Currently, there is strong pressure to use industrial byproducts and recycled materials in the construction of transportation infrastructure and geotechnical works. The reuse of these materials positively affects the environment by reducing deposits and preserving raw materials. The related geotechnical, geoenvironmental, economical, and social issues should be addressed so that these materials can be used in construction to provide sustainable development. This paper presents a study of all of these aspects and focuses on Portuguese electrical arc furnace steel slag. A huge laboratory research program was carried out that addressed four elements of geotechnical and geoenvironmental behavior: ultimate strength under monotonic loading, resilient behavior (stiffness), susceptibility to permanent deformation due to repeated loading, and leachability. These test results were compared with those from the empirical tests used in the national specifications for embankments and structural layers of transportation infrastructures. It was concluded that performance-based laboratory test results show much better material performance than the results based on empirical tests (Los Angeles and micro-Deval). Furthermore, this material shows better mechanical performance than in the mechanical tests of natural unbound granular materials used in road construction. Additionally, leaching test results show that this byproduct is inert, which caused it to become known as “inert steel aggregates for construction” (ISAC). These laboratory conclusions were validated in a full-scale field trial by end performance testing (using devices that measure in situ stiffness through spot tests and continuous monitoring, as well as lysimeters to measure leaching values). This field trial involved raw materials and ISAC. A final remark is made about some socioeconomic aspects that should be taken into account in decision making regarding the use of ISAC in public works.