*3.3. Evaluation of the Microstructure and Analysis of the Oxide Chemical Composition of the Resulting Geopolymers*

A JEOL IT200 SEM scanning microscope (JEOL, Warszawa, Poland) was used to perform oxide analysis of the resulting samples, as shown in Figure 3.



(**a**) (**b**)

**Figure 3.** Analysis of the oxide composition of the Belchatów lignite fly ash—(**a**) 10 M NaOH, (**b**) 14 M NaOH.

The above photo shows the structure of a geopolymer based on limestone fly ash Belchatów, activated with solutions of different molar concentrations. There are no visible differences in the structure and the oxide analysis performed as well [11,12].

### **4. Short Discussion**

The analysis of the phase composition performed within the scope of the article is consistent and comparable with the analysis presented in the literature [6]. Only larnite and one phase of C3A cement clinker were not identified during the study.

Oxide chemical composition analysis was carried out using a scanning microscope with an EDS system. A comparison of the literature condition with the test results (Figure 3) showed that the same oxides are mostly present in the base material [6].

The presented tests of mechanical properties, i.e., bending and compressive strength, showed that there is no major difference between the activator 10 M and 14 M. The obtained results are comparable with the results of other researchers [13]. Fly ash from lignite combustion has a less amorphous phase (glassy particles), so it is less reactive compared to hard coal ash from pulverized coal boilers. The use of an activator with a higher concentration

does not bring results, because the phases that can be solubilized and activated become active at a concentration of 10 M. For economic reasons, i.e., ever-increasing inflation and current fuel prices, a better activator is one with a lower concentration.
