**2. Materials and Methods**

The test samples were made from lignite fly ash, which came from the Bełchatów Power Plant (Bełchatów, Poland), with waste code 10 01 02, and river sand from a sand plant in Swietochłowice ( ´ Swietochłowice, Poland). The test material was made in a 50/50 wt.% ´ ratio. The base material used for the study, fly ash, was tested for phase analysis and particle size distribution. Table 5 show the XRD phase analysis, while Table 6 shows the particle size distribution analysis. The reaction activator was a 10 M and 14 M NaOH solution. Technical sodium hydroxide flakes and an aqueous solution of sodium silicate R-145 with a molar modulus of 2.5 and a density of about 1.45 g/cm<sup>3</sup> were used. The ratio of the two components was 1:2.5. Distilled water was not used—the supplementary water added was "mains" water. To prepare the alkaline solution, solid sodium hydroxide was poured over an aqueous solution of sodium silicate and water. The solution was mixed thoroughly and allowed to equilibrate until it reached a constant concentration and temperature. The compositions of the geopolymer mixture are shown in Table 7. The samples were cured in an SLW 750 laboratory dryer (POLEKO, Wodzisław Sl ˛ ´ aski, Poland), at 60 ◦C for 24 h. After 28 days of sample preparations, the geopolymers were tested for flexural and compressive strength.

**Table 5.** Phase analysis of the Belchatów lignite fly ash.


**Table 6.** Particle size analysis of the Belchatów lignite fly ash.



**Table 7.** Compositions of geopolymer mixture of the Belchatów lignite fly ash.

#### **3. Results**

*3.1. Tests of Mechanical Properties—Flexural Strength Tests*

Flexural strength tests were carried out on an MTS Criterion 43 testing machine with TestSuites 1.0 software (MTS System Corp., Eden Prairie, MN, USA) with a measuring range of up to 30 kN. The method for determining the flexural strength of cement mortar specimens is specified by EN 196-1:2016-07 (Cement test methods—Part 1: Determination of strength—Section 9.1) [10]. Test specimens in the form of cuboids are subjected to a bending moment by applying a uniform load, induced by the lower and upper rollers of the testing machine. The test continues until the maximum load is reached, which causes the failure of the component, and the bending strength is calculated based on this parameter. The bending strength is determined by the formula for the three-point method:

$$\mathbf{R}\_f = \frac{1.5 \ast \mathbf{F}\_f \ast \mathbf{l}}{b^3} \text{ [MPa]} \tag{1}$$

where:

R*f*—flexural strength (MPa)

b—lateral length of the section (mm)

F*f*—maximum load (N)

l—length between supports (mm).

For the tests, six specimens activated with 10 M NaOH and six specimens activated with 14 M NaOH with dimensions of 40 × 40 × 160 mm were prepared. Based on the results obtained, a graph (Figure 1) was made showing the results of flexural strength tests.

**Figure 1.** Flexural strength of the Belchatów lignite fly ash samples.

The above image shows that the test results for samples activated with 10 M solution and 14 M solution are comparable. The values for all samples are in a range of 2–3 MPa. The average strength value for both activators is 2.5 MPa. However, for economic reasons, a better choice would be solutions with a lower molar concentration.
