The article presents an analysis of the impact of fly ash addition, which originated from the combustion of sewage sludge in sewage treatment plants, on the properties of hardened concrete. The composition of control concrete mixture was designed using the experimental method, and then it was modified using fly ash. For all concrete series obtained, tests were performed: compressive strength of concrete after 7, 28 and 56 days of maturation, frost resistance for 100 cycles of freezing and thawing, and absorbability. Ash-modified concretes formed from the combustion of sewage sludge were characterized by comparable compressive strength in comparison to the control concrete and recorded a lower drop in compressive strength after the frost resistance test.