## 4 Conclusions  
This study aimed to develop high-strength precast concrete piles using VG as an alternative to SF. Both mortar and concrete specimens were subjected to steam curing, and their fundamental properties were analyzed. The key findings are summarized as follows:  
1. VG, like SF, consists of fine particles that enhance mortar fluidity through a ball-bearing effect when mixed with raw materials.
2. The activity index of the VG concrete increased when it was steam-cured.
3. The compressive strength of VG-mixed concrete was maximized when 20% of the OPC was replaced with VG, achieving a target strength of 123 MPa. The optimal steam curing conditions were maintained at 70 \({}^{\circ}\)C for 8 h.
4. The Young's modulus of VG-mixed concrete was slightly lower than that of concrete mixed with OPC alone or SF.  
Figure 21: SEM images of cement pastes (1 d). (**a**) OPC-70; (**b**) VG20-70; (**c**) SF10-70.  
Figure 20: Cumulative and incremental pore diameter distribution of cement pastes (1 d). (**a**) Water curing (20 \({}^{\circ}\)C); (**b**) steam curing (70 \({}^{\circ}\)C).  
These results demonstrate that VG is a viable alternative to SF for high-strength precast concrete piles. Utilizing locally available volcanic ash provides a sustainable approach to enhancing concrete strength, contributing significantly to environmentally friendly construction practices.  
For the broader adoption of VG, further investigation into its chemical properties is essential in addition to its mechanical characteristics. While this study discusses VG's role in reducing the apparent chloride ion diffusion coefficient, previous research has shown that at a 20% substitution ratio, the neutralization rate coefficient is 23-38% higher than that of OPC. However, at a substitution ratio of 10% or lower, the neutralization rate remains comparable to that of OPC [24]. Additionally, the pozzolanic reaction of VG, which contains a substantial number of Si components, may help prevent delayed ettrinigite formation (DEF), a phenomenon reported in various countries. Future research will explore the DEF suppression effect of VG.  
Conceptualization, T.T. ([PERSON]) and K.Y.; Investigation, T.T. ([PERSON]) and K.Y.; Methodology, T.T. ([PERSON]), K.Y., S.U. and T.T. ([PERSON]); Visualization, T.T. ([PERSON]); Writing--original draft, T.T. ([PERSON]); Writing--review and editing, K.Y., S.U. and T.T. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Nippon Hume Corporation.  
Not applicable.  
The datasets generated and/or analyzed during the current study are available upon reasonable request from the corresponding authors.  
The VG used in this study was provided by Principle Co., Ltd. This study used research equipment shared by the MEXT Project to promote the public utilization of advanced research infrastructure (which is a program for supporting the construction of core facilities), grant number JPMXS0440900024.  
Author [PERSON] and [PERSON] are employed by the Nippon Hume Corporation. The funding company had no role in the content or conclusion of this paper. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.  
The following abbreviations are used in this manuscript:  
AAM Alkali-activated materials  
ASTM American Society for Testing and Materials  
B British Standard Methods  
C-S-H Calcium silicate hydrate  
DEF Delayed ettrinigite formation  
JIS Japanese Industrial Standards  
OPC Ordinary Portland cement  
SEM Scanning electron microscopy  
SF Silica fume  
VG Volcanic glass powder  
XRD X-ray diffractometry