## 2 Materials and Methods
### Measurement Methods
#### 2.3.3 Ultrasonic Velocity  
The SUB-100 ultrasonic flaw detector provided by Wuxi Jiebo Instrument Technology Co., Ltd., Wuxi, China was applied in the measurement of the ultrasonic velocity. The forming sides of the specimen were evenly smeared with Vaseline. Subsequently, the SUB-100 ultrasonic flaw detectors were pressed tightly on the sides and smeared with Vaseline. Finally, the sound velocity of the ultrasonic flaw detector was recorded after ultrasonic emission. The measuring process of the ultrasonic velocity is shown in Figure 3.  
Figure 3: The schematic diagram of ultrasonic measurement.  
Figure 2: The measurement of electrical parameters.  
2.3.4. Percentage of Mass Loss  
The mass-loss rate of cement mortar can be determined as follows. During each 50 NaCl freeze-thaw cycle, the surface of the specimens was wiped dry with a rag. Subsequently, the mass of the specimen was tested. However, the steel bars should be ground before the following mass was measured. The mass-loss rate can be calculated by Equation (2).  
\[\frac{\Delta m}{m}=\frac{m-m_{0}}{m_{0}} \tag{2}\]  
where \(m\) and \(m_{0}\) are the mass of cement mortar or steel bars before and after each 50 freeze-thaw cycle, respectively.  
2.3.5. Microscopic Tests  
The scanning electron microscope (SEM) was used for observing the micro-morphology of samples. Meanwhile, X-ray diffraction (XRD) was selected to investigate the crystal component of the steel bars' passive film. To block the cement's hydration, four days' immersion in absolute ethanol was provided for the samples after reaching the planned curing age. The powdered sample whose particle size was smaller than 74 \(\upmu\)m was used for the XRD tests. Soybean-sized samples extracted from the large specimen were dried in the vacuum drying oven. After drying, the samples were taken out and sprayed with gold in a vacuum environment. The samples were coated with a gold film to conduct the SEM test. The Hitachi S-4800 cold field emission scanning electron microscope, Hitachi, Tokyo, Japan, was applied for the measurement of SEM images. The D8 ADVANCE X-ray diffractometer, manufactured by Bruker Corp., Tokyo, Japan, was used to conduct the XRD measurement.  
3. Results and Discussions  
3.1. The pH of the Cement Mortar  
The pH of the cement mortar is depicted in Figure 4 at various depths. All the specimens were cured in CO\({}_{2}\) for 28 days. Figure 4 shows that the pH value on the surface of the specimens is between 9.0 and 11. With the rising depth and lowering w/c, however, the pH value rises. This was due to CO\({}_{2}\) reacting with calcium hydroxide and lowering the alkalinity of the cement mortar. The pH value of the surface section of the cement mortar is lower because it is more prone to react with CO\({}_{2}\). Calcium carbonate, which is generated when CO\({}_{2}\) reacts with calcium hydroxide, can increase the compactness of cement mortar, inhibiting CO\({}_{2}\) penetration. As a result, as the depth of the specimen grows, so does the alkalinity, and the pH value rises. Furthermore, because of the greater compactness, the lower w/c can raise the pH value [38].  
Figure 4: The pH value of cement mortar along different depths.