## 2 Test Materials and Test Methods
### Test Method
#### 2.4.2 Microscopic Test Method  
(1) Scanning electron microscopy (SEM) and mineral composition analysis (XRD)  
After the CS-SS samples were tested for UCS, they were placed in an oven and air-dried at a low temperature (60 \({}^{\circ}\)C \(\pm\) 3 \({}^{\circ}\)C) for 24 h. The dried samples were taken out and broken apart, and representative samples were subjected to scanning electron microscopy (SEM, ZEISS GeminiSEM 300, Oberkochen, Germany); SEM images were taken at corresponding magnifications. The analysis of the mineral composition was realized by the XRD method, which is mainly used to analyze the change of mineral composition in the hardening process of cement mortar solidified soil and to study the reaction process by material change. Representative samples were subjected to X-ray diffraction (XRD) tests (Bruker, Germany), and XRD patterns of the samples were obtained. XRD patterns were processed using MDI Jade 6.0 software. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Tests** & **Tests** & **Cases** & **\(\alpha\)s/\%** & **\(d\)/mm** & **Curing** & **No/freq.** & **Soaking** & **Control** & **Number of** \\
**Series** & **Classification** & & & & **Age/d** & **Condition** & **Group/d** & **Samples** \\ \hline
1 & UCS & 0, 2, 5, 10, 15, 30 & 0.075\(\sim\)0.25 & 7, 14, 28 & 0 & / & / & 45 \\
2 & UCS & 0, 2, 5, 10, 15, 30 & 0.25\(\sim\)0.5 & 7, 14, 28 & 0 & / & / & 45 \\
3 & UCS & 0, 2, 5, 10, 15, 30 & 0.5\(\sim\)1.0 & 7, 14, 28 & 0 & / & / & 45 \\
4 & Dry–wet cycle & 0, 5, 15 & 0.075\(\sim\)0.25 & 28 & 1, 2 & Seawater, Freshwater & 58, 88 & 72 \\
5 & Dry–wet cycle & 0, 5, 15 & 0.25\(\sim\)0.5 & 28 & 1, 2 & Seawater, Freshwater & 58, 88 & 72 \\
6 & Dry–wet cycle & 0, 5, 15 & 0.5\(\sim\)1.0 & 28 & 1, 2 & Seawater, Freshwater & 58, 88 & 72 \\ \hline \end{tabular} Note: The control group in the table denotes the samples that are routinely maintained at a constant temperature and humidity without a dry–wet cycle; the sand particle content is the mass ratio of dry sand to dry soft soil; the cement content is the mass ratio of dry cement powder to dry soft soil. N denotes the number of dry–wet cycles.  
\end{table} 
Table 4: UCS tests and dry–wet cycle test schemes of the CS-SS samples. Ion chromatography test  
In the present work, the mixture of the standard solution, composed of 2, 4, 10, 20 and 40 mg/L chloride ions (Cl\({}^{-}\)) was first prepared, and the specific preparation procedures were as follows: according to the Water Quality-Determination of Inorganic Anions (HJ84-2016) [26], 1.6485 g of NaCl was accurately weighed and dissolved in water, transferred to a 1000 mL volumetric flask, diluted, fixed with water to the standard line, and shaken thoroughly. The 100 mg/L Cl\({}^{-}\) standard solution was prepared by precisely pipetting 100 mL of the mixed reserve solution into a 1000 mL volumetric flask and adding water to the scale line. Firstly, 50 g of the air-dried sample was weighted and poured into a 500 mL conical bottle. Secondly, 250 mL of ultrapure water was added and the mixture was shaken for 3 min at 25 \({}^{\circ}\)C. Thirdly, the thoroughly shaken solution was poured into a 50 mL centrifuge tube and centrifuged at a speed of 4000 r/min for 5 min. Finally, the supernatant was filtered through a 0.45 \(\upmu\)m microporous membrane into a sample tube for ion chromatography (ICS-900) analysis. Finally, 1.0, 2.0, 5.0, 10.0, and 20.0 mL of the standard solution were precisely pipetted into a 100 mL volumetric flask to prepare the 2, 4, 10, 20, and 40 mg/L of Cl\({}^{-}\) standard solutions. The ion concentration of the CS-SS sample was examined and the standard curve was constructed. They were injected into the ion chromatograph in the order of their concentration from low to high, and the peak area (or peak height) was recorded. The standard curve was plotted with the mass concentration of each ion as the abscissa and the peak area (or peak height) as the ordinate.