## 3 Results and Discussion  
### Physical and Mechanical Properties  
The results for apparent density (average and basic statistic parameters: standard deviation, SD and coefficient of variation, CV) of mortars with non-modified and modified fibers after 7 and 28 days and the density of the standard mortar after 28 days are given in Table 4. As expected, mortars with fibers made of polypropylene (with a specific gravity of 910 kg/m\({}^{3}\)--compare Table 2) were characterized by slightly lower (differences of up to 3%) apparent density compared to pure-mineral standard mortar. However, it is worth noting that the mortar containing non-modified fibers was characterized by a density with a 1.5 times higher coefficient of variation, while the mortar with fibers modified with tannic acid showed four times lower variability compared to standard mortar apparent density. The results of the mechanical tests of mortars with fibers after 7 and 28 days are given in Figure 5. In the case of standard mortar, the results obtained after 28 days were as follows: flexural strength--7.46 MPa (on average) and compressive strength--45.16 MPa (on average), which stands in line with the expectation of standard EN 196-1 for such mortars with CEM I 42.5R binder (i.e., \(f_{\text{cm}}\geq 42.5\) MPa). Both m \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Composite Type**} & \multicolumn{4}{c}{**Density after 7 Days**} & \multicolumn{4}{c}{**Density after 28 Days**} \\ \cline{2-7}  & **Mean, kg/m\({}^{3}\)** & **SD, kg/m\({}^{3}\)** & **CV, \%** & **Mean, kg/m\({}^{3}\)** & **SD, kg/m\({}^{3}\)** & **CV, \%** & **Relative Change to SM, \%** \\ \hline Standard mortar (SM) & x & x & x & 2261 & 37 & 1.6 & - \\ \hline Non-modified-fiber mortar (NM) & 2186 & 25 & 1.1 & 2240 & 58 & 2.4 & \(-\)0.9 \\ \hline Modified-fiber mortar (M) & 2198 & 5 & 0.2 & 2198 & 8 & 0.4 & \(-\)2.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Density of tested mortars: standard mortar (SM), mortar with non-modified polypropylene fibers (NM) and mortar with tannic acid-modified polypropylene fibers (M) determined after 7 and/or 28 days (SD—standard deviation, CV—coefficient of variation). Figure 4: Flexural strength and modulus test performed on specimens in the shape of beams of size 40 mm \(\times\) 40 mm \(\times\) 160 mm in the three-point bending test acc. to EN 1015-11 [22]: (**a**) scheme of the test (specimen on two supports—one stationary, the other movable, so that the system is statically determinate, the force concentrated in the middle of the span); (**b**) specimen during the test in Instron 5567 electromechanical testing machine. lower strength after 28 days, and none exceeded 42.5 MPa in compressive strength. The difference compared to the standard mortar was 5.2-5.6 MPa (11.5-12.3% reduction). In the case of 28-day flexural strength, the difference compared to the standard mortar was more significant--reduction by approx. 35% was observed. However, such weakening of the cementitious mortar after adding polypropylene fibers is unsurprising, as the connection between the mineral matrix and the polypropylene fiber is weakened. Moreover, a slight correlation was observed between the decrease in mortar density and mechanical strength. An interesting observation was that in each case--both in tests carried out after 7 days and after 28 days--higher flexural strength and compressive strength values were noted when using fibers modified with tannic acid. The differences were insignificant--they amounted to a maximum of 3%. However, authors expect that the effect of fiber modification on mechanical strength might be more remarkable in the case of composites of lower water/cement ratio and when adding plasticizing admixture that might provide a better coating of the fibers with a fluidized cement paste [23; 24]. The crucial mechanical property when assessing the effect of introducing polymer fibers into the brittle mineral composite is the elastic modulus.