**6. Conclusions**

The study investigated the possibility of using basalt flour as a substitute raw material for quartz sand during the production of fly ash-based geopolymers. The effect of the addition of basalt flour and its proportion on the durability properties of the produced geopolymers was evaluated. Based on the analysis of the obtained test results, the following conclusions can be formulated:


Basalt flour can be successfully used as a replacement for quartz river sand during the production of fly ash-based geopolymers which have application potential in construction.

**Author Contributions:** Conceptualization, B.K.; methodology, B.K.; validation, B.K.; formal analysis, B.K. and K.M.; investigation, B.K.; resources, B.K.; data curation, B.K.; writing—original draft preparation, B.K. and S.G.; writing—review and editing, B.K., S.G. and K.M.; visualization, B.K.; supervision, B.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** The publication cost of this paper was covered with funds from the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work has been supported by the National Centre for Research and Development in Poland in the framework of the project SMART-G Smart Geopolymers (ERA-MIN2- 3/SMART-G/1/2022).

**Conflicts of Interest:** The authors declare no conflict of interest.
