## 2 Materials and Methods
### Experimental Tests
#### 2.2.2 Testing Methods  
The Freeze-Thaw Cycle experiment utilized a three-dimensional freezing closed-system model in which the specimens were sealed with plastic wrap to simulate a closed environment. This setup ensured stable testing conditions and enhanced the reliability of the experimental results (refer to Figure 3). The experimental parameters were determined based on the average monthly minimum temperature in Xining during the winter season (\(-\)14.4 \({}^{\circ}\)C). Accordingly, the freezing temperature was set to \(-\)15 \({}^{\circ}\)C and the thawing temperature to 15 \({}^{\circ}\)C, replicating natural environmental conditions, while maintaining the structural integrity of the soil specimens. Preliminary tests indicated that freezing the specimens to \(-\)15 \({}^{\circ}\)C required 11.36 h, while thawing them to 15 \({}^{\circ}\)C took 9.02 h. To \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Tests**} & \multicolumn{2}{c}{**Binder Content *} & \multirow{2}{*}{\begin{tabular}{c} **Freeze–Thaw** \\ **(F-T) Cycles** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Carbonation** \\ **(h)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Moisture** \\ **Content (\%)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Dry Density** \\ **(g.cm\({}^{-3}\))** \\ \end{tabular} } \\ \cline{2-2} \cline{5-6}  &
\begin{tabular}{c} **Calcium** \\ **Lignosulfonate (\%)** \\ \end{tabular} & & & & & \\ \hline Basic Physical & & & & & & \\ Properties Test & 0, 0.5, 1, 1.5, 2, 3 & & & & & \\ \hline Thermal & & & & & & \\ Conductivity & 1 & & & & & \\ Test & 1 & & & & & \\ \hline Microscopic & & & & & & \\ tests & 1 & & 0, 15 & & 16.5 & 1.63 \\ \hline \hline \end{tabular}  
* The percent binder content is compared to the dry mass of loess; ** U = uncarbonated.  
\end{table} 
Table 2: The test program. meet experimental requirements and maintain consistency, each freeze-thaw cycle was standardized to 12 h of freezing and 12 h of thawing. This alternating temperature regime enabled researchers to monitor changes in the soil's physical and thermal properties under conditions that closely simulated the natural freeze-thaw environment. Each experiment required the preparation of three test samples with consistent specifications and uniform quality. These samples were tested separately according to the experimental requirements to ensure the reliability and statistical significance of the test data. The carbonation test was conducted using a YMS9015 carbonation chamber (Figure 4), carbonization chamber purchased from Hangzhou Jiueu Automation Technology Co., Ltd., Hangzhou, China, designed to provide precise control over CO\({}_{2}\) concentration, humidity, and temperature. The chamber was connected to a CO\({}_{2}\) gas cylinder and a water tank, allowing for adjustments to the desired experimental conditions. Sensors integrated into the system continuously monitored temperature, humidity, and CO\({}_{2}\) concentration in real time. Specimens were placed on trays inside the chamber, and the control panel located on the top of the chamber was used to configure the experimental conditions: temperature (20 \(\pm\) 2 \({}^{\circ}\)C), humidity (96%), and CO\({}_{2}\) concentration (30%). The equipment was able to reach these set conditions and operate stably within 15 min. Preliminary tests indicated that the specimens achieved near-complete carbonation after 12 h of exposure. Based on these observations, a carbonation cycle of 24 h was selected to ensure full carbonation of the specimens. It was confirmed that no additional curing was necessary after the 24 h carbonation cycle, as the experimental requirements were fully met within this period. Each set of experiments required the preparation of three test specimens with consistent specifications and uniform quality. Carbonation tests were conducted on each specimen in accordance with test requirements to ensure the accuracy of the degree of carbonation and subsequent carbonation test data.