## 3 Experimental Results
Nevertheless, at 7 d of age, the Young's modulus for VG20-70 was measured at 42 GPa, demonstrating sufficiently high strength. The effect of VG on the mechanical properties of concrete has been clarified, supporting its potential application in a wide range of secondary products beyond precast high-strength concrete piles. Additionally, ultra-high-strength concrete incorporating metallic fibers, SF, volcanic deposits, and other materials has been extensively studied for further enhancement [49; 50; 51; 52; 53; 54]. Figure 19 shows the XRD patterns of the concrete samples. No significant differences were observed in the diffraction patterns of VG20-70 and SF10-70 when compared to those of OPC-70. These results suggest that even when part of the OPC is replaced with VG, a hardened body similar to that of the OPC is produced. The intensity ratio was calculated by dividing the maximum diffraction integrated intensity (\(20=47^{\circ}\)) of Ca(OH)\({}_{2}\) in these patterns by the maximum analytical integrated intensity (\(20=43^{\circ}\)) of the standard  
Figure 11:0 Comparison of [PERSON]’s modulus of concrete. substance Al\({}_{2}\)O\({}_{3}\)[55]. The intensity ratios were in close agreement: 1.46 for OPC, 1.68 for VG, and 1.78 for SF. The intensity ratios of VG and SF were slightly higher than those of OPC, indicating that the hydration reaction had progressed further. Figure 20 illustrates the cumulative and incremental pore-size distribution of the cement paste. OPC, VG20, and SF10, which were cured in water at 20 \({}^{\circ}\)C, all exhibited numerous pores between 10 and 60 nm. The cumulative values were also in the range of 0.10 to 0.11 mL/g, showing a similar pore distribution. Contrastingly, OPC, VG20, and SF10 cured by steam curing at 70 \({}^{\circ}\)C for 8 h had many pores between 3 and 14 nm, with cumulative values ranging from 0.025 to 0.044 mL/g, also exhibiting a similar pore distribution. These results show that steam curing at 70 \({}^{\circ}\)C significantly reduces the size of the pores in the structure compared to water curing, indicating that the structure becomes densified by promoting the hydration reaction. Because pore sizes of 0.5 \(\upmu\)m or more are said to decrease the compressive strength of concrete [56], it is believed that pores large enough to decrease compressive strength will not form in the concrete prepared under the mix and curing conditions examined in this study. In addition, it has been reported that the total pore volume can be reduced if the curing method is appropriate [56]. By replacing part of the OPC with VG in this study, the total pore volume was slightly lower than in the case of OPC alone, suggesting that the curing method used was also effective. Figure 21 presents the crystal structure of the hardened cement paste. Comparing OPC, VG20, and SF10, no clear difference in the crystal structure due to the difference in the admixture was found. In this study, the effect of partially replacing OPC with VG was investigated, because SF is an imported product with a high cost. In Japan, if the quality of imported SF (specifically its particle size distribution) is suboptimal, additional adjustments are required before use. Therefore, we explored the complete replacement of SF with VG, which is both cost-effective and expected to offer high quality. Additionally, we plan to consider a partial replacement of OPC with both VG and SF in the future, because the particle size decreases in the order of OPC \(>\) VG \(>\) SF. This mixing approach is expected to result in a denser concrete structure. Figure 11:1 XRD patterns of cement pastes (1 d). The ultimate objective of this study is to utilize VG in high-strength precast concrete piles. To achieve this, it is essential to validate the actual production of centrifugally compacted piles and establish an optimal mix design.