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Fly ash (FA) is a waste product generated in a thermal power plant in huge quantitie has been posing problems of disposal. Therefore, the purpose of this study is to make a meaningful utilization of FA as a binder in concrete composites, while the research involved low volume fly ash concrete (LVFAC). The cements used in the studies were ordinary Portland cement (OPC), (FA-00) and two blended OPC containing FA in the amount of: 20% (FA-20) and 30% (FA-30). This research confronts the following concrete properties: water absorptions by immersion (wa) and compressive strength (fcm). The methodology of the wa tests was to reflect the conditions prevailing in the case of reinforced concrete structures operating below the water table. Both evaluated properties were measured at 28 days. In addition, the microstructure of all materials was also assessed. The results show that, the best fcm, i.e. 48.96 MPa can be obtained in FA-20 concrete. However, in the case of LVFAC containing 30% FA the compressive strength is decreases to 45.10 MPa. On the other hand the average values of wa was 4.6% for FA-30 and 5.3% for FA-20, respectively. Based on the microstructural studies it was found that this behavior is attributed to reduced pore diameters and densification of cement matrix microstructure in FA-30 concrete. On the other hand concrete of series FA-20 showing a loose and porous microstructure, and wa in this composite increased accordingly. Based on the conducted studies it can be stated that, the LVFAC containing 30% FA may increase durability of reinforced concrete structures subjected to immersion conditions. Furthermore, from an application point of view, the obtained research results may be helpful in understanding the impact of the FA additive on the level of wa in concretes with this waste
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