## 3 Results and Discussion
### Effects on Engineering Performance
#### 3.1.2 Engineering Property Analysis  
Figure 5 shows the slump and air content measurement results based on the GGBS replacement ratio. The slump satisfied the target range corresponding to \(180\pm 25\) mm, and the air content also satisfied the target range corresponding to \(4.5\pm 1.5\) mm. In the study, to secure data with high field applicability, the content of the chemical admixture was fixed and the unit water content required to secure the target slump was measured based on the GGBS replacement ratio. The results indicate that the unit water content exhibited a tendency to decrease by approximately 2.75 kg when the GGBS replacement ratio was increased by 10% as shown in Figure 6. When the GGBS replacement ratio was 70%, the unit water content to secure the same workability decreased to 90.7% when compared to that when the replacement ratio corresponded to 0%. This result was similar to the previous studies [22; 39], and the surface of GGBS was denser than OPC and had the characteristics of particle shape, which is advantageous for increased fluidity. Therefore, it is known to have an effect of reducing friction between the paste and the aggregate interface. This indicated that the substitution of GGBS in the field under the same total binder weight condition is helpful in improving the workability of concrete.  
Figure 7 shows the compressive strength with respect to age for each GGBS replacement ratio. At 3, 7, 28, and 56 d of age, the minimum-maximum values of compressive strength were observed as 5.0-12.0, 15.1-24.9, 30.4-37.3, and 35.0-40.0 MPa, respectively. Figure 8a shows the compressive strengths based on the GGBS replacement ratio. At 3, 7, and 28 d, the compressive strength tended to decrease by 1.0, 1.4, and 0.9 MPa, respectively, when the GGBS replacement ratio increased by 10%.  
Figure 5: Result of slump and air contents with respect to replacement ratio.  
Figure 6: Result of unit water content with respect to replacement ratio.  
Conversely, the compressive strength at 56 d remained similar even though the GGBS replacement ratio increased. When the ratio of the compressive strength at 56 d to that at 28 d was analyzed, the results indicate that the ratio exhibits a tendency to increase when the GGBS replacement ratio increases, as shown in Figure 8b. The ratio ranged from 101.3% to 101.6% when the GGBS replacement ratio ranged from 0% to 10%, thereby indicating an insignificant increase in compressive strength. However, the ratio increased to 106.4%-123.3% when the GGBS replacement ratio exceeded 20%. This confirmed that the long-term strength after 28 d of age improved when the GGBS replacement ratio increased. These results and reasons can also be found in a previous study [23]. The development of the compressive strength of GGBS based concrete depends on the GGBS replacement ratio and concrete curing age. GGBS's glassy compounds react slowly with water, and it takes time to obtain hydroxyl ions to break the glassy slag parcels from the hydration products of OPC at an early age. However, GGBS concrete had a property where the compressive strength was higher than that of OPC after the GGBS hydration and pozzolanic reactions were almost completed.