**5. Results and Discussion**

Analyzing the results of density measurements of the tested geopolymers (Figure 4), it can be observed that the replacement of quartz sand with basalt flour has a slight effect on the obtained values—a slight increase in density can be observed with an increase in the share of basalt meal in the mixture. This is related to the approximate densities of sand and basalt flour. The obtained results do not differ from the values of densities of fly ash-based geopolymers presented in the literature [2,4,17].

**Figure 4.** The density of the tested geopolymers.

As can be observed in Figure 5a,b, replacing quartz sand with basalt flour has a favorable effect on the strength properties of the produced geopolymers. As the proportion of basalt meal in the mix increases, there is a significant increase in compressive strength and a gentle increase in flexural strength. The highest values of compressive strength and flexural strength were obtained for the sample which was made from a mixture consisting of fly ash and basalt flour in a ratio of 1:1. The recorded increase in these values relative to the reference sample, made from a mixture of fly ash and sand in a ratio of 1:1, was about as high as 106% and 11% for compressive strength and flexural strength, respectively. It is also worth noting that for samples with 37.5% and 50% basalt meal, the resulting compressive strengths are higher than the compressive strengths of average concretes used in residential and commercial construction, typically ranging between 17 MPa and 28 MPa [18]. The particles of basalt powder are small, so they can fill the voids in the geopolymer structure, which is confirmed by SEM images, and thus effectively improve the strength properties of the tested composites. In addition, the particles of basalt powder dispersed in the geopolymer structure can contribute to passivation and stress dissipation, which allows the delay of the appearance of plastic deformation and the appearance of

cracks, which increases the bending strength [19,20].

**Figure 5.** Results for tested geopolymers of (**a**) compressive strength test, (**b**) flexural strength.

The obtained results of compressive strength are also higher in comparison with the results from measurements of compressive strength of geopolymers produced on the basis of basalt meal activated with aqueous sodium hydroxide solution, as presented by Saray et al. [21].

In their work, Venyite et al. [22] studied geopolymers produced based on a mixture of metakaolin, calcined laterite, and basalt flour activated with a 6 mol sodium hydroxide solution and cured at room temperature. The authors showed that the incorporation of basalt into geopolymers based on calcined laterite and metakaolin resulted in compressive strengths of 41.14, 34.46, 40.46, and 24.93 MPa for 20, 30, 40, and 50 wt.% basalt addition.

Beskopylny et al. [23] studied, among other things, the statistical compressive and flexural strengths of fine-grained geopolymer concrete with different types of stone flours. They obtained compressive strength values between 34.1 MPa and 52.2 MPa, and values between 4 MPa and 6.7 MPa for flexural strength for the produced geopolymers.

Figure 6 shows the values of height loss for all tested geopolymers after abrasion tests. Analyzing the results obtained, it can be concluded that all the geopolymers produced are very hard-to-wear materials, as evidenced by the height loss not exceeding the value of 2.5 mm. The highest abrasion resistance was demonstrated by samples made from a mixture consisting of fly ash and basalt flour in a ratio of 1:1, for which the measured average value of sample height loss was 0.81 mm.

Similar results for a geopolymer made from a mixture based on fly ash and quartz sand, also activated with a 10 mol sodium hydroxide solution, were obtained by Bazan et al. [24], where the average value of sample height loss after abrasion tests was 0.9 ± 0.02 mm, which also classifies the tested material as very difficult to abrade.

Figure 7 shows microphotographs of the structures of the produced geopolymers. For all samples, typical features of the structure of geopolymers produced based on fly ash [25] can be observed, including unreacted spheroidal fly ash particles and dissolved fly ash particles. In addition, it is possible to observe the presence of sand and basalt particles, present in the geopolymer gel in the samples for the production of which they were used. During the study of the structure of the tested geopolymer composites, the influence of the proportion of basalt flour on porosity could be observed—with the increase in the proportion of basalt flour in the mixture, the size of the voids, as well as their share in the structure, decreases.

**Figure 7.** *Cont*.

**Figure 7.** The microstructure SEM photography of produced geopolymers.
