## 2 Materials and Methods  
### Qualitative and Qualitative Composition of Tested Composites  
The experiment involved comparing the behavior of surface-modified and unmodified polypropylene fibers applied to a composite with a cement matrix, which was then subjected to destructive mechanical tests. To eliminate the influence of other components on the test results, the composite selected as the base one, i.e., reinforced with dispersed reinforcement in the form of fibers, was the so-called comparative mortar, i.e., standard mortar made of high-early strength Portland cement class 42.5, i.e., CEMI 42.5 R (CEMEX, Chelm, Poland). The applied cement fulfilled the requirements of European Standards EN 197-1 [18] and EN 196-1 [19] in terms of the composition (at least 95% Portland clinker) and properties. The aggregate used in the mortars was standard sand (meeting requirements of EN 196-1). It was natural quartz sand with rounded grains, SiO\({}_{2}\) content of at least 98%, and grain size up to 2 mm. The size distribution of sand grains recommended by the EN 196-1 standard is presented in Table 1. The mixing water was tap water (meeting the requirements of EN 1008 [20]).  
The applied fibers (presented in Figure 2a) were pure polypropylene macrofibers of a length of 25 mm (DIIF, Dneepr, Ukraine) designated to be used in structural concrete and mortars. Table 2 contains the basic physical properties of the fibers listed by the manufacturer [21]. The fibers were also characterized by high acid, alkali, and salt resistance and low thermal and electrical conductivity.  
Standard EN 196-1 includes the composition of the standard mortar, which assumes a water/cement mass ratio of 0.50 and a cement/aggregate mass ratio of 1:3. For the experiment, both a standard mortar (without fibers) and mortars with an identical composition of essential components, but supplemented with chemically modified/unmodified fibers, were prepared. The amount of added fibers was based on the manufacturer's recommendations [21]. The manufacturer recommended dosing fibers in various amounts depending on the potential use of composites, i.e., 2-4 kg per 1 m\({}^{3}\) of the concrete mix in the case of the production of industrial floors, screeds or sprayed concrete, and 2-6 kg per 1 m\({}^{3}\) of the concrete mix in the case of concrete structural elements of residential and industrial  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Square Mesh Size, mm** & **2.00** & **1.60** & **1.00** & **0.50** & **0.16** & **0.08** \\ \hline Total residue on the sieve, \% & 0 & \(7\pm 5\) & \(33\pm 5\) & \(57\pm 5\) & \(87\pm 5\) & \(99\pm 1\) \\ \hline \end{tabular}
\end{table}
Table 1: Size distribution of standard sand grains according to EN 196-1.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Type of Polymer** & **Specific Gravity, kg/m\({}^{3}\)** & **Melting Point, \({}^{5}\)C** & **Ignition Point, \({}^{5}\)C** \\ \hline Polypropylene (PP) & 910 & 162 & 593 \\ \hline \end{tabular}
\end{table}
Table 2: Properties of macrofibers used in tested composites (manufacturer’s data [21]).  
buildings. For the experiment, it was assumed that the fiber content would be close to the upper limit of the range recommended for structural concrete, i.e., 4.5 kg per 1 m\({}^{3}\) of the mortar mix. The compositions of the tested mortars (calculated per mix following the standard procedure, assuming a specific gravity of 2650 kg/m\({}^{3}\) for standard sand and of 3100 kg/m\({}^{3}\) for the cement) are listed in Table 3.