To reduce the environmental impact of steel production, multiple European companies are planning a shift toward electric arc furnace (EAF) process using hydrogen-based direct-reduced iron (H₂-DRI) as a raw material. The changes in the process and the raw materials are reflected on the generated slags, and ideally the slag could be used as a supplementary cementitious material. However, original air-cooled EAF slag has poor cementitious properties, so an amorphous material must be generated by means of slag modification. In this study, an H2-DRI-based EAF slag was modified using mine tailings and rapidly cooled by water granulation. Combination of modification and partial reduction of iron oxides by carbon produced a highly amorphous material, which is promising for the valorization of the slag. Additionally, the possibility of using electron backscatter diffraction technique to quantify the amorphous phase content was studied and compared to conventional quantitative x-ray diffraction. While the obtained percentages differed depending on the method, the trend of increased amorphous content with increasing modification was clear with both methods.