The demand of green and recycling technology has become, nowadays, a crucial issue for the scientific community. In this work, a preliminary approach of safe and reliable “black slag” use is discussed on chemical and microstructural characterization basis. Different EAF slag type, coming from several Italian steelmaking plants, are analyzed. The characterization is aimed to explain the diffusion, dissolution and releasing mechanisms of the polluting elements that affect the black slags and avoid their recycle as alternative filler materials. Furthermore, the work focuses on the leaching inhibition of these dangerous species in the environment. Experiments involve SEM-EDS and SEM-EBS analysis and elution tests, according to the UNI EN 12457 standard, on reinforced bars slag, on slag featured by chromium high content and on slag with low FeO content. The analysis allows to recognize the phases responsible for the polluting elements releasing and to identify the phases to be promoted in order to obtain a safe and inert by-product.