# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
## 4 Conclusions  
The presented article focuses on the mechanical, fracture, and thermal properties of geopolymer composites reinforced by flax fibers and glass fibers, both in the form of a single-type reinforcement and a hybrid connection of fibers. Two compositions were used as the geopolymer matrix: one based on fly ash and river sand, and the second containing fly ash and glass spheres. The fiber content in each case was a maximum of 1% by mass. The following conclusions can be drawn from the conducted research:  
* The production of geopolymer materials modified by glass spheres reduced the compressive strength almost twice compared to the material made based on fly ash and river sand.
* The introduction of single-type fibers to the composite with river sand caused a more significant decrease in compressive strength than hybrid reinforcement.
* Single-type fibers in composites based on fly ash and glass spheres did not affect compressive strength. However, introducing hybrid reinforcement increased compressive strength by about 10% compared to the reference specimens.
* Flax fibers and hybrid reinforcement ensured higher fracture toughness and energy.
* Materials modified with glass spheres have a thermal conductivity twice as low as that of materials containing river sand.
* The introduction of single-type fibers and hybrid reinforcement slightly reduces the thermal conductivity of geopolymer materials modified with river sand, which is most likely related to the density of the material. In contrast, a slight increase in thermal conductivity characterized composites based on fly ash and glass spheres compared to the reference sample.  
The presented results indicate the potential of the application of glass spheres in geopolymer materials both in terms of cracking mechanics, as a factor inhibiting brittle cracking, and in terms of reducing the thermal conductivity of materials that can be used in the construction sector, for example, for thermal panels, which should be characterized by low thermal conductivity, fire resistance, and crack resistance.  
**Author Contributions:** Conceptualization, P.B. and K.K.; methodology, H.S., K.S. and B.K.; software, P.N. and D.M.; validation, M.L., M.N. and K.S.; formal analysis, P.B., D.K. and H.S.; investigation, P.B., D.K., D.M., M.N. and B.K.; resources, M.L.; data curation, P.B., M.L. and P.N.; writing--original draft preparation, P.B. and H.S.; writing--review and editing, B.K., M.N. and D.K.; visualization, K.S. and P.N.; supervision, K.K.; project administration, H.S. and K.K.; funding acquisition, H.S. and K.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Polish National Agency for Academic Exchange and by the Ministry of Education, Youth and Sports Czech Republic under the bilateral exchange of researchers between Poland and the Czech Republic: the grant Fibers Reinforced Geopolymers (Polish designation PPN/BCZ/2019/1/00005/U/00001, the Czech designation 8J20 PL073).  
Not applicable.  
**Data Availability Statement:** The data presented in this study are available in article.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.