Research has been carried out on multicomponent slag from out-of-furnace steel processing with the purpose of its use as an active mineral additive replacing a part of cement. The tricalcium form of C3S silicate is represented in the form of three polymorphic structures: triclinic, monoclinic and rhombohedric in the total amount of 10.28%. The high-temperature form of α-C2S is represented in the form of “traces”. Most likely, it is the β + α form located within β-C2S. The total β-C2S content was 13.26%. One third of the composition is γ-C2S, it is considered inert in natural conditions as it has a very stable crystalline structure that does not react with water. The total calcium silicate content was about 23–24%. Aluminate phases Mayenite C12A7 and tricalcium aluminate C3A cubic were detected. Tricalcium aluminate hydrates with water to form calcium hydroaluminates, which harden rapidly, but the hardening products have low strength. Mayenite plays an important role in retarding the early hydration of cement with steel slag because it retards the precipitation of portlandite (CH) and C-S-H [8, 14]. Wollastonite 2M Ca3(Si3O9), Gehlenite C2AS, merwinite Ca3Mg(SiO4)2 and microcline maximum KAlSi3O8 are found in small amounts and have no binding activity at normal concrete curing temperatures. Periclase MgO content in the mixture is not more than 5%. Dolomite Ca,MgCO3 is an inert substance, so it does not participate in the hydration process. Iron alpha and Iron Silicide cubic are residues from magnetic separation of iron by iron separators of initial slag. Hydration activity of calcium ferrite C2F is insignificant, increases when their structure is modified with sulfate ion and basicity increases. When studying the strength of cement-sand compositions with different content of slag it was shown that the compositions with pure slag at the age of 28 days have 9.5MPa compressive strength, which meets the requirement of GOST R 56592-2015 on the strength for active mineral additives with binding properties. Replacing 25% of cement by weight with slag gives strength at the age of 28 days 41MPa compared to 42.2MPa of the composition without slag. When obtaining low-grade binders it is possible to save up to 50% of cement with compressive strength not less than 25MPa. A sharp increase in the strength of compositions “cement + slag” in the early age gives reason to believe that the surface of slag grains is covered with silicon-oxygen particles of amorphous structure, which interact with cement, playing the role of alkaline activator, with the formation of hydrosilicates. At a later age slag begins to show hydraulic properties.