The sustainable use of resources requires new strategies to transform industrial byproducts into raw materials for other applications. This approach, inspired by circular economy fundamentals, can be successfully applied in transportation infrastructures where a significant amount of natural raw materials is generally required. Slags from the steelmaking process in electric arc furnaces are generally of two types: oxidizing (black colored) and reducing (lighter colored). In Portugal, the oxidizing steel slag or electric arc furnace (EAF) with controlled production has been certified as “inert steel aggregate for construction,” having high strength and stiffness when compared to conventional natural aggregates. However, because of the lack of fines, EAF usually requires a milling process with increased costs and CO2 emissions. This work studies the mixture of the two types of slags, because the reducing slag (also called ladle slag) is a fine powder with cementing properties that can enhance the behavior of the mixture while avoiding the milling process. Toward the application in granular layers of transport infrastructures, the testing procedures to study the geotechnical behavior of this innovative stabilized mixture are discussed combining chemical, durability, environmental, and hydromechanical analysis. Seismic wave velocity measurements with ultrasonic transducers provide the elastic stiffness evolution with time derived from the cementation given by the ladle slag. Unconfined compression strength and permeability results were used to propose an index parameter to correlate the mixture dosage to the observed hydromechanical performance. Durability measurements with wetting and drying cycles, as well as swelling and leaching test results, are also presented.