## 4 Conclusions  
This study evaluated the effects of the unit weight of cement on concrete that uses OPC on early strength development. HFS_OPC and EPC were selected, and the effects of cement fineness, SO\({}_{3}\) content, and chemical admixture were analyzed. The results are summarized as follows.  
1. The slump and air content necessary for securing the workability on concrete met the set target ranges both at the beginning and after 60 min. The strength development of concrete was further accelerated by the cement type and the addition of EPC than when the unit weight of cement was increased. In addition, the strength development rate of concrete was higher at lower temperatures.  
Figure 13: Variation in the elapsed time at strength of 5 MPa for concrete.  
2. For the maturity of concrete, the strength development tended to increase according to the maturity. This is because the unit weight of cement increased for the concrete mixtures that used OPC. The effect, however, was not significant when the HFS_OPC and EPC were used.
3. When the amount of heat of each mixture was calculated based on the microhydration heat results of cement, no significant differences were found in the total amount of heat within 24 h depending on the unit weight of cement. In addition, analyzing the results of the amount of heat for each cement type showed that HFS and EPC exhibited significantly high values compared to OPC until 24 h, indicating that they are favorable for the development of early strength.
4. The relation between concrete strength and the cement fineness showed that the compressive strength tended to increase with the fineness of cement, and an optimal fineness between 4300 m\({}^{2}\)/kg and 4500 m\({}^{2}\)/kg was obtained. The microhydration heat was found to be higher when the fineness of cement was high than when the unit weight of cement was increased. The results of analyzing the amount of heat for each cement type showed that HFS and EPC exhibited significantly higher values compared with OPC up to 24 h. Thus, they are expected to be favorable for early strength development.
5. A linear relationship was observed between the SO\({}_{3}\) content and relative strength of cement when SO\({}_{3}\) content was less than 4%. In addition, when the unit weight of cement was increased, the early strength was slightly increased owing to the increase in overall SO\({}_{3}\) content.
6. The criterion of each country for the formwork removal time on concrete were examined and applying the Asian criterion was judged to be effective, which presents a clear compressive strength of 5 MPa. The use of EPC and HFS_ePC can shorten the formwork removal time by 20-24 h at 10 \({}^{\circ}\)C and by 14-16 h at 20 \({}^{\circ}\)C.  
Conceptualization, T.L., J.L., and H.C.; methodology, T.L., J.L., and H.C.; investigation, T.L., J.L., J.K., H.C. and D.-E.L.; resources, T.L. and J.L.; writing--original draft preparation, T.L. and J.L.; writing--review and editing, T.L., J.L., J.K., H.C and D.-E.L. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT), grant number NRF-2018R1A5A1025137.  
The authors declare no conflict of interest.