## 3 Results
### Compressive Strength  
For samples using only SF without RHA, the strength of compression is proportionally increased with the SF content increase. The results show that samples with 15% and 20% SF exhibit a significant increase in compressive strength compared to the sample with 10% SF,  
Figure 4: Entrapped air content of UHPC mixture.  
Figure 3: Slump flow of UHPC mixture.  
with respective increments of 6.9% and 12.7% at the 28-day stage. It aligns with previous studies by [13; 14], where it was explained that SF can enhance hydrolysis reactions. This results in a reduction in unreacted cement, and less stable components such as Ca(OH)\({}_{2}\) are neutralized through ettringite reactions, leading to the formation of new C-S-H gels with improved quality.  
Regarding samples incorporating RHA at levels of 5-10%, it has been observed that this enhances the compressive strength of UHPC at both the 5-day and 28-day stages compared to the sample with 20% SF. The rationale behind this improvement may be attributed to RHA's ability to fill microscale voids, creating a denser state within the UHPC matrix. According to [PERSON] and colleagues [46], RHA particles fall between the size range of cement particles and SF particles, facilitating the formation of a more compact and robust UHPC mixture. In this particular situation, the substitution of cement with SF accounted for a 20% replacement rate. However, this proportion might fall short of effectively occupying all the system's empty spaces and voids [47]. The insufficiency of this replacement percentage to adequately fill these gaps could potentially impact the overall performance and characteristics of UPHC. Figure 5 shows the compressive strength of UHPC at 5 days and 28 days of age when using different ratios of SF and RHA.  
The increase in the compressive strength of UHPC with the incorporation of RHA is primarily due to its impact on workability and entrapped air content. RHA enhances the densification of the concrete matrix through its pozzolanic reaction with Ca(OH)\({}_{2}\), resulting in the formation of additional C-S-H gel. This reaction not only contributes to a denser microstructure but also improves the packing density of the concrete mix. Consequently, voids within the concrete are more effectively filled, leading to reduced porosity and enhanced compressive strength.  
The finer particle size of RHA further contributes to this effect by improving workability. Better packing density due to finer RHA particles allows for a more uniform distribution and consolidation of the mix, which reduces the likelihood of entrapped air. Since entrapped air can create internal voids that weaken the concrete, reducing its content is crucial for achieving higher compressive strength.  
Additionally, RHA's ability to enhance pozzolanic reactions, fill microscale voids, and mitigate autogenous shrinkage also plays a significant role in increasing compressive strength. However, when RHA is used at higher proportions, such as 15-20%, a decrease in  
Figure 5: Effect of SF and RHA on compressive strength of UHPC.  
compressive strength compared to a 10% RHA sample may occur. This reduction could be due to an increase in entrapped air, insufficient silica fume (SF) to fill microscale voids, and the limited compensatory effect of RHA particles in addressing these issues.