## 2 Materials and Methods
### Study of the Microstructure of Hardened Cement Paste, Dispersion-Reinforced with Coconut Fibers  
Microstructural changes in cement paste, dispersion-reinforced with coconut fibers, were investigated to find the relationship between the concrete structure and its SC. Figures 16-18 show the microstructure photos of the hardened cement paste of the control composition and with coconut fibers in amounts of 1.75 % \(wt.\) and 2 % \(wt.\)  
Figure 14: Stress–strain curves in compression. Figure 15: Stress–strain curves in tension. Figure 16: Photographs of the microstructure of a sample of the hardened cement paste of the control composition: (**a**) 1000\(\times\); (**b**) 5000\(\times\). Figure 17: Photographs of the microstructure of a sample of hardened cement paste with dispersed reinforcement with coconut fibers in an amount of 1.75% by weight of cement: (**a**) 1000\(\times\); (**b**) 5000\(\times\). Figure 18: Photographs of the microstructure of a sample of hardened cement paste with dispersed reinforcement with coconut fibers in an amount of 2%: (**a**) 1000\(\times\); (**b**) 5000\(\times\). Figure 16 demonstrates the microstructural changes in the control composition after collapse. The structure was loose and had many voids. Figures 17 and 18 demonstrate that coir particulate reinforcement shortened the number of microcracks in the microstructure of the hardened cement paste. They restrained the stack propagation and, thanks to the bridging effect created by the fiber, bound the concrete matrix. The microstructure of hardened cement paste samples with a percentage of fiber reinforcement of 1.75% was the densest, practically not containing structural defects in the form of voids and cracks (Figure 17). However, at a dosage of coconut fiber in an amount of 2%, due to their high water absorption, cracking was observed at the phase boundaries, which reduced the anchoring of the fiber in concrete (Figure 18). Figure 18 deserves special attention, since this area illustrates the nature of microcracking at the interfaces of the phases \"coconut fiber--cement-sand matrix\". By the nature of this microcracking, good compatibility of coconut fiber with the mineral matrix was visible. SEM analysis thus proved the expediency of dispersed reinforcement of cement systems with coconut fiber at the microstructural level. Thus, at the micro level, in the course of studying the structure formation of concretes reinforced with coconut fiber, it was established that the proper quality of the structure was maintained, the degree of cracking at the interface was low, and the fibers were well compatible with cement paste and other concrete components. Microstructural studies also confirmed the effectiveness of concrete reinforcement with dispersed fibers. In view of the formation of the concrete structure at the micro and macro levels, under the condition of a uniform distribution of coconut fibers in the mixture, a rational concentration of crystallization centers occurred throughout the entire volume of the composite. Good chemical compatibility of coconut fibers with hydration products and a high degree of mechanical anchoring of the fibers in the cement-sand matrix contributed to the high final strength characteristics of concrete and the expansion of its serviceability. Compared with the results of other authors, we achieved high values of the SC of concrete reinforced with coconut fibers. In [5], the authors achieved an increase in the axial tensile strength of concrete up to 30.63% due to the FDRC with coconut fibers. The technology we used and composition recommendations gave an increase in the axial tensile strength of concrete up to 43%. In [43], the authors managed to obtain an increase in tensile strength in bending up to 45% by adding coconut fibers and recycled filler from 16% to 45%. In our study, by using only coconut fibers, it was possible to achieve an increase in the tensile strength in bending of concrete up to 42%, which approximately corresponded to the increase achieved in [43].