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The studies were carried out to find a relation between important physical property and the main mechanical parameter of concretes containing fly ash (FA) in order to develop indications for the production of durable composites with the addition of this waste. The tests included low volume fly ash concrete (LVFAC). The cements used in the studies were following: ordinary Portland cement (OPC), (FA-00) and two blended Portland cements containing FA in the amount of: 20 %, (FA-20) and 30 % (FA-30). Measurements of water adsorption (wa) by immersion and compressive strength (fcm) of analysed composites have been performed. The microstructure of all three materials was also assessed. Due to the fact that, the methodology of the wa tests was to reflect the conditions prevailing in the case of concrete- and reinforced concrete structures operating below the water table these tests were very significant from both a scientific and engineering point of view. Test results indicated that LVFAC mixes developed 28-day fcm between 45.10 for FA-30–48.96 MPa for FA-20 with a value of 47.51 MPa for the reference concrete. Neverless wa of LVFAC mixes decreased with the increase in FA content. The average values of wa was 4.6 % for FA-30 % and 5.3 % for FA-20, respectively, while for concrete without FA it was 5.0 %. Thus, it was found that wa by immersion was inversely proportional to the fcm. 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 showed a loose and porous microstructure, and wa in this composite increased accordingly.
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