*Sodium chloride solution*

A conductivity of 0.333 S/m ± 0.001 S/m (mean ± std) was measured in 0.17 % NaCl solutions with the ProfiLine Cond 3310 (0.5 % uncertainty) with a temperature compensation to 25 °C, which is in accordance with previously published data [21]. Without temperature compensation, the 0.17 % NaCl solution provided conductivities of 0.299 S/m ± 0.005 S/m on the ProfiLine Cond 3310 at temperatures of 20.04 °C ± 0.70 °C. Considering these conductivities and the geometry, with an inner electrode distance of 25 mm and a tube diameter of 58 mm, the reference impedance for 0.17 % NaCl solution for our cell configuration was 31.69 Ohm at 20 °C and 28.45 Ohm at 25 °C (Z25ref). The measured impedance of the reference 0.17 % NaCl solutions in the four-electrode [22] setup was 31.41 Ohm ± 0.14 Ohm (Zmeas) which resulted in a conductivity value of 0.301 S/m ± 0.002 S/m (cf. equation 2). During these measurements, the temperature in the 0.17 % NaCl solutions was 21.04 °C ± 0.18 °C (ϑmeas). We determine the cell constant α for the four-electrode setup to -0.026. The cell constant was calculated from equation 1 using Z25ref, Zmeas, and ϑmeas from the impedance measurements with only 0.17 % NaCl solution in the cell.

With the temperature compensation applied (cf. equation 1), the impedance of 0.17 % NaCl solution in the four-electrode setup was 28.39 Ohm ± 0.19 Ohm at 25 °C, resulting in a conductivity value of 0.332 S/m ± 0.003 S/m. Figure 1 depicts the averaged conductivity traces from the three sample solutions with their mean, standard deviation, and the range deviating 1 % of the mean.

*Agar hydrogel*

The three samples of 2 wt% agar hydrogel doped with 0.17 % NaCl solution provided a conductivity of 0.284 S/m ± 0.009 S/m at a room temperature of 21.05 °C ± 0.23 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.314 S/m ± 0.01 S/m was obtained.

The samples with 3 wt% agar provided a conductivity of 0.272 S/m ± 0.005 S/m at a room temperature of 20.99 °C ± 0.19 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.302 S/m ± 0.005 S/m was obtained.

The samples with 4 wt% agar provided a conductivity of 0.281 S/m ± 0.02 S/m at a room temperature of 20.88 °C ± 0.41 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.311 S/m ± 0.018 S/m was obtained.

Figure 2 presents the averaged conductivity data from the samples of 2 wt%, 3 wt% and 4 wt% agar in 0.17 % NaCl solution with their standard deviation, both for the frequency range and the constant frequency of 1 Hz.

Even though measurements were conducted in a grounded Faraday cage, the spectra showed a deviation at 50 Hz reflecting power-line interference.

Figure 3 presents the averaged conductivity data from the three samples of 2 wt% agar in 0.17 % NaCl solution with their standard deviation, both for the extended frequency range from 0.01 Hz to 100 kHz and the constant frequency of 10 Hz. In this measurement regime, the samples provided a conductivity of 0.276 S/m ± 0.006 S/m at a room temperature of 20.76 °C ± 0.1 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.306 S/m ± 0.007 S/m was obtained.

*Gypsum*

Figure 4 presents the conductivity data from one sample of Stewaform gypsum without NaCl in the casting compound infiltrated with epoxy resin and immersed in a 0.17 % NaCl solution. The sample was repeatedly tested with at least 20 h at ambient air between measurements. Each test contained three measurement series.

The gypsum sample demonstrated a capacitive character with increasing conductivity for frequencies above 1 kHz, such that the phase decreased from -8 degrees at 1 kHz to -50 degrees at 100 kHz. For quantitative evaluations, the frequency range up to 1 kHz was considered. Given the dry initial condition of the gypsum, the sample demonstrated a conductivity of 0.0008 S/m ± 0.0001 S/m at 25 °C in the first measurement series. In the following measurement series, the previously dried sample demonstrated a conductivity of 0.0017 S/m ± 0.00004 S/m at 25 °C.

Figure 5 presents the conductivity data from the sample of Stewaform gypsum with NaCl in the casting compound infiltrated with epoxy resin and immersed in a 0.17 % NaCl solution. The sample was repeatedly tested with at least 20 h at ambient air between measurements. Each test contained three measurement series. Across the three measurement series, the sample provided a conductivity of 0.037 S/m ± 0.0012 S/m at a room temperature of 18.85 °C ± 0.19 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.043 S/m ± 0.0015 S/m was obtained.

Figure 6 presents the averaged conductivity data from the three samples of gypsum with NaCl in the casting compound with their standard deviation, both for the extended frequency range from 0.01 Hz to 100 kHz and the constant frequency of 10 Hz. In this measurement regime, the samples provided a conductivity of 0.037 S/m ± 0.002 S/m at a room temperature of 19.75 °C ± 0.26 °C. After the temperature compensation to 25 °C (cf. equation 1), a conductivity value of 0.042 S/m ± 0.003 S/m was obtained.

*Conductivity anisotropy*

In Figure 7 we present the conductivity data and the conductivity anisotropy ratio from a tube cell configuration holding 80 reed sticks and filled with 0.17 % NaCl solution. The setup was kept sealed during the complete measurement series. Measurements 1 and 2 were performed on the same day with a break of 6 h in between. The subsequent measurements 3 and 4 were performed on consecutive days and the last measurement 5 was performed after a break of 3 days.

In longitudinal direction, the reed sticks provided an average conductivity of 0.32 S/m with a mean standard deviation of 0.02 S/m (6.2 %) at 10 Hz over 60 min and a mean conductivity of 0.32 S/m with a standard deviation of 0.003 S/m (0.9 %) across the tested frequency spectrum.

In transversal direction, the reed sticks provided an average conductivity of 0.12 S/m with a mean standard deviation of 0.001 S/m (0.8 %) at 10 Hz for 60 min and a mean conductivity of 0.12 S/m with a standard deviation of 0.003 S/m (2.5 %) across the tested frequency spectrum.

The spectra again showed deviations at 50 Hz, reflecting power-line interference. The noise level was generally higher for frequencies above approximately 300 Hz.

These conductivity differences between longitudinal and transversal direction resulted in a conductivity anisotropy ratio (mean ± standard deviation) of 1:2.8 ± 0.02 at 10 Hz over 1 hour and 1:2.7 ± 0.05 across the tested frequency range.