## 3 Results
### Electrical Resistivity of the CNT-Cement Composites  
Electrical resistivity measurements were performed for the cement-based composites containing 0.00%, 0.10%, and 0.40% CNT by mass of cement and the four SPs studied. Five specimens were tested for each condition. The CNT amounts by mass of cement were chosen as the maximum achievable using the CNT concentration in the aqueous dispersions for the selected a/c. The 0.10% CNT-cement composites were produced with the 0.25% CNT and 1.00% SP dispersions, while the 0.40% CNT-cement composites were produced with the 0.80% CNT and 1.60% SP dispersions. All SPs were tested in order to verify if the best dispersion conditions identified through UV-Vis spectroscopy were translated into lower electrical resistance values. Typical obtained resistivity versus time results are presented in Figure 8. A transient behavior, also called the drift effect [35], was identified in all samples and associated with polarization phenomena due to the dielectric nature of the cement matrix [36]. This effect has been found to appear when performing measurements with a DC power source [37]. Nevertheless, allowing a given amount of time for measurement stabilization has been reported as a satisfactory solution [28]. After 20 min of constant voltage application, two different types of behaviors could be identified: one corresponding to a fast stabilization of the resistivity measurement, and one corresponding to a slow stabilization. The stabilization time between 4 and 20 min of constant voltage application, presented as a fraction of their initial resistance (R/R\({}_{0}\)) [35] for 0.40% CNT-cement composites, is shown in Figure 9. The R/R\({}_{0}\) values were obtained starting from the fourth minute of testing in order to discard the initial sharp curvature and analyze the more linear part of the result. It was found that the electrical resistance of the samples produced with SPs A and C, which presented lower dispersing efficiency, also prese  
Figure 8: Typical electrical resistivity results over time obtained for CNT–cement composites. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Dispersion** & **SP Type** & **CNT (\%)** & **SP (\%)** & **Water (\%)** & **CNTESP** & **Sonication Energy** \\  & & & & & & **(/g\({}_{dispersion}\))** \\ \hline
0.25\%CNT/0.25\%SP A & A & 0.25 & 0.25 & 99.50 & 1:1 & 600 \\
0.25\%CNT/3.00\%SP A & A & 0.25 & 3.00 & 96.  
behavior. On the other hand, the electrical resistance of samples produced with SPs B and D, which presented higher dispersing efficiency, could be considered stable after very short stabilization periods. This indicates that faster stabilization times are associated with better CNT dispersion in the cement matrix. Electrical resistivity results of all cement-based composites after 20 min of constant voltage application are presented in Figure 10. It was found that all the cement-CNT composites had a lower electrical resistivity than the reference cement sample, and that this resistivity decreased proportionally to the amount of CNT present in the matrix. At the highest CNT dosage (0.40% by mass of cement), it was found that the samples produced with superplasticizers A and D presented an average resistivity of 150 k\(\Omega\).cm, while the samples produced with superplasticizers B and D presented and average resistivity of 50.0 k\(\Omega\).cm, a 130 times decrease when compared to the resistivity of the reference sample (6.5 M\(\Omega\).cm). This confirms the trend identified in the UV-Vis results, where SP B and D were found to be more efficient dispersing agents than SPs A and C. Special attention should be given to the resistivity results obtained for the 0.40% CNT-cement-SPB composites. These samples were prepared with the 0.80% CNT and 1.60% CNT dispersion, which did not present satisfactory UV-Vis results. Their low obtained resistivity can be associated with conduction through CNT agglomerates rather than through individual nanotubes [38; 39] and may be the cause of the higher dispersion of the results.