**3. Results and Discussion**

*3.1. Phase Composition of Precursors Results*

Table 3 shows the identified phases for calcium fly ash from Belchatow.


**Table 3.** The phase composition of fly ash from Belchatow.

Calcium fly ash from Belchatow Power Plant is a product of lignite coal combustion in pulverized coal furnaces. Due to the coal's mineral part composition, the ash has an aluminous–silicon–calcium character [18]. The material has a high gehlenite content (around 30%). Anhydrite, anorthite, and mullite oscillate within 14–16%. Hematite occurs within 10%. About 3% of the phase composition consists of chlormayenite and lime. The smallest percentage of the composition is occupied by quartz—about 1%. Similar phases have been shown by other researchers [11].

Table 4 shows the phase composition of non-calcined and calcined diatomite dust.

The analyzed diatomite dust was characterized by the presence of phases such as silicon oxide, kaolinite, albite, and aluminum oxide. Similar phases were obtained in the investigations by Ediz et al. and Ren et al. [19,20]. As a result of the calcination of diatomite, the percentage of albite increased to about 30%, while the percentage of kaolinite decreased to about 31%. In addition, in the calcined diatomite dust, the percentage of the silicon oxide phase increased to almost 38%. The non-calcined diatomite dust had a higher percentage of phases, such as kaolinite (about 49%) and aluminum oxide (0.5%), in its phase composition, compared to calcined diatomite dust.


**Table 4.** Phase composition of non-calcined and calcined diatomite dust.

### *3.2. Mechanical Properties*

Compressive strength is one of the basic methods for evaluating the proper course of the geopolymerization process. The strength results depend on several factors, such as the structure, the presence of a crystalline phase, and the distribution and hardness of insoluble Al-Si particles. In addition, the type of alkali used and the %CaO and %K2O also affect the mechanical properties of the geopolymer composite [21,22]. Figure 2 shows a summary of the average results of compressive strength investigations performed for each type of geopolymer material.

For geopolymers activated with a 10-mole alkali solution, the highest compressive strength value was obtained for the 10MDN10% sample—almost 35 MPa. The lowest value was obtained for the 10MDN30% material—about 12 MPa. The values of the average compressive strength for samples 10MDK10% and 10MDK15% oscillate at a similar level about 25 MPa. The addition of 30% calcined diatomite (10MDK30%) caused a decrease in compressive strength by almost 50% compared to the reference sample (R10). The largest mean standard deviation was recorded for the reference material (R10) and 10MDK10%. However, the lowest values were for 10MDN30% and 10MDK30%.

For geopolymers activated with 14 molar alkali solution, the highest compressive strength value was obtained for sample R14 (reference material)—almost 30 MPa. The lowest value was obtained for the 14MDN30% material—about 11 MPa. The values of the average compressive strength for samples 14MDN10% and 14MDN15% oscillate at a similar level—about 20 MPa. The addition of 30% calcined diatomite (14MDK30%) caused a decrease in compressive strength by almost 50% compared to the reference sample (R14). The highest mean standard deviation, which remained similar, was recorded for the reference material (R14) and the 14MDN15% material. On the other hand, the smallest was for 14MDN30% and 14MDK30%.

#### *3.3. Microscopic Observations*

Scanning electron microscopy (SEM) allows a visual examination of the material to obtain morphological information and allows the evaluation of structures that cannot be revealed by other examination methods [23].

Figure 3 shows the microstructure of geopolymers based on limestone fly ash and sand activated with a 10 mol alkali solution (Figure 3a) and a 14 mol alkali solution (Figure 3b).

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

**Figure 3.** Microstructure of reference geopolymers in magnifications 1000×: (**a**) 10R, (**b**) 14R.

In Figure 3a, the inconsistent structure of the porous material can be observed.

These are grains of unreacted limestone fly ash from Belchatow. However, as opposed to silica ash, limestone fly ash grains are characterized by very large particles of unburned carbon, porous and poorly sintered [11]. Furthermore, in addition to the geopolymer matrix, we can observe C-S-H. A similar structure was analyzed by Zhang et al. in their work [24].

Figure 3b shows the microstructure of a geopolymer based on lime fly ash from Belchatow and sand, activated with a 14 mol alkali solution. In this case, a much smaller amount of unreacted fly ash can be observed. The structure is more compact than that of the 10R material. Alehyen et al., in their work, focused on studying the microstructures of fly-ash-based geopolymer mortars. They described them as porous heterogeneous mixtures in which some of the ash grains did not react or had reacted partially. In addition, they indicated the possible presence of residual alkaline deposits and geopolymer gel [25].

Figure 4 shows the morphology of all geopolymer materials based on fly ash, sand, and diatomite dust (calcined and non-calcined).

As a result of the SEM analysis, it can be observed that materials based on calcium fly ash, sand, and diatomite dust (calcined and non-calcined), activated with a 10 mol alkali solution (Figure 4a–f), have more unreacted lignite particles (clusters of porous structures) in their structure. Compounds of geopolymers activated with 14 molar alkali solution show significantly fewer particles of unreacted ash (Figure 4g–l). According to the presented microstructure images, the fly ash-based materials are characterized by an amorphous structure and contain undecomposed fly ash particles. It was also noted that there is a changeable pore content in the microstructure of the materials.

**Figure 4.** Microstructure of geopolymers in magnification 1000x: (**a**) 10MDN10%, (**b**) 10MDK10%, (**c**) 10MDN15%, (**d**) 10MDK15%, (**e**) 10MDN30%, (**f**) 10MDK30%, (**g**) 14MDN10%, (**h**) 14MDK10%, (**i**) 14MDN15%, (**j**) 14MDK15%, (**k**)14MDN30%, (**l**)14MDK30%.
