## 3 Results and Discussion
### Upv  
UPV is a non-destructive testing method for concrete quality commonly used in practical engineering [39]. The porosity of the material mainly affects compressive strength, while the main influencing factors of UPV include Young's modulus, Poisson's ratio, and density. Figure 4 shows the UPV test results of mortar specimens of each component. The highest UPV values for C100 were obtained at 1, 3, and 7 days of hydration, with values of 3050, 3636, and 3738 m/s, respectively. The early UPV value decreased significantly with the increase in QZ and FA substitution. For example, when the curing age was 1 day, the UPV value decreased by 14.9% with a 20% FA content. While with a 20% QZ content, the UPV value decreased by 7.6%. QZ exhibits physical effects (nucleation effect and filling effect) in the early stage of hydration, accelerating cement hydration [18; 22; 40]. However, the UPV value of the QZ group was still lower than that of the control group in the early stage. This is due to the reduction in cement mass and the insufficient physical effect of QZ to compensate for the negative effect caused by the reduction in the amount of cement used. \begin{table}
\begin{tabular}{c c c c} \hline
**Item** & **Coefficient** & **F Value** & \(p\) **Value** \\ \hline Constant & 50.701 & 105.75 & \textless{}0.001 \\ FA & \(-\)0.166 & \(-\)1.48 & 0.198 \\ QZ & \(-\)0.866 & \(-\)7.74 & 0.001 \\ FA\(\times\)FA & 0.00006 & 0.01 & 0.992 \\ QZ\(\times\)QZ & 0.01206 & 2.24 & 0.075 \\ FA\(\times\)QZ & 0.0604 & 6.47 & 0.001 \\ \hline \end{tabular}
\end{table} 
Table 4: Coefficients and test results. Figure 4: UPV test results. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Mean** & **S.D.** & **Min** & **Max** & **R\({}^{2}\)** & **R\({}^{2}\) (Adjusted)** & **R\({}^{2}\) (Predicted)** & **F Value** & \(p\) **Value** \\ \hline
46.25 & 3.98 & 38.50 & 50.60 & 99.08\% & 98.16\% & 77.15\% & 107.62 & \textless{}0.001 \\ \hline \end{tabular}
\end{table} 
Table 5: Model fit indices. The UPV value of the QZ group at 28 days was lower than that of the other groups, possibly because QZ's filling and nucleation effects accelerated the hydration reaction mainly in the early stage. Additionally, QZ has a different pozzolanic reaction compared to FA in the late stage, resulting in a lack of apparent improvement in porosity [41]. In the early stage, the FA group's UPV value is low, and in the late stage, the UPV value becomes significantly higher. As the curing time increased, the UPV value of the FA group eventually became equivalent to that of the control group. This may be due to the low reactivity of FA in the early stage, which limits its ability to facilitate the growth of hydration products. However, the secondary hydration effect increases the number of secondary hydration products as the curing period increases, densifying the internal structure of the mixed mortar and increasing the UPV value. It is important to note that the 10 FA10 QZ group exhibits higher strength and UPV values both in the early and late stages of development. When these two materials are used in combination, QZ enhances strength through its filling and nucleation effects in the early stages, while FA's secondary hydration reaction contributes to strength gain in the later stages. This synergistic effect allows both binder materials to leverage their respective advantages at different ages, thereby collectively promoting the development of cement strength. Factors that affect the UPV value, such as the amount of hydration products and pore size, also have a crucial impact on compressive strength [42]. As the hydration products increase and the capillary pores are filled, the UPV value and strength increase during the hydration process. Therefore, UPV is applicable in engineering to evaluate and predict the compressive strength of concrete. Previous studies have revealed a strong correlation between UPV and compressive strength, with an exponential relationship [42; 43].