STUDY OF CHANGES IN THE POROSITY OF FILLED CEMENT-SAND SAMPLES BY THE KINETICS OF THEIR WATER ABSORPTION

The article deals the effect of fillers on the change in the porosity of cement-sand mortar samples based on the kinetics of their water absorption. Kinetics of water absorption of cement-sand samples is a research method that simulates the operation of concrete structures under real operating conditions. The water permeability of such materials is a function of porosity and the main factor that ensures the durability of structures and buildings. The following parameters of the pore structure of the cement-sand mortar were evaluated according to the kinetics of water absorption: total porosity, open and closed porosity, and the average size of open capillary pores. As a filler, a fine fraction of crushed recycled concrete sleepers was used. The results showed a change in the ratio of total, open and closed porosity, as well as the average size of open capillary pores. The ratio of open porosity to closed porosity is 2.95, i.e. open porosity is almost three times bigger than closed porosity for the unfilled sample. For filled samples, this ratio fluctuates within 2.1…0.98 and indicates that the number of closed pores increases due to the number of open ones. The biggest water absorption is demonstrated by samples with the biggest percentage of sand or cement replacement – 15.6 % and 11.4 %, respectively. This is because of both the overall loose structure due to the non-optimal arrangement of sand, cement and filler particles and the increased porosity of the filler itself. The smallest water absorption from 9.2 % to 8.4 % is found in filled samples. Consequently, the densest particle packing is achieved for these samples, which reduces water absorption even when using porous fillers. It was experimentally established that replacing 40 % of sand or 10 % of cement leads to a shift in the porosity of samples to the area of micropores. This has been confirmed by a twofold decrease in the average size, a decrease in the volume of total pores and an increase in the number of closed pores, as well as an almost threefold decrease in open porosity compared to closed porosity. Under these conditions, free movement of water from outside concrete structures into concrete is difficult. In turn, this rises the durability. Further studies will be aimed at determining the change in porosity of cement-sand samples over time to confirm the hypothesis of a self-compacting mechanism due to the slow release of water in the pores of secondary aggregates.