•Effect of fly ash (FA) addition on changes in the Interfacial Transition Zone (ITZ) were investigated.•Analysis of the relationship between the microcracks width in the ITZ and mechanical parameters of composites.•The environmental and utilitarian aspects were taken into account.•The 20% fly ash cement binder causes favorable changes in the mature concrete.•Ecological benefits resulting from the presented studies are given. The paper presents results of tests on the effect of the addition of siliceous fly ashes (FA) in the amount of: 0, 20 and 30% by weight of cement on the interfacial microcracks and mechanical parameters in plain concrete. The environmental and utilitarian aspects of the analyzed problem were taken into account in the studies. The analyses were carried out based on the results of the macroscopic and microstructural tests. During macroscopic investigations fracture toughness (KIIc) and compressive strength (fcm) of concretes were assessed, whereas based on microscopic examinations, the average width of the microcracks (Wc) in the Interfacial Transition Zone (ITZ) of coarse aggregate with cement matrix was evaluated. In the own testing, it was demonstrated that there is a direct relationship between the observed Wc and designated fcm and KIIc. Based on these results, it was found that with the decrease of the Wc, the mechanical parameters increases. Using the 20% FA cement binder causes favorable changes in the microstructure and affects the improvement of mechanical parameters of mature concrete. Moreover, such treatments also causes a lot of benefits – both from the point of view of ecology and sustainable construction.