*3.2. Tests of Mechanical Properties—Compressive Strength Tests*

Compressive strength tests were carried out on an MATEST 3000 kN testing machine (Matest, Treviolo, Italy). In the building and construction sector, the document that regulates the method for determining the compressive strength of cement mortar specimens is PN-EN 196-1:2016-07 (Methods for testing cement—Part 1: Determination of strength—Section 9.2) [10]. Compression testing involves loading specimens until a critical value is reached that will cause the material to fail. The maximum load is the basis for calculating the compressive strength of the concrete material according to the following formula:

$$\mathbf{R}\_{\mathbf{\tilde{c}}} = \frac{\mathbf{F}\_{\mathbf{\tilde{c}}}}{1600} \begin{bmatrix} \text{MPa} \end{bmatrix} \tag{2}$$

where:

R*c*—compressive strength (MPa) 1600—surface of tiles (or auxiliary tiles) (mm2) F*c*—maximum load (N).

Specimens formed after flexural strength tests—12 specimens activated with 10 M NaOH and 12 specimens activated with 14 M NaOH with dimensions of 40 × 40 × 40 mm were used for the tests. Based on the results obtained, a graph (Figure 2) was made showing the results of compressive strength tests.

**Figure 2.** Compressive strength of the Belchatów lignite fly ash samples.

The above picture shows that the test results for samples activated with 10 M solution and 14 M solution are comparable, as is the case for the testing of crushing strength. The values for all samples are in a range of 20–35 MPa (the last result for the 10 M activator is only a deviation from this norm). The average strength value for both activators is 28 MPa. Again, for economic reasons, a solution with a lower molar concentration would be a better choice.
