## 3 Results and Discussion
### Effect of Cement Fineness on Concrete Early Strength  
Figure 5 shows the particle size distribution of OPC, HFS, and EPC. The volume was found to be in the following order when the particle size distribution of each cement type was analyzed: OPC (mean size: 16.49 \(\upmu\)m; fineness: 330 m\({}^{2}\)/kg) \(<\) HFS (mean size: 16.31 \(\upmu\)m; fineness: 380 m\({}^{2}\)/kg) \(<\) EPC (mean size: 14.01 \(\upmu\)m; fineness: 488 m\({}^{2}\)/kg).  
Figure 4: Relation between maturity and concrete strength at early ages: (a) Cement amount and (b) cement type and chemical admixture.  
Figure 3: Maturity of concrete according to the elapsed time.  
Figure 6 shows the relation of fineness of cement and concrete strength. The relative strength of concrete was calculated as follows:  
\[Relative\ strength=\frac{f_{cu}}{f_{330p}}, \tag{2}\]  
where \(f_{cu}\) = compressive strength with mixture and \(f_{330p}\) = compressive strength of 330p.  
The relation between the fineness of cement and concrete strength showed that the compressive strength tended to increase with the fineness of cement. The optimal fineness, however, was found to be between 4300 m\({}^{2}\)/kg and 4500 m\({}^{2}\)/kg. In addition, the high fineness of cement had a higher effect on the concrete strength at low temperatures.  
In particular, when 330 HFS was used, the strength was approximately twice that of 330P at 13 \({}^{\circ}\)C, and this effect was higher at early ages, i.e., less than 24 h. Although 330 HFS was found to be favorable for applying the concrete strength of 5 MPa, its strength development was found to be slightly lower than that of 330 EP after 24 h. At temperatures higher than 20 \({}^{\circ}\)C, the relative strength increased with the fineness of cement in a manner similar to that of the 13 \({}^{\circ}\)C curing.  
This difference in fineness is significantly related to the hydration reaction of cement. Figure 7 shows the variation in heat of microhydration with elapsed time for each cement type. HFS and EPC had higher microhydration heat values than OPC at early ages less than 24 h. Figure 8 shows the  
Figure 5: Particle size distribution of ordinary Portland cement (OPC), high fineness and SO\({}_{3}\) ordinary Portland cement (HSPC), and early Portland cement (EPC).  
Figure 6: Relation between fineness of cement and compressive strength development.  
cumulative heat of microhydration for each cement type. The plot of the cumulative microhydration heat of cement also shows that HFS and EPC exhibited values more than twice higher than those of OPC from the point of contact with water. Based on this, the total amount of heat of each mixture can be calculated according to the unit weight of cement.  
There was no significant difference in the total amount of heat according to the unit weight of cement within 24 h, which was also related to the maturity and compressive strength results (see Figure 4a). In addition, after 24 h, the amount of heat significantly increased as the unit weight of cement increased, indicating that the unit weight of cement affects the concrete strength. Figure 9 shows the variation in the cumulative microhydration heat according to the cement type.  
Figure 8: Cumulative microhydration heat according to the cement type.  
Figure 7: Heat of microhydration according to elapsed time for each cement type.  
In addition, the results of analyzing the amount of heat according to the cement type showed that HFS and EPC exhibited similar values until 24 h, but the amount of heat from EPC tended to decrease after 24 h. In general, for concrete that uses EPC, fast early strength development is secured but the long-term strength tends to decrease [9; 10].