## 3 Results and Analysis
### MIP Analysis of Test Results  
Figure 8 is the pore and pore diameter distribution curve of cement soil (90 d immersion time, pH = 6.0 FA solution, 15% HA reagent content, 20% cement mixing ratio, 0%, 10%, 30%, 40%, and 50% UFC proportion). For the convenience of analysis, based on the curve characteristics in Figure 8, the pore diameter can be categorized into four classifications: (1) micropore d < 0.01 \(\upmu\)m; (2) small pore 0.01 < d < 0.1 \(\upmu\)m; (3) medium pore 0.1 < d < 10 \(\upmu\)m; (4) macropore 10 \(\upmu\)m < d < 100 \(\upmu\)m. Figure 8a reveals that the distribution curve has three distinct peak-shaped distribution areas. The peak-shaped distribution area with a pore diameter of about 30 \(\upmu\)m is the highest, followed by the peak-shaped distribution area with a pore diameter of about 2 \(\upmu\)m and the lowest peak-shaped distribution area with a pore diameter of about 0.03 \(\upmu\)m. The curve peak shows a downward trend with an increase in the UFC proportion, as the pore diameter is about 30 \(\upmu\)m. The above indicates that the pore diameter percentage in cement soil between 10 \(\upmu\)m and 100 \(\upmu\)m decreases. When the UFC proportion is 50%, the macropores' percentage in the cement soil is almost zero. It indicates that the pore diameter of the macropores continues to decrease, and the connectivity of the macropores continues to decline. Thus, the macropores have been filled and transformed into smaller, close pores. The peak shows an upward trend with an increase in the UFC proportion as the pore diameter is approximately 2 \(\upmu\)m. The above shows that the percentage of pore volume in the cement soil for pore diameters between 0.3 \(\upmu\)m and 4 \(\upmu\)m increases. The varying regularity of the pore diameter is similar between 0.01 \(\upmu\)m to 0.1 \(\upmu\)m and 0.3 \(\upmu\)m to 4 \(\upmu\)m.  
In summary, the percentage of macropore volume in the cement soil shows a downward trend with an increase in the UFC proportion, and the percentage of medium and small pores volume presents a gradual upward trend. Increasing the UFC proportion improves the pore structure and enhances the compactness of the cement soil microstructure. The reason is that the macropores provide a larger space for hydration, and the cementitious products continuously cement the soil particles and fill pores. Hydration products fill the macropores preferentially as the UFC proportion increases, transforming into medium and small pores and increasing the volume percentage of medium and small pores in the cement soil. Figure 8b shows that the cumulative volume greater than 10 \(\upmu\)m in pore size decreases from 7.68% to 0.17% with an increase in the UFC proportion. When the UFC proportion is less than 30%, the slope in the macropore range is steeper than that of the medium  
Figure 8: Pore diameter distribution curves of cement soil with different UFC proportions. (**a**) The distribution curve of pore volume percentage. (**b**) Accumulation curve of pore volume percentage greater than a certain diameter (HA 15%, pH = 6.0, 90 d). and small pores. However, when the UFC proportion is greater than 30%, the slope in the macrorope range gradually becomes gentle. The slope in the medium pore range is significant as the UFC proportion is less than 30%. After the UFC incorporation is more than 30%, the slope of the medium pore also tends to be gentle. The small pores and micropores show a relatively gentle trend. The pore diameter range of each pore decreases with an increase in UFC proportion. The specimens' internal pores have been filled with hydration products, the cement soil macropores have been refined, and the cement soil's internal pore morphology and structure have changed. This trend favors developing the macroscopic mechanical properties of cement soil from microscopic perspectives. In summary, incorporating UFC decreases the percentage of macropores' volume in cement soil. Macropores gradually transform into medium and small pores.