## 2 Materials and Methods
### Preparation of Samples  
#### 2.2.1 Mix Design  
In the proportioning test, mainly, the effects of steel slag substitution rate and bentonite dosage on the strength properties of barrier materials were considered. In the case of a certain water-to-binder ratio (\(\mathrm{R}_{\mathrm{w/b}}=6.25\)), the configuration of five kinds of steel slag substitution rates \(\mathrm{R}_{\mathrm{s}}\) (the ratio of steel slag mass to the composite cementitious material) and three kinds of bentonite dosages \(\mathrm{R}_{\mathrm{B}}\) (the ratio of bentonite mass to the total mass of the solids) of the steel slag-cement-bentonite soil samples at different curing ages were used to study the effect of different ratios and ages on its strength properties.  
Definitions are shown in Equations (2)-(4):  
\[\mathrm{R}_{\mathrm{w/b}}=\mathrm{m}_{\mathrm{w}}/(\mathrm{m}_{\mathrm{s}}+ \mathrm{m}_{\mathrm{c}}) \tag{2}\]  
\[\mathrm{R}_{\mathrm{s}}=\mathrm{m}_{\mathrm{s}}/(\mathrm{m}_{\mathrm{s}}+ \mathrm{m}_{\mathrm{c}}) \tag{3}\]  
\begin{table}
\begin{tabular}{c c} \hline \hline Olivine slag (geology) & 0.9–1.4 \\ Magnesium Rosette & 1.4–1.6 \\ Dicalcium silicate slag & 1.6–2.4 \\ Tricalcium silicate slag & \textgreater{}2.4 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Relationship between basicity and mineral composition of steel slag.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Testing** \\ **Items** \\ \end{tabular} } & \multirow{2}{*}{**Stability**} & \multirow{2}{*}{**3-Day** \\  & & **\begin{tabular}{c} **3-Day** \\ **Flexural** \\ **Strength** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **3-Day** \\ **Compressive** \\ **Strength** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **Initial Condensation** \\ **Time (min)** \\ \end{tabular} & \begin{tabular}{c} **Final Condensation** \\ **Strength** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **28-Day** \\ **Compressive** \\ **Strength** \\ **(MPa)** \\ \end{tabular} & \
\begin{tabular}{c} **28-Day** \\ **Compressive** \\ **Strength** \\ **(MPa)** \\ \end{tabular} \\ \hline
\begin{tabular}{c} National \\ standard \\ Measured \\ value \\ \end{tabular} & \(\geq\)300 & Eligible & \(\geq\)3.5 & \(\geq\)17 & \(\geq\)45 & \(\leq\)600 & \(\geq\)6.5 & \(\geq\)42.5 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Main performance indexes of cement.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ingredient** & **CaO** & **Fe\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **MgO** & **MnO** & **Cr\({}_{2}\)O\({}_{3}\)** & **Na\({}_{2}\)O** & **P\({}_{2}\)O\({}_{5}\)** & **TiO\({}_{2}\)** & **SO\({}_{3}\)** \\ \hline Content (\%) & 34.30 & 26.22 & 19.64 & 7.72 & 4.14 & 2.49 & 2.08 & 1.06 & 0.76 & 0.52 & 0.49 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Chemical composition of steel slag.  
\[\mathrm{R_{B}=m_{B}/(m_{s}+m_{c}+m_{B})} \tag{4}\]  
where m\({}_{w}\) is the mass of water (g); m\({}_{s}\) is the mass of steel slag (g); m\({}_{c}\) is the mass of cement (g); m\({}_{B}\) is the mass of bentonite (g); R\({}_{s}\) is the substitution rate of steel slag (%); and R\({}_{B}\) is the dosage of bentonite (%).  
Through pre-design tests and reading of the related literature [20; 21], the total amount of 750 g of water, cement, and steel slag was 120 g, i.e., the water-to-binder ratio was 6.25 and the steel slag substitution rate was 0, 12.5%, 25%, 37.5%, 50%. Bentonite dosage in 100 g, 120 g, 140 g, i.e., bentonite admixture in 46%, 50%, 54%. The specific proportioning scheme is shown in Table 7.  
A total of 15 proportioning schemes, each proportion of the production of nine internal diameters of 39.1 mm with height of 80 mm soil samples, respectively, for the determination of age 7d, 14d, 28d unconfined compressive strength, was used in order to avoid test errors, and each design used three parallel sample ages.