## Experimental Results and Discussion  
### Compression Strength  
Figure 3 shows the compressive strengths of the specimens at 14 days according to the CO\({}_{2}\) concentration. The variation in compressive strength was found to be within 8%. The compressive strength of normal cured specimens significantly decreased as the replacement level of the AOD slag increased. For STP60, in particular, the compressive strength was lower than 10 MPa. This was because \(\gamma\)-C\({}_{2}\)S contained in the AOD slag did not react owing to the absence of CO\({}_{2}\) in the curing process, and only the OPC facilitated the hydration reaction. In the case of CO\({}_{2}\)-cured specimens, specimens with the AOD slag showed improved compressive strength. As the CO\({}_{2}\) concentration  
Figure 2: CO\({}_{2}\) curing chamber.  
increased, the compressive strength showed a tendency to increase. For STP0, which used 100% OPC, the compressive strength increased slightly after CO\({}_{2}\) curing, but its impact was not significant. For STP30 and STP60, the compressive strength increase rates were highest for the CO\({}_{2}\) concentration of 5% compared to the specimens without CO\({}_{2}\) curing. When the CO\({}_{2}\) concentration was increased to 15%, the increase rates decreased compared to those at 5%. Compressive strength of STP30 was similar to that of STP0 when the CO\({}_{2}\) concentration was 15%. Its compressive strength was expected to be higher than that of STP0 if the CO\({}_{2}\) concentration exceeded 15%. STP60 exhibited higher compressive strength increase rates than STP30, but the difference was not in proportion to the mixed amount of AOD slag.  
Figure 4 shows the compressive strengths of the specimens according to the curing temperature. In general, a high curing temperature promotes the initial hydration of cement. Therefore, the compressive strength of specimens should increase with high curing temperature. However, compressive strength tends to decrease when specimens were cured at high temperature [29; 30]. The specimen size of this study was relatively small, and the compressive strength STP0 decreased with temperature.  
The compressive strengths of STP30 and STP60 showed different results from that of STP0. In the cases of STP30 and STP60, the compressive strength increased as the curing temperature increased, regardless of the CO\({}_{2}\) concentration.  
The compressive strength of STP0 increased with CO\({}_{2}\) curing at all curing temperatures. As the curing temperature increased, the strength increase tended to increase slightly. Similar results were found by [PERSON] et al. [31]. For STP30 and STP60, the compressive strength significantly increased owing to CO\({}_{2}\) curing as the curing temperature increased. The compressive strength of STP30 exceeded that of STP0 when CO\({}_{2}\) curing was performed at 40 \({}^{\circ}\)C. This result confirms that the increase in the curing temperature accelerates the carbonation of the AOD slag. STP60 showed a similar trend to STP30, and its rate of increase in compressive strength owing to CO\({}_{2}\) curing was the largest.  
Figure 3: Compressive strength of specimens according to CO\({}_{2}\) concentration.