**4. Conclusions**

The obtained results show the possibility of using a coal shale from Marcel mining to create new eco-friendly materials, geopolymers. The geopolymer was prepared using a sodium activator. The obtained material has a structure, chemical and mineralogical composition for geopolymer. The material had a compressive strength of 12.7 MPa and a bending strength of 3.4 MPa. These kinds of mechanical properties are relevant for construction applications such as various types of foundations, walls, columns, lintels, terraces, stairs, ceilings, small building elements, and small architecture. However, further applications required additional research, including durability studies. According to an environmental point of view, the most promising seems to be an application onsite for the mining industry to minimalize the cost of transportation.

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

**Funding:** This research was funded under the project: "Development of geopolymer composites as a material for protection of hazardous wrecks and other critical underwater structures against corrosion" under the M-ERA.NET 3 program by the Polish National Centre for Research and Development, grant number M-ERA.NET3/2021/71/MAR-WRECK/2022.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The publication cost of this paper was covered with funds of 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.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
