# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
## 3 Results and Discussion
The addition of GF increased the variability of results in comparison with the REF composite and other types of fibers. On the other hand, in the case of the GS composite, the addition of fibers had a positive effect on the effective fracture toughness. The highest increase of about 30% was observed when the MIX of fibers was used. The last investigated parameter was specific fracture energy. At first, a part of the fracture energy \(G_{f,1}\), considering the area under the _F-d_ diagram up to the peak load \(F_{\text{max}}\), is discussed. In the case of S composites, the addition of fibers caused a decrease of about 10% in comparison with REF composites. It should be noted that the variability of results is quite high when the GF was used. On the contrary, a positive effect of the addition of the fibers was observed in the case of GS composites. The addition of fibers increased the value of fracture energy in the pre-peak part of the _F-d_ diagram by more than 60%. Afterward, the area under the whole _F-d_ diagram was considered for evaluation. As is evident from Figure 2, the addition of FF and the MIX of fibers had a positive effect on post-peak behavior for both types of composites. The fracture energy increased more than two times and three times in the case of S and GS composites, respectively, in comparison with the REF one. The addition of fibers led to a higher variability of the results, especially  
Figure 5: Values of modulus of elasticity and fracture energy for tested materials. in the case of the MIX one, which could be connected with the non-uniform distribution of fibers. Studies on the mechanical and fracture properties of geopolymer composites stated that most strength parameters increase after introducing fibers already at 0.5% by mass. Flexural strength, tensile strength, fracture toughness, and fracture energy increase in relation to reference materials. The growth in properties is attributed to the crack bridging mechanism, which contributes to larger cracks at maximum loads, with more plastic behavior resulting in increased strength and greater energy absorption capacity. The energy is usually dispersed by separating the material and overcoming the friction forces while pulling the fibers out of the matrix. The fiber bridging effect in micro- and macroracks effectively maintains the integrity of the composite and limits catastrophic cracking. These properties, however, depend on the shape, length, and amount of introduced reinforcement and can also cause a diminishing effect in compressive strength due to an increase in the porosity of the material [47; 48; 49; 50; 51; 52; 53]. Figure 6a-d present microscopic images of composite materials based on fly ash with river sand. The microstructure analysis showed a relatively compact geopolymer structure with visible places left by undiluted fly ash particles. Cracks in the composite matrix were also visible, which indicated the brittleness of the geopolymer matrix (Figure 6a,c). The microstructure of the composite with glass fiber revealed an amorphous character with visible places of the crystal structure. Glass fibers were pretty well placed in the composite matrix, but they presented an agglomeration character. In the case of the microstructure of composites with flax fiber, a typical surface of natural fibers embedded in the composite matrix with local cracks deep into the material could be observed. Figure 6: SEM images of the S composite: (**a**) reference specimen based on fly ash and sand, (**b**) composite with GF, (**c**) composite with FF, and (**d**) composite with hybrid reinforcement. The microstructure of the composites in which the matrix consisted of fly ash and glass spheres (Figure 7a-d) demonstrated a different character. The deposition of glass and natural fibers and hybrid reinforcement is very similar; fiber agglomerates were visible, but the fibers were also well embedded in the geopolymer matrix. However, the matrix was characterized by significantly higher areas of undiluted fly ash particles and areas of glass bubbles.