## 3 Results and Discussion
### Effects on Durability Performance  
Figure 13 shows the carbonation depth measurement results based on the GGBS replacement ratio. In the case of the specimens at four weeks of age, the carbonation depth remained at similar levels until the replacement ratio reached 50%, although it significantly increased when the replacement ratio exceeded 50%. In the case of the G7 specimen, the carbonation depth increased to approximately 266.7% when compared to the lowest value. For specimens at eight weeks of age, the carbonation depth also remained at similar levels until the replacement ratio reached 50%, although it significantly increased when the replacement ratio exceeded 50%. With respect to the G7 specimen, the carbonation depth increased to approximately 161.7% when compared to the lowest value. These results indicate that it is desirable to set the GGBS replacement ratio to 50% or less to minimize the carbonation depth in the field application. This finding is complemented by the existing literature. It has long been known  
Figure 11: Compressive strength with respect to basicity. Figure 12: CaO content and CaO/SiO\({}_{2}\) ratio with respect to replacement ratio. that concrete is adversely affected by carbonation when large amounts of GGBS are used [40; 41]. Some studies reported that the amount of GGBS that started to be affected by this negative effect was about 30-40% of the total binder content based on field data [42; 43]. Figure 14 shows the chloride penetration depth measurement results based on the GGBS replacement ratio. The chloride penetration depth significantly decreased until the replacement ratio reached 25%, although it remained at similar levels when the replacement ratio ranged from 25% to 70%. In the case of the plain specimen, the chloride penetration depth was approximately 442.1% when compared to the lowest value. Therefore, it is deemed as desirable to secure 25% or higher GGBS replacement ratios during concrete mix design at construction sites where the chloride attack must be considered. According to a previous study [44], laboratory accelerated tests resulted in lower chloride transfer coefficients with higher replacement ratios of GGBS in concrete. Therefore, cement-free slag concrete was found to have the greatest resistance to chloride ion penetration compared to other specimens. In addition, it was similar to the previous studies [24; 25] where the depth of degradation can be minimized and resistance to acid fluids can be improved by adding raw materials such as GGBS. To examine the effect of the GGBS replacement ratio on freeze-thawing or other chemical reactions, the relative dynamic elastic modulus was measured based on the GGBS replacement ratio, as shown in Figure 15a. The relative dynamic elastic modulus exhibited a tendency to increase when the replacement ratio increased. It was as low as 64.6% when the replacement ratio ranged from 0 to 45%, although it increased to 86.8%-94.5% when the replacement ratio exceeded 45%. This indicated that the GGBS replacement ratio must be 45% or higher to secure resistance to freeze-thawing. Figure 14: Chloride penetration depth relative to the replacement ratio. Figure 13: Carbonation depth ratio with respect to replacement ratio. Figure 15b shows the weight reducing ratio based on the GGBS replacement ratio. The test was conducted to examine the effect of the replacement ratio on the surface degradation of the specimens. When the replacement ratio increased, the weight reducing ratio exhibited a tendency to increase. While the weight reducing ratio of the plain specimen was measured as 96.8%, the weight reducing ratio reached 99.9% or higher when the GGBS replacement ratio exceeded 20%. This indicated that it is necessary to secure 20% or higher GGBS replacement ratios in the field to minimize surface degradation of structures. This result is similar to that of the previous study [44]. The slag replacement concrete had a higher coefficient of durability as the slag replacement ratio was higher.