*2.2. Sample Preparation*

First, Mefisto L05 and Ron D460 HR precursors were mixed with three waste aggregate fractions. Pellets of carbon fibers were dispersed in water with non-ionic surfactant Triton X-100 and siloxane-based air-detraining agent Lukosan S to reduce the surface tension of the carbonaceous admixture and defoam the suspension. The vessel with the suspension was then treated in an ultrasonic bath for 10 min to effectively crumble the pellets and disperse individual fibers. The well-prepared suspension was poured into a dry mixture of precursors, aggregate, and alkali activator and mixed for 10 min. Fresh mortar was finally placed into molds (Figure 4): 160 × <sup>40</sup> × 40 mm3—mechanical properties, 100 × <sup>100</sup> × 100 mm3—electrical properties, 70 × <sup>70</sup> × 70 mm3—thermal properties. The <sup>100</sup> × <sup>100</sup> × 100 mm3 samples were additionally embedded with copper-grid electrodes using a 3D-printed plastic board for precise positioning. After 24 h of curing in laboratory conditions (22 ◦C, 50% RH), samples were unmolded and left in equal conditions for a further 28 days. The composition of the studied geopolymer is summarized in Table 2.

**Figure 4.** RMCF1 mortar samples.

**Table 2.** Composition of RMCF1 mortar.


*2.3. Methods*

The bulk density *<sup>ρ</sup><sup>v</sup>* [kg·m<sup>−</sup>3] was determined on the 40 × <sup>40</sup> × 160 mm3 samples using the gravimetric method (Equation (1)). The matrix density *<sup>ρ</sup>mat* [kg·m−3] was evaluated by helium pycnometry (Pycnomatic ATC EVO). The total open porosity *ψ* [%] was then calculated using Equation (2).

$$
\rho\_v = \frac{m}{V} \tag{1}
$$

$$
\psi = \left(1 - \frac{\rho\_v}{\rho\_{\text{mat}}}\right) \cdot 100 \tag{2}
$$

The dynamic modulus of elasticity *Edyn* [MPa] was determined on the 40 × 40 × 160 mm<sup>3</sup> samples via a non-destructive method using the Pundit ultrasonic device according to Equation (3).

$$E\_{dyn} = \rho\_v \cdot v^2 \tag{3}$$

where *<sup>ρ</sup><sup>v</sup>* [kg·m<sup>−</sup>3] is the bulk density of the material, and *<sup>v</sup>* [m·s<sup>−</sup>1] is the speed of ultrasonic wave propagation through the sample.

Mechanical properties represented by the flexural and compressive strength were determined according to the CSN EN 196-1 [ <sup>ˇ</sup> 16] using FP 100 and ED60 presses on the 40 <sup>×</sup> <sup>40</sup> × 160 mm3 samples after 7 and 28 days.

The thermal properties, the thermal conductivity *<sup>λ</sup>* [W·m−1·K−1] and the specific heat capacity *cp* [J·kg−1·K<sup>−</sup>1] were determined on the 70 × <sup>70</sup> × 70 mm<sup>3</sup> samples via nonstationary measurements using the ISOMET 2114 device equipped with a flat surface probe.

The DC electrical conductivity was determined on the 100 × <sup>100</sup> × 100 mm3 electrodeembedded samples using a GW Instek GPR-11H30D power source and two Fluke 8846A multimeters for the voltage and current measurements. The electrical conductivity *σ* [S·m<sup>−</sup>1] was determined for three different input voltage levels according to Equation (4).

$$
\sigma = \frac{I}{\underline{U}} \cdot \frac{l}{\underline{S}} = \frac{1}{\underline{R}} \cdot \frac{l}{\underline{S}} \tag{4}
$$

where *I* [A] is the electric current, *U* [V] is the voltage, *S* = 0.0072 [m2] is the area of electrodes, *l* = 0.07 [m] is the distance between electrodes and *R* [Ω] is the resistance of the material.

AC electrical properties represented by the magnitude of the impedance *Z* [Ω] and the phase shift *θ* [ ◦] were determined in the range of 10 Hz–10 MHz on the 100 × 100 × 100 mm<sup>3</sup> samples using a GW Instek 8210 LCR bridge.
