## 3 Results and Analysis
### Analysis of SEM Test and PCAS Test Results  
SEM and PCAS tests are further conducted on cement soil specimens with various UFC proportions to explore the pore structure change in cement soil. It is the typical SEM image of cement soil specimens magnified 500 times and 2000 times. The microscopic pore segmentation images processed by PCAS software V2.3 are shown in Figure 9 (15% HA content, pH = 6 FA solution, 90 d immersion time, 0%, 10%, 20%, 30%, 50% UFC proportion). After the framed position in Figure 9a(1)-f(1) is enlarged 4 times, the 2000 times SEM image of Figure 9a(2)-f(2) is obtained. The SEM image (500 times) in Figure 9a(1)-f(1) is processed by PCAS software V2.3 to obtain the pore segmentation image in Figure 9a(3)-f(3). Figure 9: _Cont._  
Figure 9a shows that the microstructure of the unadulterated UFC cement soil specimens exhibits evident aggregate-macropore-aggregate cohesion, and there are not enough hydration products wrapped and filled between aggregates. Thus, the pores of a larger overhead structure are formed in the cement soil. The skeleton of the cemented soil is loose, and the overall structural connection is feeble. In Figure 9b, the SEM image shows that the hydration products grow intricately on agglomerate surfaces and between pores. The fibrous hydration products are visible on them. There is a significantly reduced macropore, weakening the interconnectivity between the pores. The cement soil's overall structural cohesion is enhanced compared to the unadulterated UFC cement soil specimens. The fibrous and massive hydration products are visible, as shown in Figure 9c. The fibrous hydration products gradually accumulate and overlap to form massive hydration products, filling between aggregates and pores. There are further reduced macropores, further weakening the interconnectivity between pores. The cement soil forms a dense overall structure. Figure 9d-f show that the aggregates and pores are filled with hydration products, accumulating the hydration products in an orderly manner. The pore size is gradually invisible, and the pores are gradually disconnected. Cement soil forms a significantly denser overall structure, and its cohesion is powerful. In summary, the UFC's incorporation gradually increases the relative content of hydration products. There is no sufficient hydration product between aggregates and pores, and hydration products gradually fill them. The hydration products accumulate from disorderly to orderly between the aggregates and the pores. The connectivity between pores changes from a large-area connection to a small-area connection to basically no  
Figure 9: Microstructure images of cement soil with various UFC proportions [19] and PCAS pore segmentation processing images. **(a)** UFC proportion 0%. **(b)** UFC proportion 10%. **(c)** UFC proportion 20%. **(d)** UFC proportion 30%. **(e)** UFC proportion 40%. **(f)** UFC proportion 50%. (l) 500 times SEM image. (2) 2000 times SEM image of yellow frame. (3) PCAS pore segmentation image. connection between pores. Its overall structure is from very loose to dense and presents as significantly denser. PCAS can be used for the identification and semi-quantitative characterization of fractures and pores. The pore structure is analyzed by the pore data parameters obtained by PCAS. This experiment is only for macropores' analysis (pore diameter \(>\) 10 \(\upmu\)m). The relationship curve of the macropores percentage and UFC proportion is shown in Figure 10. Figure 10 shows that the percentage of macropores' volume (pore diameter \(>\) 10 \(\upmu\)m) shows an integrally downward trend with an increase in UFC proportion. The UFC proportion rises between 0% and 10%, and the reduction rate in the percentage of macropore volume is the largest, at 38.84%. When the admixture of UFC is 30%, the cumulative reduction in the percentage of macropore volume reaches 71.55%; however, when the admixture of UFC exceeds 30%, the reduction in the percentage of macropore volume gradually tends to level off.