**4. Conclusions**

The obtained results show the influence of precursor particle size for geopolymerization using a sodium activator on mechanical properties. The best result for mechanical strength, both for compression and flexural strength, was obtained for geopolymers where particle sizes were below 200 μm—19 MPa and 5.7 MPa, respectively. A slightly worse result, about 4% lower, was obtained for samples with particle sizes in the range of 200–900 μm. The worst results were obtained from samples with the largest particle sizes from 900 μm to 1.25 mm. A large amount of fine-fraction particles in the geopolymer G 200–900 μm, below 100 μm, can affect the high strength of the geopolymer, giving the effect of a filler. The high mechanical strength of G 200 μm resulted from the presence of very small particles below 90 μm, which facilitate the solubility of SiO2 and Al2O3 in the NaOH solution [24,28]. Grinding precursor particles to sizes below 900 μm allows improved strength properties to be achieved, and at the same time saving energy, without the need to further grind particles to smaller than 200 μm.

Coal gangue from the mining industry, for example Wieczorek, is a material that is still being researched and has the potential to be used in geopolymerization.

It is an environmentally friendly solution, especially if the method for its preparation as a raw material to produce geopolymers is energy-saving.

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

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was supported by the Polish National Centre for Research and Development under the LIDER Grant no: LIDER/31/0168/L-10/18/NCBR/2019. 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 material engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

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

#### **References**


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