In this study, the effects of clay content in aggregates used in conventional concrete production on concrete properties were experimentally investigated in order to determine how the amount of clay in different ratios in the aggregate affects the mechanical properties of concrete. For this purpose, five series of standard cube and beam samples with clay substitution of 5%, 10%, 15%, 20% of the fine aggregate amount in various proportions were produced together with the reference concrete samples. Within the scope of the studies, consistency tests were carried out on fresh concrete (slump), and compression strength tests with ultrasonic pulse velocity on hardened concrete cube samples and bending tests on beam samples were performed. According to the results of the experiments, it was determined that the workability level increases with the increase of the clay ratio in fresh concrete, and the pressure and bending strengths of the hardened concrete decrease significantly as the amount of clay increases. It has been determined that with the increase in the amount of clay-silt, the amount of settling in fresh concrete increased up to 43%, the compressive strength decreased by 39% and the bending strength decreased by 84%, and it was determined that the ultrasonic pulse velocity decreased with the increasing clay ratio.