*3.2. Mechanical Properties*

The results for compressive and flexural strength are presented in Table 2.

**Table 2.** Mechanical properties.


The obtained values for mechanical properties are quite reasonable. The obtained results are not as good as for fly-ash-based geopolymers [5]. However, the values allow for planning some possible applications for this material, especially as a material for construction purposes, such as pavements onsite at the mining place [19].

### *3.3. Microstructure Investigation*

The obtained diffractogram is presented in Figure 3. Based on this investigation a quantitative analysis was performed and the mineralogical composition was determined— Table 3.

**Figure 3.** XRD analysis for coal shale from Marcel mining (before calcination).


**Table 3.** Phases identified along with their percentage share in the sample.

The results show a relatively small amount of quartz, a high amount of kaolinite, and a slightly increased amount of illite and typical for coal shales amount of muscovite [20]. The relatively low mechanical properties of the designed geopolymers can be associated with a small amount of quartz that affects the mechanical properties [21]. At the same time, illite could also decrease the mechanical properties of geopolymer [21]. The common influence of a small amount of quartz and an elevated amount of illite probably decides the relatively low mechanical properties of the investigated material. The high amount of kaolinite, that under temperature is transformed into metakaolinite, should be a factor that makes the geopolymerization process possible. It is an advantage taking into consideration the geopolymerization process [22].

Moreover, SEM observation was provided for the investigated samples—Figure 4.

The visible microstructure of the material is typical for geopolymers [2,5]. In Figure 4a,c there is visible cracking which is an effect of previous mechanical strength tests. Additionally, an EDS analysis was performed for the selected points. Exemplary results are presented in Figure 5.

The oxide composition obtained as a result of EDS measurements is typical for a geopolymer material. It is characterized by a large amount of SiO2, about 60%, and Al2O3 of more than 21% [2,5]. Another important compound, Na2O at 17%, probably comes from the alkali activation process. There are also small amounts of MgO and K2O (less than 1%), in the structure.

**Figure 4.** SEM images of geopolymer: (**a**) at 100× magnification; (**b**) at 200× magnification; (**c**) at 1000× magnification; and (**d**) at 2000× magnification.

**Figure 5.** Geopolymer sample: (**a**) SEM image with the point of measurements; and (**b**) results of the EDS analysis.
