*3.1. CO2 Uptake by Pressure Monitoring*

Figure 2a shows the pressure loss of each pressure vessel during the 3 bar CO2 curing of Mortar (W/C = 0.35) and Mortar (W/C = 0.5) as an example. The values were calibrated by considering a CO2 leakage of 0.696 kPa/h in the pressure vessel. The CO2 pressure decreased over time, where the initial pressure was approximately 350 kPa as injected. Taking the slope at each point, the carbonation rate in a unit of kPa/h was evaluated as shown in Figure 2a. CO2 in the pressure vessel was also dissolved in the water phase of a sample, but the dissolution in water phase went to equilibrium quickly. The rate of the pressure decrease, disregarding the initial pressure records right after the initiation of the monitoring, therefore directly indicated the CO2 amount being carbonated.

**Figure 2.** Pressure loss and CO2 uptake of Mortar (W/C = 0.35) and Mortar (W/C = 0.5). (**a**) Decrease of CO2 pressure, (**b**) Carbonation rate, (**c**) CO2 uptake.

The consumed CO2 was then calculated with the ideal gas equation, *PV* = *nRT*, where *R* is the gas constant of 8.314 J/mol/K. The volume of the pressure vessel was *V* = 0.004 m3. The temperature slightly changed over time, but it was averaged at *T* = 24 ◦C or 301 K. The consumed CO2, *n* = *PV*/*RT* per unit time, represented the carbonation rate in a unit of mol/h. Normalizing the carbonation rate with the cement mass required for producing the samples in the pressure vessel yielded its value per cement mass, as shown in Figure 2b. Integrating the carbonation rate for the time of the CO2 curing gave the CO2 uptake of each sample, where the percentage was calculated with the molecular weight of CO2 (44.01 g/mol) as shown in Figure 2c.

In Figure 2a, it can be seen that Mortar (W/C = 0.5) had a fast loss of pressure. However, as shown in Figure 2c, the actual amount of carbonation should be divided by the cement mass compared to the total mass. The result indicates that the CO2 uptake of the Mortar (W/C = 0.35) is larger than the CO2 uptake of Mortar (W/C = 0.5). As a result, it could be confirmed that the Mortar (W/C = 0.35) compact showed quicker carbonation, and with that, CO2 uptake, compared with the Mortar (W/C = 0.5) sample.

Table 4 compares the CO2 uptake per cement mass, where its value at 3 h is representatively reported. First of all, as expected, the Paste (W/C = 0.15) and Mortar (W/C = 0.35) compacts

proportioned by a low W/C ratio had a higher CO2 uptake compared with the Paste (W/C = 0.4) and Mortar (W/C = 0.5) samples. The compacts made of low W/C had a high air-filled porosity, resulting in easy CO2 diffusion inside. Second, there was a size effect on the CO2 uptake of the paste samples. The 40-mm cube specimens showed a higher CO2 uptake than the 25-mm cubes of the replicated samples. Lastly, incorporating colloidal silica increased the CO2 uptake of the mortar samples. This will be discussed later in detail.


**Table 4.** Results of CO2 uptake.

#### *3.2. Compressive Strength*

Figure 3 compares the 1-day strengths of the 40-mm cube samples. The CO2 curing was influential in the early-age strength development of the paste samples. The strengths of Paste compact (W/C = 0.15) and Paste sample (W/C = 0.4) subjected to CO2 curing were higher than the control sample (moisture curing for 24 h). The 20% CO2 curing for 24 h was much more effective in the Paste samples (W/C = 0.4), while the 3 bar CO2 curing was better in the Paste compacts (W/C = 0.15). The effectiveness of the CO2 curing condition bifurcated with the mortar samples. The 20% CO2 curing resulted in a higher strength regardless of the casting method, whereas the 3 bar CO2 curing failed.

**Figure 3.** Strength of 1-day samples fabricated by (**a**) compacting and (**b**) consolidating-in-mold method.

Figure 4 shows the strength development of the 40-mm cube samples of Mortar (W/C = 0.5), where each trend was fitted in a hyperbolic equation. Following the trend of the early-age strength, as illustrated in Figure 3, the 20% CO2 curing provided a higher strength gain than the other curing conditions. Incorporating colloidal silica intensified the effect of the 20% CO2 curing, which resulted in higher 28-day strength. However, the effect of the 3 bar CO2 curing was negligible, as shown in Figure 4, and even negative for Mortar (W/C = 0.5) incorporating colloidal silica.

**Figure 4.** Strength development of Mortar (W/C = 0.5).

#### **4. Discussions**
