**5. Conclusions**

The effect of elevated temperature ranging from 200 ◦C to 800 ◦C on samples with 100% fly ash and 50% fly ash, and ground granulated blast furnace slag blended geopolymer mortar was investigated in the presented research by evaluating compressive strength, tensile strength, apparent density, specific density, porosity, and the ultrasound pulse velocity. Based on the results, the following conclusions are drawn:


According to the obtained results, both materials can be used as the basis for developing fire insulation materials or thermal barriers. They remain stable in terms of integrity under high-temperature conditions, presenting porosity at an affordable level. Despite the drop in mechanical properties, compressive strength, even after exposure to 800 ◦C, remained at a satisfactory level in terms of the structural application.

**Author Contributions:** Conceptualization, T.M.M.I., I.H., M.S., K.M., E.S.; methodology, I.H, T.M.M.I. and K.M.; software, M.S., K.M. and T.M.M.I.; validation, I.H., K.M. and M.S.; formal analysis, T.M.M.I., K.M. and M.S.; investigation T.M.M.I., K.M., E.S. and M.S.; data curation, T.M.M.I., K.M. and M.S.; writing—original draft preparation, I.H., K.M. and M.S.; writing—review and editing, T.M.M.I., I.H., M.S., K.M., E.S.; visualization, K.M. and M.S.; supervision, I.H.; project administration, I.H.; funding acquisition, I.H., K.M. and M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Cracow University of Technology and the Polish National Agency for foreign students in the framework of the project NAWA Spinaker, POWR.03.03.00-00- PN16/18. 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).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This research was part of the work provided by two interdisciplinary research group of Building Materials Engineering, Faculty of Civil Engineering (Cracow University of Technology, 31-155 Cracow, Poland) & Buildings and Public Works (School of Engineering—Cesi, 76800 Saint Etienne du Rouvray, France). Publication cost of this paper was covered with founds 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.
