**2. Material**

The base material from which the test samples were made was fly ash from coal combustion (Figure 1a) from the Skawina Combined Heat and Power Plant (Skawina, Poland). The ash used is classified as class F. Obtained by XRD analysis on an X-ray diffractometer from Panalytical Aeris (Malvern PANalytical, Lelyweg 1, Almelo, the Netherlands), the proportion of phases comprising the fly ash used in the study is shown in Figure 2. The main components are quartz and mullite, with a small proportion of hematite and magnetite, not exceeding 4%. Quantitative analysis was carried out using the Rietveld method in HighScore Plus software (version: 4.8, Malvern PANalytical B.V, Almelo, the Netherlands) with a PDF-4+ database.

**Figure 1.** SEM micrographs of (**a**) fly ash, (**b**) quartz sand, (**c**) basalt flour.

**Figure 2.** Diffractogram for fly ash.

Materials added in different proportions to the fly ash were quartz sand (Figure 1b) and basalt flour (Figure 1c). Basalt is a volcanic rock that is formed from magma melted in the Earth's mantle. Basalt flour is produced as a result of processing during the extraction of the raw material. The oxide compositions of the fly ash, quartz sand, and basalt powder used for the tests, which were determined with the use of the JEOL JSM-820 scanning electron microscope (IXR Inc., Austin, TX, USA) with the EDS attachment, are shown in Table 1.


**Table 1.** Oxide composition of raw materials.

The summary plots of particle size distribution and cumulative curves for ash, sand, and basalt meal are shown in Figure 3a,b, respectively. Particle size distribution tests were carried out using an instrument from Anton Paar GmbH (Graz, Austria).

**Figure 3.** (**a**) The particle size distribution curves for fly ash, quartz sand, and basalt flour; (**b**) the cumulative curves for fly ash, quartz sand, and basalt flour.

### **3. Sample Preparation**

The first step in the preparation of the geopolymer mortar was to mix dry ingredients in a GEOLAB cement mortar mixer (GEOLAB, Warsaw, Poland), which were previously weighed in the appropriate proportions for each mixture according to Table 2, which shows the composition of the mixtures used for the geopolymers, along with sample determinations.

**Table 2.** Percentage of individual dry components in the mixtures used to produce geopolymers, along with designations.


Then, an alkaline 10 mol solution of sodium hydroxide and an aqueous solution of sodium silicate R-145, whose mass ratio was 1:2.5, was prepared and used to activate the prepared mixtures. The dry ingredients with the addition of an alkaline solution were mixed until a uniform paste was obtained. The ratio of dry ingredients to the solution was 0.4. The prepared geopolymer paste was poured into molds, which were then cured in an SLW 750 STD laboratory oven (POL-EKO-APARATURA, Wodzislaw Slaski, Poland) at 75 ◦C for 24 h. The samples were seasoned under laboratory conditions for 28 days.

#### **4. Methods**

The density of the produced geopolymers was measured using a geometric method measurements were made of the volume and weight of the samples made for compressive strength tests.

Compressive strength and flexural strength measurements were carried out in accordance with PN-EN 196-1:2016-07 [14]. The tests were performed on a Matest 3000 kN testing machine (Matest, Treviolo, Italy) equipped with heads for compressive and flexural strength measurements. For compressive strength tests, six perpendicular specimens of <sup>50</sup> × <sup>50</sup> × 50 mm<sup>3</sup> each were prepared. In contrast, four 50 × <sup>50</sup> × 200 mm<sup>3</sup> specimens were made for flexural strength testing. The specimens were made in accordance with PN-EN 12390-1:2021-12.

The abrasion resistance of the produced geopolymers was determined using the Boehme method. The test was carried out in accordance with PN-EN 14157:2017-11 [15]. The samples prepared for the measurements had dimensions of 71 × <sup>71</sup> × 71 mm3. The abrasive used during the test was alumina (20 g was used for each sample pass). The clamping force of the sample to the disc was equal to 294 N. For each specimen (three specimens were abraded for each compound), 16 cycles were performed for 20 rotations of the disc per cycle. After the cycle was completed, the sample was rotated 90◦ and another cycle was started. One of the methods described in the standard for determining abrasion resistance is the method for measuring sample height loss, which was used in this study. This method involves measuring the height of the sample before and after the test and determining their difference. Based on the literature [16], the classification of the tested geopolymers in terms of abrasion resistance was made.

The standard deviation was determined for all obtained results, which was plotted on graphs in the form of error bars.

The microstructure of the fabricated geopolymers was also observed using a JEOL JSM-820 scanning electron microscope (IXR Inc., Austin, TX, USA). A JOEL DII-29030SCTR vacuum coating machine (IXR Inc., Austin, TX, USA) was used to sputter gold onto the samples.
