**2. Experimental**

*2.1. Materials*

2.1.1. Carbon Nanotubes

Multi-walled carbon nanotubes, provided by Sigma-Aldrich in Spain, were used. Their properties are given in Table 1. Figure 1 shows its transmission electron microscope (TEM) image [33].

**Table 1.** Properties of multi-walled carbon nanotubes (MWCNTs).


**Figure 1.** Transmission electron microscope image of MWCNT, adapted with permission from Sikora et al. (2019) [33].

#### 2.1.2. Cement, Sand and Mortar

CEM I 42.5 R type cement was used to manufacture the mortar. Table 2 summarises composition and some physical-chemical properties of the cement used in this work, suppled by Portland Valderrivas, in Spain. Siliceous sand from a nearby quarry, with continuous granulometry and 2 mm maximum size was used. The sand equivalent was 87.35% and the density was 2.63 g/cm3.

Mortar mixes with 0.00 wt.%, 0.01 wt.%, 0.015 wt.% and 0.02 wt.% of MWCNTs were prepared. The mix proportions are shown in Table 3. All samples were mixed in a laboratory drum mixer. Then, the fresh mortar samples were placed in 40 × 40 × 160 mm<sup>3</sup> prismatic moulds and homogenized using a vibrating table. Three specimens of each mix were manufactured. The specimens were cured in a curing chamber at 100% humidity and 20 ◦C temperature. Two curing periods were considered: 28 and 90 days. Additionally, to study the thermal variation of electrical resistivity of MWCNT-mortars, one sample of each MWCNT proportion, for every curing period, was manufactured with a thermocouple inserted at the center of the beam. These specimens were cast and cured in the same conditions as stated before.


**Table 2.** Elemental composition (wt.%) of source cement and physical-chemical properties (according to the supplier).

**Table 3.** Mortar mix proportions.


Inclusion and dispersion of MWCNTs in the mortar were achieved by prior preparation of MWCNT nanofluids. The corresponding amount of MWCNTs was dispersed in 100 g mixing water by mechanical stirring for 3 min, followed by magnetic stirring for 30 min and sonication for 1 h. Model 505 Sonic Dismembrator (Fisher Scientific, Waltham, MA, USA) was used for sonication. This probe is designed as 20 kHz operating frequency and a maximum power of 500 W. During the sonication, 50% amplitud, and 2 s ON and 2 s OFF pulses were applied. This so-prepared nanofluid was then incorporated to the cement-sand-remain water mix, that kept mixing in the drum mixer for the necessary time (approximately 4 min) until homogeneity.
