Use of fly ash and ground tuff as pozzolanic additives in lightweight structural

This paper investigates the effect of partial replacement of cement with fly ash and ground volcanic tuff on the physical and mechanical properties of concrete. The main focus is on the changes in average density and compressive strength at different contents of replacement materials (from 10% to 35%). The investigated concrete composition (27PPPF) without admixtures has an average density of 1925.5 kg/m³ and compressive strength of 40.1 MPa. The results show that when fly ash is added, the concrete strength first increases, reaching a maximum value of 41.6 MPa at 10% cement replacement and then decreases to 29.1 MPa at 35% replacement. A similar trend is observed when tuff is introduced, but the peak strength (40.7 MPa) is also reached at 10% replacement, after which the strength gradually decreases to 27.9 MPa at 35%. The average density of the specimens changes insignificantly, being in the range of 1910.4-1928.5 kg/m³, which indicates that the dense structure of the concrete is maintained. Thus, the optimum content of fly ash and tuff in the concrete composition is 10-15%, as these values provide the best mechanical characteristics. Higher dosages of substitutes result in lower strength due to thinning of cement stone and lack of binding properties. This study confirms the possibility of using fly ash and volcanic tuff as effective pozzolanic additives to improve the environmental friendliness and sustainability of concrete.