*2.2. Methods*

#### 2.2.1. Testing Procedures

Technological tests were performed to evaluate the mechanical behavior and durability properties of the cement mortars, namely: consistency, density and setting time of fresh mortars, and flexural strength and compressive strength of hardened mortars at the ages of 7, 28 and 90 days, following UNE-EN 1015-3:2000/A2:2007, UNE-EN 12390-3:2020 and UNE-EN 12390-5:2020 [34–36], respectively.

## 2.2.2. Electrical Resistivity Measurements

The electrical resistivity was measured by means of the four point probe Wenner technique [37] which is an electrical resistivity determination test performed on the surface of the structure or test body. According to Wenner method, by using four equally-distanced electrodes placed on the mortar surface, an alternating current *I* (A) is sent through the two outer electrodes and the voltage *V* (V) is measured through the two inner electrode to finally calculate the apparent resistivity *ρ<sup>a</sup>* (*k*Ω· cm) through Ohm's law, as expressed in Equation (1)

$$
\rho\_a = \frac{2\pi aV}{I} \tag{1}
$$

where *a* (cm) is the distance between electrodes.

This experimental methodology to calculate resistivity assumes that the mortar is semi-infinite and homogeneous. As mortar confined in laboratory specimens cannot be considered semi-infinite, a correction should be made according to the geometry of the specimen [38], by means of a cell constant *K*, so that the real resistivity *ρreal* is obtained from the following expression given in Equation (2):

$$
\rho\_{real} = \frac{\rho\_a}{K} \tag{2}
$$

In this work, the electrode spacing used was 2.5 cm, and all measured resistivity values were corrected for the geometry effect with a cell constant *K* = 3.1, according to [39]. An alternating current was applied by a waveform generator (National Instrument, 9263) which imposed a peak amplitude voltage of ±10 V at 500 kHz frequency. Values of the voltage and current intensity were recorded by National Instrument 9227 and 9222 respectively.

Figure 2A shows a graphic scheme of the described measurement method. In addition to the thermocouple, four pure copper pins were fixed in the mortar samples surface using a conductive epoxy. Figure 2B shows a schematic of the beam dimensions, spacing and position of the electrodes and temperature probe.

**Figure 2.** Diagram of Wenner method for resistivity measurements in mortar specimens (**A**), and (**B**) electrode spacing and dimensions.

To investigate the electrical transport within the MWCNT-mortar microstructure in terms of activation energy, we introduce the effect of temperature on the resistivity of MWCNT-mortar composites. The resistivity/temperature response is presented in an Arrhenius format thereby allowing evaluation of the activation energy for electrical conduction processes. Specimens were placed in a temperature controlled heater and their resistivity tested over the range 20–60 ◦C, following the so-called dynamic temperature test (DTT) as described by Liu and Presuel–Moreno [40]. This procedure consists of the monitoring the thermal-electrical behavior of the mortar samples when a thermal gradient is applied. The test was carried out in a controlled environment of constant temperature and relative humidity.
