## 3 Results and Discussion  
The optimal ratio of chrysotile nanofibers and carbon black in the composition of a complex additive was stated based on the particle dispersion level in the suspension as well as mechanical properties of the modified cement matrix. Primarily, the ratio of chrysotile to carbon black by mass was taken equal to 10:1, 7:5:1, 5:1, 2.5:1 and 1:1. The chrysotile amount in the suspension was 10% of the full suspension mass. Higher concentrations lead to the fiber agglomeration that made it harder to provide the uniform particle distribution in cement composite that adversely affected the properties of the material [33]. The particle size distribution analysis of the complex additive was carried out on Shimadzu SALD-7500 nano laser analyzer. The results are presented in Figure 3.  
The highest dispersion level of the complex additive was obtained at the ratio of chrysotile fibers to carbon black equal to 5:1. In this case the average particle diameter was around 65 nm. It should be noted that carbon black particles do not significantly change the size of agglomerates in the complex additive apparently due to the stabilizing effect of another component [19; 34; 35] in this case the chrysotile fibers. Thus, it is hard to make specific conclusions on the effect of additives on the properties of cement composites even considering the sufficient clarity of the results of the particle size analysis of the complex additive since the dispersion levels vary insignificantly. In this case, the most reliable option for stating the optimal amount of additives that provides the maximum increase in strength of cement composites is the preparation of standard samples and their mechanical testing. The mechanical tests of samples modified with complex additives containing the different chrysotile to carbon black ratios were carried out on fine-grained concrete beams with the standard dimensions of 40 \(\times\) 40 \(\times\) 160 mm at the age of 1 day after steam curing. To determine the strength, a series of samples was prepared, containing three samples  
Figure 3: Particle size distribution in the complex additive: (**a**) at different additives ratios; (**b**) at chrysotile to carbon black ratio equal to 5:1.  
of each composition. The average compression and flexural strength values obtained via statistical analysis of the mechanical test results are indicated in Table 1. In all cases, the deviation from the average value did not exceed 10%. As can be seen from Table 1, the maximum increase of concrete strength is achieved with the combined addition of chrysotile fibers in the amount of 0.05 % by cement mass and carbon black in the amount of 0.01% by cement mass. In this case, the compressive and flexural strengths of composite increased by up to 31.9% and 26.7%, respectively. A further increase of the amount of chrysotile fibers leads to a strength decrease, supposedly due to the tendency of fibers to re-agglomerate due to the action of van der Waals forces. These local fiber agglomerates are held together only by the forces of intermolecular interaction and can promote the destruction of the material. In order to interpret the results obtained during physical and mechanical tests, the chemical analysis of concrete samples was carried out as well as the microstructure study. IR spectral analysis of cement matrices of the reference composition (Figure 4a), compositions modified with carbon black in the amount of 0.02% by the cement weight (Figure 4b) and chrysotile fibers in the amount of 0.05% by the cement weight (Figure 4c), as well as the composition modified with the complex additive based on chrysotile and carbon black (Figure 4d) was carried out using Shimadzu IRAffinity-1 spectrometer in the frequency range 400-4000 cm\({}^{-1}\).