Accelerated tests were conducted to evaluate the potential reactivity of one of the potentially dangerous aggregates from the point of view of combination with high alkaline cement with the measurement of linear expansion deformations in order to study the impact of nano-silica on the process of alkaline corrosion of the concrete and formation of cement matrix. It was revealed by using electron microscopy that the addition of nano-silica in concrete mixture promotes the formation of alkali-silica gel in the cement matrix, but not in the pores of the sample, which contributes to slowing, but not prevention of an alkaline corrosion of concrete.