## 2 Experimental Procedures  
### Materials and Specimens  
Five cement mixtures of varying carbon nanotube concentrations, namely 0.2%, 0.4%, 0.6%, and 0.8% by weight of cement, were prepared according to standard protocol \"BS EN 196-1:2005\" [45] pertaining to the determination of physical, mechanical, and electrical properties as well as chloride content in cement. A reference mixture without nano-inclusions was also prepared for comparison purposes. All mixtures contained ordinary Portland cement, regular tap water and natural sand. Long multi-wall carbon nanotubes (MWCNT), synthesized via catalytic chemical vapor deposition were used as nano-reinforcements. The tubes, commercially available by Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China), had nominal purity higher than 97% and amorphous carbon content which is less than 3%. Nominal tube diameter ranged from 20 to 40 nm, while their length ranged from 5 to 15 \(\upmu\)m. Viscocrete Ultra 300 (Sika AG, Baar, Switzerland), which is a native water-based concrete additive comprised of polycarboxylate polymers, doubled as a superplastisizer and nanotube dispersing agent. Superplastisizer selection was based on its high CNT dispersion efficiency [8; 46], excellent resistance to mechanical and chemical attack, as well as its ability to inhibit air entrapment inside the specimens. A second superplastisizer, Viscocrete Ultra 600 (Sika AG, Baar, Switzerland) was added in all fresh mortars, in amounts of approximately 1.5 g, to aid air content stability and workability, hence eliminating contributions of these phenomena to the overall behavior and enabling attribution of findings exclusively to nanotube concentration. In all cases, water to cement ratio was maintained at 0.5.  
The experimental procedure for the preparation of nano-reinforced cement mixtures, at tube loadings of 0.2%, 0.4%, 0.6%, and 0.8 wt. % of cement, adopted the three following successive steps. Initially, the superplasticizer and nanotubes were mixed at a ratio of 1.5:1 under magnetic stirring, in regular tap water. Given the fact that for a single mix, BS EN 196-1 requires usage of 1350 g of sand, 450 g of cement, and 225 g of water, the relevant water/CNT ratios were 250 at 0.2% CNT concentration and 125%, 83.5%, and 62.5% at 0.4%, at 0.6%, and 0.8% CNT concentration, respectively. Accordingly, the sand/cement ratio was constant at 3:1. The resultant aqueous suspensions of MWCNTs were ultrasonicated for 90 min at room temperature with a Hielscher UP4008 device (Hielscher Ultrasonics GmbH, Teltow, Germany) equipped with a cylindrical 22 mm diameter sonotrode delivering a power throughput of 4500 J/min at a frequency of 24 kHz. The selected combination of ultrasonication duration and energy rate was established as optimum for the suspension homogeneity achievement without tube aspect ratio impairment [47]. The ultrasonicated suspensions were mixed with ordinary Portland cement type \"I 42.5N\" and natural sand using a laboratory rotary mixer, in low and high speeds sequentially for a total of 4 min, according to standard test method BS EN 196-1:2005 [45]. Immediately after mixing, the workability and air content of the fresh mortars were measured. The remaining fresh mixture was poured into oiled metallic molds with internal dimensions of 40 \(\times\) 40 \(\times\) 160 mm for flexural and compressive test specimen production and 150 \(\times\) 150 \(\times\) 150 mm for chloride penetration and electrical conductivity measurement specimen, where they were left for 24 h before demolding. The specimens were subsequently placed into a 100% humidity room for 28 days before being transferred into a laboratory corrosive environment.