This paper shows that the Electric arc furnace (EAF) slag is a byproduct material composed mainly from different types of calcium silicate minerals. This study deals with chemical, mineralogical, microstructure and geotechnical impacts of Egyptian EAF slag at slow cooling rate. From the boiling test it was clear that the percent of Free CaO and/or MgO in Egyptian EAF was negligible, so that they have volume stability to be used as construction materials. The microstructural behavior and free swelling parameters of slag-treated expansive soil with or without lime activation are also examined. XRD analysis of slag treated clay showed the existence of calcium silicate hydrate (C–S–H), calcium aluminate hydrate (C–A–H), which facilitated the strength increment. Lime activated cooled EAF slag is more effective in soil improvement, where swelling percentage decreases to 10% compared to the native soil, which recommended that EAF slag is used as additive in soil improvement.