## 2 Experimental Procedures
### Chloride Penetration
#### 2.3.2 RCT Method  
Evaluation of the chloride penetration features of nano-modified mortars was also carried out by exposing specimens to salt spray environments according to ASTM B117 [51] specifications. As laboratory conditions are much more aggressive than natural ones, to accelerate the phenomenon, severe corrosion states in short times were anticipated. Compared to other laboratory-grade \begin{table}
\begin{tabular}{c c} \hline \hline
**Charge Passed [Cb]** & **Chloride Ion Penetrability** \\ \hline \(>\)4000 & high \\ 2000 \(\div\) 4000 & moderate \\ 1000 \(\div\) 2000 & low \\ 100 \(\div\) 1000 & very low \\ \(<\)100 & negligible \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chloride ion penetrability based on charge passed. Figure 1: Experimental set up for the rapid chloride permeability test method. accelerated corrosion tests, the salt spray test was considered more representative of the natural coastal environment [52]. Therein, after removal from the 100% humidity room, nano-modified mortars were surface dried and then naturally dried in the laboratory environment. Subsequently, they were sealed with insulating exterior paint on four sides, including the top side of the mold and two opposite open sides permitted chloride diffusion through a predefined path. Coated samples were placed in a VSC 450 salt spray corrosion test chamber (Votsch Industrietechnik, Balingen, Germany) (Figure 2a) for 100 days, with a sealed side resting at the bottom of the chamber (Figure 2b). Salt solution prepared by dissolving five parts, by mass, of sodium chloride (NaCl) into 95 parts of deionized water, was introduced into the chamber's solution reservoir. The utilized salt contained less than 0.3 percent impurities to avoid impurities acting as corrosion inhibitors. Salt spray solution acidity, measured at 25 \({}^{\circ}\)C, was maintained in the pH range from 6.5 to 7.2, while the temperature inside the chamber was maintained at 35 \({}^{\circ}\)C [52]. After the end of the 100-day exposure period, nano-reinforced mortars were carefully removed from the chamber and gently washed with clean tap water to remove any surface salt deposit before being naturally dried. The dried mortars were prepared for testing by the following methodology. Initially, a cylindrical core of the exposed sides to the NaCl was drilled out of each cubic specimen. The diameter of the cores was maintained within 95 mm to 105 mm while sectioning of the total length provided cylindrical samples of an approximate height of about 65 mm, such as the ones shown in Figure 3.  
Subsequently, 1.5 g of powder ground from incrementally increasing depths inside the nano-modified material were removed using a profile grinder (Germany Instruments--Copenhagen, Denmark) and collected for chloride ion measurement. The grinding depth increment was kept as low as 1 mm, to allow for high accuracy measurement; 60 successive depths were interrogated. Next, ground powders originating from mortars with different CNT content were tested separately according to the rapid chloride test (RCT) method. In this way, the amount of chloride calculated using the RCT  
Figure 3: Testing cores drilled off from mortars exposed to accelerated corrosion. Figure 2: (**a**) Salt spray chamber and (**b**) sealed specimens inside the chamber. electrode, which was initially calibrated, was related to CNT loading. Profiles of chloride content versus the depth below the exposed surface were determined. Assuming that diffusion is a one dimensional process, diffusion of chloride ions into the materials is governed by [PERSON]'s second law of diffusion (steady state diffusion) as follows:  
\[\frac{\partial\mathrm{C(x)}}{\partial\mathrm{t}}=\mathrm{D}\frac{\partial^{2} \mathrm{C}}{\partial\mathrm{x}^{2}} \tag{1}\]  
where C represents chloride ion concentration, x is the depth from the surface of concrete, t represents time, and D is the diffusion coefficient.