In order to evaluate the usefulness of unground granulated slag for the concrete production, as the partial replacement of fine aggregate – namely sand, the detailed studies of the influence of this addition on the properties of the fresh and hardened concrete were conducted. These studies included the compression strength and the modulus of elasticity determination, as well as the evaluation of durability by the determination of frost resistance. The evaluation of the properties of concretes hardened under winter conditions, at +10oC and summer conditions at +20oC. These studies also covered the resistance to chloride attack and carbonization. The long-lasting examination covered also the ASR. At the end, under real construction conditions in the Austrian prefabrication plant, the unground granulated slag was examined. Under these conditions especially the long-lasting measurements of the strength development of the precast elements were verified. In order to evaluate the usefulness of the unground granulated slag, as partial replacement of fine aggregate, the concretes with the additions of slag of 10% and 15% were produced and examined. For this evaluation, the slags of two Austrian plants from Upper Austria and Styria were used. The old and fresh slags from these two plants were used because the old slags can also be adopted for concrete production. Presented in the paper results are confirming that the natural aggregate of the fraction < 4 mm can be successfully replaced by unground granulated slag, without any negative influence on the properties of concrete.