## 4 Conclusions  
The following research work was aimed at comparing the effect of the addition of different forms of reinforcement (glass fiber, carbon fiber, basalt grids) on the strength properties of fly ash and sand-based geopolymers and fly ash, sand, and cement-based geopolymers. In addition, basaltic nets were used as reinforcement for metakaolin and sand-based geopolymers. The tests carried out showed the following:  
* The highest compressive strength values among all tested geopolymers were achieved by fly ash and sand-based geopolymers activated with 10 M NaOH solution and reinforced with two layers of basalt grid (66.2 MPa), and fly ash and sand-based geopolymers activated with 5 M NaOH solution and reinforced with a single layer of basalt grid (66.6 MPa).
* The addition of carbon fiber reinforcement (long or chopped) did not significantly improve the compressive strength of geopolymers (values oscillated around 48 MPa) and geopolymer-cement hybrids (values oscillated around 50 MPa). However, the highest value was obtained for the hybrid material with 0.5% carbon fiber reinforcement in cut form (5 mm).
* The addition of glass fiber type E to geopolymers and geopolymer-cement hybrids increased the compressive strength in comparison to the reference samples. However, the highest value was obtained for a material based on fly ash and sand with the addition of Portland cement, reinforced with 2% chopped glass fiber, with a length of 12 mm (42.3 MPa).
* The highest flexural strength values among all tested geopolymers were achieved by fly ash and sand-based geopolymers, which were activated with 10 M NaOH solution and reinforced with two layers of basalt grid (12.0 MPa), and fly ash and sand-based geopolymers, activated with 10 M NaOH solution and reinforced with a single layer of basalt grid (12.0 MPa).
* The use of E-glass fiber reinforcement contributed to a decrease in flexural strength values for all the fly ash and sand samples tested compared to the values obtained by the reference material (9.7 MPa).
* The use of E-type chopped glass fiber reinforcement, 12 mm in length, contributed to an increase in flexural strength values for the geopolymer-cement hybrids: 6.5 MPa for 1% addition and 8.2 MPa for 2% addition to the matrix, respectively.
* The addition of carbon fiber resulted in a slight decrease in the strength parameters (flexural strength) of the geopolymer-cement hybrids, compared to the reference material (11.4 MPa).
* The addition of carbon fiber decreased the strength parameters (flexural strength) of fly ash and sand-based geopolymers compared to the reference material (10.4 MPa).
* 5 M NaOH solution is a poor alternative for the production of geopolymer materials based on metakaolin and sand due to unsatisfactory results from strength tests.  
Conceptualization, M.L., K.K. and K.P.; methodology, B.K., D.K. and D.J.; formal analysis, M.L. and K.K; resources, K.K. and M.L.; writing--preparation of the original draft, K.P. and M.C; writing--review and editing, M.L. and K.K.; supervision, M.D.G. and M.C.; fundraising and proofreading of the article, M.D.G. and M.C. All authors have read and agreed to the published version of the manuscript.  
This work has been financed by the Polish National Agency for Academic Exchange under the International Academic Partnership Programme within the framework of the grant: E-mobility and sustainable materials and technologies EMMAT (PPI/APM/2018/1/00027).  
Not applicable.  
This research was supported by a French Government Scholarship.  
The authors declare no conflict of interest.