**3. Results and Discussion**

The summary of the material properties of the geopolymer mortar is given in Table 3. The total open porosity of ~17%, corresponding to the bulk and matrix density of 1954 kg·m−<sup>3</sup> and 2362 kg·m−3, was mainly affected by the amount of mixing water (water/binder = ~0.37) due to the use of waste aggregates and carbon fibers [17]. The thermal conductivity and specific heat capacity were of 1.18 W·m−1·K−<sup>1</sup> and 884 J·kg−1·K−<sup>1</sup> which are typical values for such composites. Thermal conductivity was reasonably high for an effective spreading of the heat, which is an important presumption for self-heating ability.

**Table 3.** Basic physical, thermal and electrical properties of RMCF1 mortar.


The flexural and compressive strengths at 7 d and 28 d are summarized in Figure 5. An increase in flexural strength from 2 MPa to 2.5 MPa and 12.5 MPa to 15 MPa in compressive strength was observed. The relatively low strength values were mainly due to the use of a significant amount of waste metashale binder, as well as fine waste aggregate (input waste materials ~85 wt.%). The decrease in the strength of cementitious and geopolymer composites due to the substitution of natural aggregate with waste counterparts is well known and mentioned in the literature, e.g., Nuaklong et al. [18], who investigated fly ash geopolymers with limestone and recycled aggregate, observing that the recycled aggregate led to a reduction in strength from 40 MPa to 30.6 MPa. Zaid et al. [19] studied natural aggregate replacement with recycled aggregate in steel-fiber-reinforced concrete and concluded that with an increasing amount of the recycled aggregate, the strength of the composites decreased. It was justified by a higher porosity of the recycled aggregate, resulting in an increase in material drying shrinkage and the formation of microcracks.

AC electrical properties represented by the frequency-dependent magnitude of the impedance and phase shift are presented in Figure 6. The magnitude of the impedance, involving both the resistive and capacitive component (resistance, capacitance), decreased from the initial ~175 Ω (10 Hz) to ~100 Ω (10 MHz), which due to the low values, revealed the potential for new functional properties, such as self-heating. The phase shift was from −3◦ to −10◦ in the tested frequency range and a noticeable decrease was observed at higher frequencies of 1–10 MHz. The phase shift close to 0◦ confirmed the resistive nature of

the mortar and its potential for the self-heating function in an AC electric field. Since the DC electrical conductivity of the mortar (*<sup>σ</sup>* = 3.9·10−<sup>2</sup> <sup>S</sup>·m−1) was reasonably high, the self-heating potential was confirmed also in a DC electric field.

**Figure 5.** (**a**) Flexural strength, (**b**) Compressive strength of RMCF1 mortar.

**Figure 6.** AC characteristics of RMCF1 mortar.

### **4. Conclusions**

The study was focused on the design and basic material characterization of a geopolymer mortar with a special emphasis on the utilization of a significant amount of waste input materials. The waste metashale binder and waste cementitious aggregate originating from crushed defective cementitious products and cement mix used within the design of the geopolymer composite ensured an ~85 wt.% waste origin of input materials.

The mechanical properties of the mortar (28 d: *Rf* ~ 2.5 MPa, *Rc* ~ 15 MPa) are acceptable for some civil engineering applications. Nevertheless, it is important to target further efforts on the optimization of the geopolymer mortar composition, ensuring better mechanical performance. The thermal and electrical properties were favorable for the self-heating function in a DC and AC electric field, even with a low amount of carbon fibers. Nevertheless, it should be noted that the samples were characterized in a partially water-saturated state (curing in laboratory conditions, successive measurements without the preceding drying). Since the porous system in a heterogeneous geopolymer matrix is partially filled with water/salt solutions, which is beneficial in view of the thermal and electrical conductivity increase, the evaluated self-heating potential is slightly higher than in the case of dry material.

**Author Contributions:** Conceptualization, L.F. and R.C.; methodology, P.H. and Y.-H.C.; validation, ˇ L.F. and W.-T.L.; investigation, P.H. and Y.-H.C.; resources, L.F. and W.-T.L.; data curation, P.H. and Y.-H.C.; writing—original draft preparation, L.F. and P.H.; supervision, L.F., W.-T.L. and R.C.; project ˇ administration, L.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** The outcome has been achieved with the support of M.era-Net Call 2021, Project No. 9262 and financial support from the Technology Agency of the Czech Republic under the project No. TH80020002 and the Grant Agency of the Czech Technical University in Prague under the project No. SGS22/137/OHK1/3T/11. Publication cost of this paper was covered with funds from the Polish National Agency for Aca-demic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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