*3.2. Chemical and Mineralogical Properties*

Alkaline activation increased the amorphous matter content and also caused the formation of a zeolitic structure, diffracting at low angles. As the slag content increased, the amount of amorphous matter increased as well (see Figure 3). It may be observed that the phase composition of alkali-activated S+R mixtures (Table 4) reflects the gradually changing composition of the mixed precursors.

**Figure 3.** Comparison of two extreme activated products: 20S+80R and 80S+20R. Explanatory notes: Kao, kaolinite; Ill, illite; Zeo, zeolitic structure; Hyd, hydrotalcite; Ak, akermanite; C, calcite; Q, quartz; M, mullite.



The amount of R-related phases (kaolinite, illite, mullite) decreased, while the amount of slag-related minerals (akermanite, merwinite) increased with the slag content. Interestingly, the calcite content was about the same in all samples. This might indicate that akermanite also undergoes some kind of alteration in an alkaline environment. The zeolitelike structure was clearly apparent only in the 20S+80R; the lower R content caused its merge in low-angle intensity to increase. Hydrotalcite was observed as clear crystalline product of activation; its content increased with the slag dosage (magnesium and carbonate anions both come from slag).

### *3.3. Mechanical Properties*

The mortar specimens (the samples were 40 × 40 × 160 mm in dimension) were tested for basic mechanical properties, i.e., flexural tensile strength and compressive strength. The mechanical properties were investigated both on specimens produced without the addition of WFF and on separate specimens with WFF added. Both strengths (flexural and compressive tensile) were tested over a standard period of 28 days.

Figure 4 below shows the results of the individual flexural tensile strength tests. As expected, the specimens without waste fibers showed lower values for these strengths. The highest values were achieved by the 60S+40R blend in both cases (without and with WFF). The flexural tensile strength of the samples with WFF fibers was 8.96 MPa, an improvement of nearly 27% in these strengths.

**Figure 4.** Comparison of the flexural strength of samples without and with WFF fibers.

For the mixture labeled 20S+80R, the final strengths deteriorated and decreased after the addition of fibers. This phenomenon was caused by the poor precursor ratio. A large amount of RON D460 HR metashale did not produce good strengths under alkaline activation.

The compressive strength (Figure 5) of the specimens was tested on fractions of the specimens after the flexural tensile strength test. Again, the compressive strength of the fiber specimens without WFF was highest for the mixture marked 60S+40R, which achieved 46.9 MPa. There was a slight increase in the compressive strength after the addition of WFF fibers. For the 60S+40R blend, the increase was below 10 MPa.

**Figure 5.** Comparison of the compressive strength of samples withoutand with WFF fibers.

The best blend in terms of mechanical properties appeared to be a blend of the alkaliactivated composite with 60% GGFBS and 40% RON D460 HR within the precursor blend. The fibers helped to improve the structure and thus increased the flexural and compressive tensile strengths of the alkali-activated material.

#### *3.4. Scanning Electron Microscopy (SEM)*

Scanning electron microscopy (SEM) was used to investigate the surface of the alkaliactivated composite. The aim was to capture the interface between the WFF fibers and

the sample matrix, and to map any surface changes or micro-cracks in the material (see Figure 6).

**Figure 6.** (**a**) The interface between the fiber and the matrix. (**b**) Structure of the material with microcrack-trapping fibers.

In Figure 6a, a single strand of the fibers can be seen, in which the individual glass fibers can be seen due to the cut. The quality of the interconnection between the fibers and the sample matrix can be seen. This resulted in a stronger and more durable material structure. In Figure 6b, the ability of the fibers to prevent the propagation of microcracks (shown in purple in the figure) is more evident. The size of the microcracks in the samples was around 7 μm. The largest microcrack captured was 20 μm.

### **4. Conclusions**

This study considered the utilization of various waste materials in the form of upcycling to design environmentally friendly reinforced materials that may be viewed as an alternative to cementitious materials. For this purpose, a blended precursor composed of waste metashale and ground granulated blast furnace slag was activated by a waste cleaning solution and reinforced by waste fiberglass. The results obtained indicated the successful integration of the WFF into the material's matrix without any distinct segregation or the formation of microcracks on the interface. Increasing the metashale from 20% to 40% resulted in a positive effect on the mechanical properties, specifically from 42 MPa to 47 MPa in compressive strength without fibers and from 45 MPa to 57 MPa with fibers. However, a further increase in the metashale content reduced both the flexural and compressive strength. The most beneficial effect of applying fibers was seen for the combination of 60% slag and 40% metashale. In general, the mechanical strength of the designed materials reached a sufficient level and can be deemed to be a viable way for future follow-up work aiming to preciselydetermine the optimal ratio of the components.

**Author Contributions:** Conceptualization, J.F. and R.C.; methodology, J.F. and R. ˇ C.; software, M.M.; ˇ validation, J.F. and R.C.; formal analysis, J.F. and R. ˇ C.; investigation, J.F. and M.M.; resources, J.F.; ˇ data curation, M.M.; writing—original draft preparation, M.M. and J.F.; writing—review and editing, J.F.; visualization, M.M.; supervision, J.F.; project administration, R.C.; funding acquisition, R. ˇ C. All ˇ authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the support of M.era-Net Call 2021, Project No. 9262 and financial support from Technology Agency of Czech Republic under project TH80020002 and by the Czech Technical University in Prague under project No. SGS22/137/OHK1/3T/11.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The publication cost of this paper was covered by funds from the Polish National Agency for Academic Exchange (NAWA): "MATBUD 2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD 2023.

**Conflicts of Interest:** The authors declare no conflict of interest.
