*4.1. Optimization for CO2 Curing Condition*

Both the initial pressure level and the duration in which the samples remained in the pressure vessel affect the efficiency of the CO2 curing. A high pressure of CO2 reportedly accelerates the carbonate reaction at an early age [25,26]. In this study, the initial pressure was therefore controlled for all samples: 3 bar, strictly inbetween 340 to 380 kPa, as stated in the previous section. For the effect of the duration, a preliminary test was conducted to optimize the CO2 curing condition using the pressure vessel. The duration of the CO2 curing then took the period in which the carbonation rate slowed to a crawl. As shown in Figure 2, the 50-mm cube Mortar (W/C = 0.35) and Mortar (W/C = 0.5) samples were cured for more than 6 h. The carbonation rate of Mortar (W/C = 0.35) is 1.71 <sup>×</sup> <sup>10</sup>−<sup>4</sup> mol/h/<sup>g</sup> at 3 h, and 0.98 <sup>×</sup> <sup>10</sup>−<sup>4</sup> mol/h/g at 6 h. The carbonation rate of Mortar (W/<sup>C</sup> <sup>=</sup> 0.5) is 5.40 <sup>×</sup> <sup>10</sup>−<sup>5</sup> mol/h/g at 3 h, and 3.02 <sup>×</sup> <sup>10</sup>−<sup>5</sup> mol/h/g at 6 h. The carbonation rate decreased from 3 h to 6 h was within 5% compared to the carbonation rate decreased during the initial 1 h. The carbonation under the initial pressure of 350 kPa was almost accomplished within 3 h.

The time of demolding is also critical for the effectiveness of CO2 curing on the sample produced by the conventional consolidating-in-mold procedure [27]. The time of sealed curing in a mold affects the air-filled pore system [16]. Cement hydration is expected to consume water and at the same time, also produce solid hydrates in the pores. The former increases the volume of air-filled pores, but the latter adversely decreases the total amount of pores. Another preliminary test was conducted for this case. The Paste (W/C = 0.4) and Mortar (W/C = 0.5) were sealed in 40-mm cube molds for 6, 12, 18, and 21 h, and then they were subjected to 3 bar CO2 curing after their demolding. Table 5 lists their CO2 uptakes. Note that the Mortar (W/C = 0.5) sample was broken when it was demolded at 6 h. Paste (W/C = 0.4) and Mortar (W/C = 0.5) had the highest CO2 uptake with the demolding time of 12 h and 18 h, respectively. After that, the CO2 uptake monotonically decreased with the demolding time. The air-filled pores were expected to decrease stably with the cement hydration. The time of 21 h for the demolding was therefore taken for the period when the air-filled pores showed stable change.


**Table 5.** CO2 uptake depending on sealed time.

#### *4.2. E*ff*ect of Specimen Size*

The size effect on the strength of cement-based materials is inherent, and CO2 curing affects the size effect of sample because of inconsistent CO2 diffusion. Figure 5 shows the carbonation depth of Mortar (W/C = 0.5) subjected to 20% CO2 curing for 28 days, where the area of carbonation can be clearly compared. The 25-mm cube specimen was fully carbonated, but its 40-mm and 50-mm cubes were not fully carbonated, displaying a crimson color inside (pH > 9).

**Figure 5.** Carbonation depth of Mortar (W/C = 0.5) cured in 20% concentration CO2 for 28 days.

The size effect law [28,29] helps us to understand the measurement of the compressive strengths of concrete. A large concrete cylinder provides a lower strength than that of a small cylinder which is geometrically similar to the large one. The tendency could be fitted with a size-effect equation [30]. Applying it to the current measurement generates an equation predicting the strength of a *D*-sized cube, *fcu*(*D*), based on that of a 25-mm cube:

$$f\_{\rm Cu}(D) = \frac{f\_{\rm cu}(25)}{\left[1 + \left(\frac{D}{A\_{\rm d}d\_{\rm d}}\right)\right]^{1/2}} \mathbf{B} + af\_{\rm cu}(25) \tag{6}$$

where α, *B*, λ0, and *da* can be considered empirical constants. Each parameter, notably λ<sup>0</sup> and *da*, has a physical meaning; however, here it is important that the parameters are constant. The variation of each strength is then explained by a linear relation of:

$$\frac{\partial f\_{\rm cu}(D)}{\partial f\_{\rm cu}(25)} = \frac{1}{\left[1 + \left(\frac{D}{\lambda \rho d\_4}\right)\right]^{1/2}} \mathbb{B} + \alpha \tag{7}$$

Using Equation (7) allows us to consider the strength development of each sample. For example, Δ*fcu* (25) is calculated by the difference in the 25-mm cube strength at a certain age compared with 28 days (the reference age). Figure 6 comparatively shows the strength variation (development) of Mortar (W/C = 0.5). The linear trend lines, whose slopes correspond with the constant in Equation (6), do not change according to CO2 curing when the 25- and 40-mm cube strengths are compared: Δ*fcu*(25) and Δ*fcu*(40) in the left figure. However, the linear trend lines of the samples subjected to the 20% CO2

curing go off on the trend with the 50-mm cube strength (Δ*fcu*(25) and Δ*fcu*(50) in the right figure). The resultant nonlinear trend indicates that the size-effect parameters in Equation (6) need corrections or its reformulation. Partial carbonation on the edge of the 50-mm cube specimen, as shown in Figure 5, breaks the assumption of a geometrically similar specimen, which results in the nonlinear trend. In order to fit into the size effect law, the degree of carbonation in specimens with different size should be similar under the same CO2 curing condition. However, in this study, as the specimen size is larger, the depth of carbonation is smaller. This result may show that the size effect law of the specimen in CO2 curing does not fit.

**Figure 6.** Comparison of the size effect on the strength development of mortar (W/C = 0.5).
