## 5 Conclusions  
In this article, the engineering performance and durability properties of binary blended concrete containing a large amount of fly ash (FA) were evaluated, and the optimum CaO content range to ensure adequate durability was deduced by each admixture type. In addition, the results of this study and the results of a previous study by [PERSON] [25; 27] on high-volume GGBFS concrete were compared with each other on the basis of various properties. The conclusions of this article are as follows:  
(1) When the unit content of water required to ensure the target slump range of 180 \(\pm\) 25 mm was measured after fixing the unit binder content and chemical admixture content in fresh concrete, it was found that the unit water content decreased by 3.00 kg/m\({}^{3}\) in the case of FA (approximately 2.75 kg/m\({}^{3}\) for GGBFS) as the admixture mixing rate increased by 10%.  
(2) In hardened concrete, the compressive strength decreased when the admixture mixing rate increased, and FA more significantly reduced the compressive strength than GGBFS. In addition, it was found that both GGBFS and FA increased the compressive strength development rate after 28 days of aging when the admixture mixing rate exceeded 15%.  
(3) The overall compressive strength increased as the CaO content increased, regardless of the admixture type. When the CaO content was between 49.49% and 58.67%, however, the compressive strength of GGBFS was lower than that of FA at ages of 3 and 7 days. The GGBFS specimens exhibited higher compressive strength development rates at the early ages.  
(4) When the durability properties were examined, it was found that the depth of carbonation and relative dynamic elasticity modulus decreased, whereas the depth of chloride penetration tended to increase as the CaO content increased for both GGBFS and FA. The weight loss rate, however, was relatively unchanged.  
(5) The optimum CaO content satisfying all of the desired engineering and durability properties required in this study was found to be between 39% and 48% for FA (i.e., an FA mixing rate of 21.7% to 37.7%). Therefore, the optimum CaO content of FA was found to be slightly lower than the results of the previous study (i.e., between 53% and 56% for GGBFS).  
The engineering and durability property evaluation data comparing FA and GGBFS derived in this study and the optimum CaO content ranges derived to secure durability will be able to provide guidelines for mixing proportions when applying high-volume FA-based concrete in local construction fields in future projects. They are also anticipated to be utilized as a basis for determining chemical composition standards for developing binders to improve the durability of the fabricated concrete.  
Figure 19: Optimum CaO content range of FA for ensuring adequate durability performance.  
**Author Contributions:** [PERSON], [PERSON], and [PERSON] equally contributed to the research conceptualization and all investigations performed under its purview. Resource procurement was handled by [PERSON] and [PERSON]. The original draft of the manuscript was prepared by [PERSON] and [PERSON], whereas the final draft was reviewed and edited by [PERSON], [PERSON], D.-E.L., and [PERSON] All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1A5A1025137).  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.