## 3 Results and Analysis
### Analysis of UCS Test Result  
The curves of the strength related to the UFC proportion are given in Figure 11 (15% HA content, pH = 6 FA solution, 90 d soaking time.) The histogram of the strength growth  
Figure 10: PCAS test results (percentage of macropore volume).  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **XRD, MIP, SEM, and PCAS Comparison** \\ \hline XRD test & The XRD patterns indicate the presence of hydration products, reflecting the many phases in the cement soil. \\  & The MIP test shows the change in the internal structure of the pores of the cement soil. \\ MIP test & With the increase in the UFC proportion, the macropores are transformed into small and medium pores, the cumulative pore volume percentage of large pores decreases, and the cumulative pore volume percentage of small and medium pores increases. \\  & The SEM test presents the number and morphological changes of hydration products \\  & and the changing forms of pores. As the UFC proportion increases, the number of hydration products increases and the lap hydration products are gradually ordered and flat. Fibrous hydration products gradually lap into massive hydration products—the pore space changes from visible to almost invisible. \\  & The PCAS test demonstrates the connectivity of the internal pores’ structure. With the increase in the UFC proportion, the internal structure of cement soil gradually changes from large-area connectivity to small-area connectivity. Finally, it shows that the pores are not connected.  
\end{table}
Table 5: Summary of microtests.  
of specimens versus the immersion time for different UFC proportion ranges is given in Figure 12. The growth of the cement soil strength is 12.99%, 4.17%, 2.84%, 0.53%, and 1.74% for a UFC content of 0-10%, 10-20%, 20-30%, 30-40%, and 40-50%, respectively. From Figure 11 to Figure 12, it can be seen that the strength of the specimen increases continuously with an increase in the UFC proportion. Its strength growth is a maximum at 0-10% of the UFC proportion. The strength growth is slowed as the UFC proportion exceeds 10%. With the increase in the UFC proportion, the fine particles and Blaine area increase, the surface energy rises, and ultra-fine particles are unstable. The diameter of ultra-fine particles is small. The Van der Waals force between the particles is much larger than their gravity--part of the fine particles agglomerate reduces the comparative area of the agglomerate [30]. The surface reaction activity is reduced, the degree of hydration is lowered, and the amount of hydration products is reduced accordingly. The cement soil's strength development depends on the specimen's interior pore and the cementation between the hydration products and the cohesive soil agglomerates [39; 40]. According to the above comprehensive microscopic test results, the proportion of gradually increased UFC increases the number of fine particles and Blaine area, promotes surface reaction activity, accelerates hydration reaction rate, promotes hydration's extent, and increases the hydration product's quantity. Cementation between soil agglomerates and hydration cementitious products constitutes the soil skeleton, and its cohesion is proportional to the number of hydration products. In the meantime, the specimen's interior pores are continuously filled with hydration products, changing the connection of mineral aggregates and the pore structure of the soil. The number of pores and the pore connectivity is reduced, the pore diameter distribution is uniform, the pore structure is refined, and the cement soil forms a relatively denser overall structure. The intrinsic reason for the gradual increase in cement soil strength depends on improving the compactness of the microscopic pore structure.  
Figure 11: UCS curves of cement soil with various UFC proportions.