*2.3. CO2 Curing and Successive Hydration*

Table 3 details the schedule of the casting and curing conditions. We considered two conditions for the CO2 curing after demolding. The first CO2 curing condition was at 20% CO2 concentration, where the relative humidity (RH) was 75% ± 5% and the temperature was 25 ◦C under ambient pressure. Each sample was placed in a controlled chamber, and the 20% concentration CO2 curing continued for 28 days.

The other was 3 bar pressure CO2 curing. The samples in a pressure vessel, vacuum-sealed beforehand, were subjected to 99.9% purified CO2 gas for 3 h. The initial CO2 gas pressure was set above 340 kPa, but not to exceed 380 kPa (approximately 350 kPa). Each sample in Table 2 was subjected to 3 bar CO2 curing for 3 h, and then successive hydration followed: 21 h moisture curing for Paste (W/C = 0.15) and Mortar (W/C = 0.35), and 28 days water curing for Paste (W/C = 0.4) and Mortar (W/C = 0.5). The moisture curing was conducted under 85% ± 5% RH and a temperature of approximately 25 ◦C. The water curing was conducted at approximately 23 ◦C.

The control samples were produced by the moisture curing for the Paste (W/C = 0.15) and Mortar (W/C = 0.35). The control samples for Paste (W/C = 0.4) and Mortar (W/C = 0.5) took the water curing. The conditions for the controlled curing were the same as the conditions for the successive hydration after the 3 bar CO2 curing.


**Table 3.** Curing condition and sequence for samples.

#### *2.4. Measurements*

Figure 1 shows a pressure vessel designed to have a single inlet valve and a single outlet valve. The internal temperature and pressure were monitored during the 3 bar CO2 curing. Pressure loss caused by cement carbonation was measured using a pressure digital gauge (PDR1000; Pressure Development of Korea Co., Daejeon, Korea). Its measurement range was from −100 kPa to 400 kPa, and its precision was 0.1%. The sampling rate for the pressure measurement was 1 record per second.

**Figure 1.** Pressure vessel.

The compressive strengths of the samples were measured at the age of 1, 3, 7, 14, and 28 days in accordance with ASTM C109 [24]. The standard test method suggests a loading rate within 0.9 to 1.8 kN/s. In this study, the loading rate for the paste and mortar samples were assigned as 1.0 kN/s and 1.5 kN/s, respectively. The strength measurements took an average of the results of three replicated samples.

Each broken specimen following the strength measurement was used to evaluate the depth of carbonation. The depth of carbonation was determined using 1% phenolphthalein indicator in the broken specimen. The sprayed phenolphthalein solution on the specimen remains colorless when the specimen was carbonated, resulting in pH < 9.

#### **3. Results**
