## 3 Results and Discussion
Later on, in the temperature range of 650-750 \({}^{\circ}\)C, the mass loss of the samples was calculated to be 2.58% to 1.61% for the reference and modified samples, respectively. This additionally confirms the higher degree of OH\({}^{-}\) hydroxyl group binding in calcium silicate hydrates as well as a decrease of the total amount of free calcium hydroxide due to its replacement by C-S-H.  
To assess the influence of the complex additive on the morphology of hydration products in the cement composites, the microstructural analysis of samples was carried out using the Thermo Fisher Scientific Quattro S microscope. Figure 6 shows a significant change of the micromorphology of cement matrix modified with the complex additive based on carbon black in the amount of 0.01% by the cement mass and chrysotile fibers in the amount of 0.05% by cement mass, resulting in the replacement of the porous structure of the cement hydration products by the gel phase of calcium silicate hydrates C-S-H. In this case, the addition of the complex additive contributes to the formation of a dense microstructure, creating cement composites with increased strength. The microstructure of the sample in Figure 7 clearly shows that chrysotile nanotubes are evenly distributed among the phases of calcium silicate hydrates and their surface is fully covered with cement hydration products. This confirms the tendency of new formations to grow along the modifier fibers and evidences for the high adhesion between chrysotile fibers and cement hydration products. The location of fibers in the pore space as well as the formation of hydration products on their surface contribute to a change in the pore structure of cement matrix increasing its density and leading to an improvement of mechanical characteristics of material. X-ray microanalysis of crystalline hydrates formed on the surface of chrysotile fibers (Figure 8) confirmed that the formations belong to calcium silicate hydrates. Moreover, the SEM of the cement matrix modified with the complex additive (Figure 9) shows that the main hydration products are presented by calcium hydroxide, C-S-H and spherical formations about 100 nm in diameter. The EDX analysis was carried out on samples modified with the complex additive in order to study the elemental composition of the observed cement hydration products (Figure 10). Figure 6: Microstructure of cement composite at 1000-fold magnification: (**a**) reference sample, (**b**) modified sample. Figure 8: X-ray microanalysis of crystalline hydrates on the surface of chrysotile fibers. Figure 7: Microstructure of cement matrix modified with the complex additive: (**a**) at 14000-fold magnification, (**b**) at 70,000-fold magnification. Based on X-ray spectral analysis the hydration products of spherical shape can be attributed to thaumasite formations (CaSiO\({}_{3}\)-CaSO\({}_{4}\)-CaCO\({}_{3}\)-15H\({}_{2}\)O) obtained in the course of the reaction of calcium silicate hydrates (C-S-H) with calcite and unbound sulfate ions or reaction of ettringite with C-S-H and carbonates/bicarbonates [36; 37; 38; 39]. The noted hydration products of spherical shape are mainly formed on the surface of ettringite needle crystals and on the surface of calcium hydroxide plates. The results of mechanical tests, microstructure studies, thermal analysis and IR spectroscopy prove that the addition of the complex additive based on chrysotile and carbon black increases the intensity of the cement hydration and promotes the formation of a  
Figure 10: X-ray microanalysis of spherical hydration products. Figure 9: Microstructure of cement matrix modified with the complex additive: (**a**) at 14000-fold magnification; (**b**) a fragment at 70,000-fold magnification. dense structure of cement matrix containing thaumasite globular hydration products and calcium silicate hydrates of a lower basicity that increases the mechanical characteristics of the material.