## 3 Results and Discussion  
### Effect of Coir Fiber Addition on the Compressive Strength of Geopolymer Specim  
The compressive strength is a key property for measuring the mechanical properties of building materials. Figure 6 shows the compressive strength results of the metakaolin-based geopolymer reinforced with coir fibers at different fiber contents (0.5-2%). Upon the introduction of coir fibers at 0.5% fiber content, the compressive strength value increased from 16.9 1 N/mm\({}^{2}\) (MGP Control) to 21.25 N/mm\({}^{2}\). The addition of 0.5% PCF fiber may have enhanced the formation of a denser composite than the control, with less porosity and cracks, thus increasing the compressive strength. The 0.5% fiber content was evenly distributed in the matrix. Crack arrest at the metakaolin/fiber interface and the high aspect ratio of the fiber improved the mechanical strength. This is consistent with the result of [PERSON] et al. [19], who reported that 0.5% coconut fiber content improved the compressive strength of high calcium fly ash geopolymer. This result suggests that pedestrian paving bricks can be fabricated using coir fiber-reinforced metakaolin geopolymers since it satisfies the minimum compressive strength requirement standard of the American Society for Testing and Materials (ASTM C216) [62]. Surprisingly, 1% coir fiber addition decreased the compressive strength, suggesting that at 1% fiber, the dispersion of the coir fiber in the geopolymer matrix was not good. Fiber clustering may have promoted nucleation cracks within the geopolymer matrix. A similar trend was also observed in the study of [PERSON] et al. [19], in which the compressive strength of composites decreased with increasing fiber content from 0.5% to 1%. However, this result is not consistent with the findings of [PERSON] et al. [1], who recorded improved compressive strength at 1% coir fiber addition in fly ash geopolymer composite. At higher coir fiber content (1.5%), the geopolymer composite strength corresponded to 16.94 N/mm\({}^{2}\). It is similar to the control strength of the Mk-based geopolymer without fibers. At this fiber content, the presence of coir fibers had no significant effect in contributing to the compressive strength of the synthesized coir-fiber-reinforced geopolymer composite. Worthy of note also is the decrease in strength at 2% coir fiber addition, suggesting a less dense composite structure. At higher fiber content, the density of samples tends to reduce, packing of geopolymer matrix becomes difficult, and porosities of specimen increase, which in turn reduces compressive strength [19]. Figure 7 illustrates the mode of failure of the tested cubes. The failure mode of the cubes is non-explosive [63]. The cubes almost maintained their original shape. The energy absorption capacity of the coir fibers might have helped control the cracks. Coir-fiber-reinforced metakaolin-based geopolymer composite fabricated with 0.5% coir fiber content can therefore be used to produce eco-friendly building materials as it satisfies the ASTM C62 standard [64] for building. Bricks having a compressive strength of 17.20 N/mm\({}^{2}\) is the minimum requirement. Our results suggest that 0.5% coir fiber content in the metakaolin geopolymer matrix composite is the optimum for improved compressive strength.