# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Sand**} & \multicolumn{4}{c}{**Glass Sphere**} \\ \cline{2-10}  & **REF** & **GF** & **FF** & **MIX** & **REF** & **GF** & **FF** & **MIX** \\ \hline \(E_{dyn}\) [GPa] & 12.4 & 10.4 & 10.2 & 11.1 & 4.7 & 4.5 & 4.1 & 5.2 \\ STD [GPa] & 0.26 & 0.28 & 0.38 & 0.21 & 0.05 & 0.04 & 0.07 & 0.06 \\ \hline \(\
u_{dyn}\) [-] & 0.16 & 0.17 & 0.16 & 0.13 & 0.11 & 0.14 & 0.15 & 0.16 \\ STD [-] & 0.01 & 0.03 & 0.03 & 0.00 & 0.01 & 0.01 & 0.03 & 0.02 \\ \hline \(\rho_{dyn}\) [kg/m\({}^{3}\)] & 1730 & 1680 & 1710 & 1690 & 910 & 910 & 980 & 970 \\ STD [kg/m\({}^{3}\)] & 21 & 0 & 10 & 6 & 6 & 3 & 6 & 6 \\ \hline \(f_{c}\) [MPa] & 45.9 & 37.2 & 38.3 & 41.0 & 18.3 & 17.9 & 17.8 & 20.3 \\ STD [MPa] & 2.28 & 1.53 & 0.65 & 1.05 & 1.79 & 1.06 & 0.45 & 0.99 \\ \hline \(\sigma_{c}\) [MPa] & 4.42 & 3.53 & 3.64 & 3.73 & 1.63 & 1.43 & 1.71 & 2.03 \\ STD [MPa] & 0.12 & 0.32 & 0.09 & 0.27 & 0.10 & 0.11 & 0.02 & 0.15 \\ \hline E [GPa] & 15.4 & 11.9 & 12.5 & 13.4 & 6.0 & 5.2 & 4.8 & 6.4 \\ STD [GPa] & 0.66 & 0.80 & 0.82 & 0.56 & 0.16 & 0.20 & 0.21 & 0.33 \\ \hline \(K_{Lee}\) [MPa- m\({}^{1/2}\)] & 0.473 & 0.391 & 0.410 & 0.414 & 0.170 & 0.168 & 0.181 & 0.223 \\ STD [MPa- m\({}^{1/2}\)] & 0.028 & 0.069 & 0.025 & 0.024 & 0.004 & 0.007 & 0.009 & 0.019 \\ \hline \(G_{f,1}\) [/m\({}^{2}\)] & 8.2 & 7.2 & 7.6 & 7.2 & 2.7 & 3.0 & 4.8 & 4.5 \\ STD [/m\({}^{2}\)] & 0.69 & 1.95 & 0.76 & 0.65 & 0.15 & 0.34 & 1.26 & 0.79 \\ \hline \(G_{f}\) [/m\({}^{2}\)] & 26.0 & 30.0 & 64.1 & 48.8 & 7.4 & 7.7 & 23.8 & 35.8 \\ STD [/m\({}^{2}\)] & 2.1 & 4.3 & 7.1 & 9.5 & 0.3 & 1.17 & 4.9 & 13.9 \\ \hline \(k\) [W/ (m\cdot K)] & 0.906 & 0.721 & 0.869 & 0.778 & 0.273 & 0.296 & 0.291 & 0.303 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of materials parameters of considered composites (mean values and standard deviations (STDs)). Figure 8: Thermal conductivity results of tested materials. In summary, fiber reinforcement in geopolymer materials can significantly improve the strength properties, especially in fracture behavior and limit brittle catastrophic fracture od structures. The mechanism of the reinforcement is visible in fracture toughness because the fibers join together parts of the matrix in the case of flexural strength. They give an additional force for bonding the two parts together. It is visible in this kind of force, because of the relatively low value of the fracture toughness for the matrix material. The mechanism of the reinforcement is not observed in the case of compressive strength, because in this case, the matrix material has very good properties for this kind of force, much better than natural fibers. It caused the decreasing mechanism in compressive strength to be observed. It is especially important for lightweight materials with reduced mechanical strength. Reinforcement of these kinds of materials allows for application at the same time for isolation materials that have these construction properties. It is also worth noticing that the obtained values for thermal conductivity are comparable to selected isolation materials such as aerated concrete blocks or wood (a value of about 0.2 W/(m\(\cdot\)K)) [30]. The clear advantage of using the fibers is the improvement of the mechanical properties of the materials. Moreover, the usage of short fibers does not require changes in the technology. Both selected fibers are cost-effective. The proposed technology also has some disadvantages, including the further recycling process, and especially the application of hybrid fibers, which makes this process more complex. It is also worth noticing that the material is characterized by an environmentally friendly approach and can be used as a replacement for traditional building materials based on OPC. The alkali-activated materials are considered to be a more environmentally friendly alternative that has a lower carbon footprint and is coherent with the circular economy approach.