## 4 Conclusions  
The presented research is a preliminary phase of work on improving the properties of polymer fibers for concrete available on the Polish market. Only one level of fiber dosing was used, and the modified composite was a standard mortar (the so-called comparative  
Figure 7: Fractures of PP fiber mortar specimens after flexural strength test and the image of the fibers after their pull-out from the cementitious matrix: (**a**) mortars with non-modified fibers; (**b**) mortars with TA-modified fibers (macro-observation and micro-observation under magnification: 50\(\times\)); (**c**) non-modified (upper) and modified (lower) fibers (magnification: 50.4\(\times\)); (**d**) non-modified (upper) and modified (lower) fibers (magnification: 120\(\times\)).  
mortar) with Portland cement (although there is a tendency to reduce consumption of high-emission binders). Nevertheless, the results obtained in the presented research are promising. Firstly, after adding polypropylene fibers to mortars in a significant amount, recommended by the manufacturer as reinforcement for concrete for structural applications, there was no significant deterioration in compressive strength (only 11.5-12.3% reduction). Secondly, the modification with tannic acid did not lead in any case (neither after 7 days nor after 28 days) to a deterioration in compressive strength or flexural strength (even a slight improvement was noted). It significantly improved the flexural modulus of the tested fiber mortars. Thirdly, it was confirmed that the surface modification with tannic acid, which was earlier used in the case of different polymers, positively affected the polypropylene fibers, improving their adhesion and anchoring in the cementitious matrix.  
The study aimed to evaluate the possibility of improving the performance of the polypropylene fibers in the cement composite with the method previously employed for the polyethylene fibers. The results show that the surface modification method with tannic acid can effectively improve the bonding between a cementitious matrix and the various polymer fibers.  
In the subsequent investigation phase, the authors intend to extend the research to include composites with lower water/cement ratio values and fluidizing admixtures. They also intend to shift the scale of the experiment from mortars to concretes with coarse aggregate and to determine properties such as those assessed with the wedge splitting test (WST), which is considered a good measure of adhesion [26; 27; 28]. Moreover, they want to investigate the effectiveness of tannic acid modification on other polymer fibers for concrete available on the Polish market, such as copolymer polyolefin fibers.  
Conceptualization, J.J.S. and P.L.; methodology, J.J.S.; investigation, J.J.S. and A.B.; writing, J.J.S.; visualization, J.J.S.; supervision, P.L.; project administration, P.L. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Not applicable. Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the policy of Warsaw University of Technology. The authors declare no conflicts of interest.