This paper presents the results of tests of selected physical and mechanical properties as well as the chemical composition of two types of natural aggregates: porphyry and diabase, as well as artificial aggregate based on steel slags. Based on the conducted tests, it was established that the physical and mechanical properties of the artificial aggregate exhibit slightly lower parameters as compared to the results obtained for porphyry and diabase aggregates. However, this does not limit the possibility of using the aggregate based on steel slags, as according to the applicable WT-4 and WT-5 standards, it can be used in mixtures unbound to the improved subsoil and layers of the road foundation as well as road mixtures with hydraulic binders for each category of traffic load. The chemical composition of the aggregate based on steel slags differs from the chemical composition of the tested natural aggregates. The slags contain lower amounts of SiO2 and Al2O3, while the concentration of CaO and Fe2O3 is greater. Additionally, heavy metals have also been exhibited in the slags. However, it was established that the alkaline nature of the slags, which is affected by low sulphur content and a significant proportion of CaO, as well as the way the metals occur limit the possibility of heavy metals release and migration from slags. The tested steel slags may constitute a prospective material used in road construction.