Effect of cenospheres on the proprieties of plain concrete exposed to elevated temperature

Abstract The main objective of the study was to determine the effect of the use of cenospheres for C30/37 class concretes used in construction and road construction. As part of the research, four concrete mixtures were designed: three with the addition of cenospheres and one reference mixture, without the additive. In the three modified mixtures, 20, 40 and 60% of fine aggregate were replaced by cenospheres, respectively. Selected features of the mixtures were determined—consistency and density, as well as the strength properties of concrete. Strength tests were carried out after 28 and 56 days of maturation. Experimental tests of concrete after initial thermal treatment were also carried out at three selected temperatures: 300, 500 and 700 °C, with loading times ranging from 60 to 180 min. These times were selected in such a way that the samples could be heated in their entire volume. A separate group of experimental studies was microstructure studies, the aim of which was to assess the changes occurring in concrete under the influence of high temperatures and to try to justify why CS-I concrete containing cenospheres in the amount of 20% of concrete subjected to annealing at 300 °C has a higher compressive strength than the corresponding reference concrete. The negative influence of cenospheres on the strength of concrete after initial thermal treatment was unequivocally demonstrated. An increase in the amount of cenospheres causes a decrease in compressive strength. A similar negative relationship was also demonstrated in the case of non-heat-treated samples. Graphical Abstract