## 2 Materials and Methods  
Portland cement CEM I 32.5 N produced by Timlyui Cement Plant LLC was used as a binder. Natural river sand was used as a fine aggregate obtained from sand deposit of the Kama river (Novy village, Udmurtia, Russia) with the size modulus equal to 2.0. The fine aggregate-to-cement ratio was 3:1. The water-to-cement ratio was 0.45.  
Suspensions of chrysotile nanofibers and carbon black were added into the cement-sand mortars together with mixing water. In order to stabilize the suspensions of chrysotile fibers and prevent the re-agglomeration of ultrafine particles, the C-3 superplasticizer was used that is produced on the basis of naphthalene sulfonic acid and formaldehyde.  
Chrysotile is a natural mineral of the serpentine group which can be chemically described as a hydrous magnesium silicate with the theoretical composition corresponding to 3 MgO-2 SiO\({}_{2}\cdot\)2H\({}_{2}\)O. In this research, chrysotile was added into cement-sand mortars in the form of an aqueous suspension that was prepared by mixing the chrysotile fibers in the amount of 10% of the total suspension volume with the C-3 superplasticizer in the amount of 2% by water mass using the cavitation disperser. The study of the microstructure of the chrysotile fibers (Figure 1a) shows that the diameter of individual fibers is around 30-50 nm, whereas the length of the fibers varies significantly. The pattern of the particle size distribution curve of the aqueous suspension of chrysotile fibers (Figure 1b) showed that the average particle diameter varied from 0.01 um to 0.2 um. The average value was equal to 0.046 um.  
Carbon black is a powdery product obtained in the process of controlled pyrolysis or thermal oxidative decomposition of liquid or gaseous hydrocarbons. The elementary composition of this synthetic material is represented by carbon (88.6-93.7%), hydrogen (0.7-0.8%) and oxygen (5.5-10.5). The particle size of carbon black (Figure 2) was found to be 30-120 nm.  
In order to determine the properties of complex additive as well as the modified cement composite a comprehensive study was carried out using the following methods:  
* particle size distribution analysis using the laser dispersion on a SALD-7500 nano analyzer manufactured by Shimadzu with the 7 nm to 800 um measurement range;  
Figure 1: Suspension of chrysotile fibers: (**a**) microstructure of chrysotile fibers at 20,000-fold magnification; (**b**) distribution of chrysotile particles after processing in the cavitation disperser.  
Figure 2: Carbon black particles: (**a**) microstructure at 50,000-fold magnification, (**b**) particle size distribution.  
mechanical tests (compressive and flexural strength) on hydraulic press model PGM-100 MG4-A with the maximum load of 100 kN and loading rate of 0.5 MPa per second;
* differential thermal analysis of cement composites on a TGA/DSC1 Star system manufactured by Mettler Toledo in the temperature range from 60 \({}^{\circ}\)C to 1100 \({}^{\circ}\)C with the heating rate of 30 \({}^{\circ}\)C/min;
* IR-spectral analysis of cement composites on Shimadzu IRAffinity-1 spectrometer in the frequency range 400-4000 cm\({}^{-1}\);
* microstructure analysis and energy dispersive X-ray analysis (EDX) on scanning electron microscope Quattro ESEM Thermo Fisher Scientific with the resolution up to 0.8 nm.  
The cement composite samples were tested at the age of 1 day after steam curing.