In the article the results of laboratory research on influence of addition of mineral siliceous fly ash on concrete’s properties were presented. Fly ash has been used as a substitute of cement binder in concrete compounds because of it’s Pozzolanic properties. For research 5 cement compounds were designed: in one control concrete without the addition of fly ash, in the rest the k factor (fly ash/cement) stood at: 0,05; 0,11; 0,18; 0,25. As a cement binder, Portland cement CEM I 42,5 R was used. The results of the laboratory tests have shown the usefulness of fly ash as a substitute for cement in concrete mixes.