environment. Thus, the interaction between slag and water is key, in order to classify the slag as a safe raw material. In this work, the investigation on the main factors ruling chemical leaching on about seventy EAF carbon steel slag from different steel grades productions was performed. Elution test on slag bulks and powders were performed and the concentration of leached elements was measured by ICP-OES. Morphological and microstructural characterizations were performed by means of SEM and XRD analysis in order to identify the main phases constituting the slag. The leaching tests results were correlated with the chemical composition and crystalline phases featuring the slag. The survey makes possible the identification of safe chemical composition areas on the main ternary diagrams, able to transfer stability to the slag. The analyses also allowed for the identification of the phases controlling the heavy metal leaching (larnite, Mg-Cr-spinel, brownmillerite), as well as those that were not involved in dissolution phenomena (gehlenite and kirschsteinite).