The paper summarizes changes in slag microchemical composition and its hydration products in Portland cement self-compacting concrete after 15 years of exposure in water under laboratory conditions. Scanning electron microscopy (SEM) and X-ray microanalysis (EDX) techniques show the results of the degree of hydration of blast furnace slag particles. The experiment showed a connection between the slag content and the hydration process. It was verified that the average values of the base-forming oxides ratio (MgO + Al2O3)/CaO are inversely proportional to the slag content in the mixture. The results of these procedures provide valuable information not only in the long-term predictions of the effect of slag and other additives in cement concrete, but also in the evaluation of slag hydration in historical concrete structures.