# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 2 Material and Methods
### Test Procedures  
Typical red mud samples with a pH \(>\) 12.3 (C\({}_{\rm(OH}^{-})\) = 2 \(\times\) 10\({}^{-2}\) mol/L) were collected in Jiaokou County of Shanxi Province (hereafter referred to as Shanxi red mud) and Shandong Province (hereafter referred to as Shandong red mud), which were gray and red-brown in color, respectively (see the digital photographs in Figure 1). The particle size distributions were measured by laser diffraction (LA-950 Mode, HORIBA, Kyoto, Japan; wet method), which can easily measure the solid particle size of soil in the range of nanometer to millimeter. The particles \(d\)\(<\) 0.005 mm, 0.005 mm \(<\)\(d\)\(<\) 0.075 mm, and \(d\)\(>\) 0.075 mm accounted for approximately 5%, 90%, and 5%, respectively, and thus these particles were mainly silts with \(d\)\(<\) 0.075 mm. The liquid limit was \(w_{\rm L}\) = 56.7% and 55.8%, the plastic limit was \(w_{\rm P}\) = 45.4% and 45.9% (liquid-plastic limit combined measurement method), and the plasticity index was \(I_{\rm P}\) = \(w_{\rm L}\) - \(w_{\rm P}\) = 11.3 and 9.9 (ignoring the percent sign) for the Shanxi Province and Shandong Province red mud, respectively. Silty soil samples with a pH of 7.6 were collected from the shallow surface near the red mud impoundment in Jiaokou County of Shanxi Province (hereafter referred to as Shanxi silty soil) and used for comparison, whose liquid limit was \(w_{\rm L}\) = 32.9%, plastic limit was \(w_{\rm P}\) = 19.2%, and plasticity index was \(I_{\rm P}\) = 13.7. The temperature of the laboratory was controlled at \(T\) = 22 \(\pm\) 0.2 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multicolumn{2}{c}{**Chemical**} & \multicolumn{4}{c}{**Sintering Process**} & \multicolumn{4}{c}{**Combined Process**} \\ \cline{2-9}
**Composition** & **Guizhou** & **Shanxi** & **Shandong** & **Henan** & **Average** & **Shanxi** & **Henan** & **Average** \\ \hline SiO\({}_{2}\) & 25.9 & 21.4 & 22.0 & 21.4 & 22.7 & 20.6 & 20.5 & 20.6 \\ TiO\({}_{2}\) & 4.4 & 2.9 & 3.2 & 2.6 & 3.3 & 2.9 & 7.3 & 5.1 \\ Al\({}_{2}\)O\({}_{3}\) & 8.5 & 8.2 & 6.4 & 8.8 & 8.0 & 9.2 & 7.0 & 8.1 \\ Fe\({}_{2}\)O\({}_{3}\) & 5.0 & 8.1 & 9.0 & 8.6 & 7.7 & 8.1 & 8.1 & 8.1 \\ NaOH & 11.1 & 8.0 & 11.7 & 16.3 & 11.8 & 8.1 & 8.3 & 8.2 \\ CaO & 38.4 & 46.8 & 41.9 & 36.0 & 40.8 & 45.6 & 44.1 & 44.9 \\ Na\({}_{2}\)O & 3.1 & 2.6 & 2.8 & 3.2 & 2.9 & 3.2 & 2.4 & 2.8 \\ K\({}_{2}\)O & 0.2 & 0.2 & 0.3 & 0.8 & 0.4 & 0.2 & 0.5 & 0.4 \\ MgO & 1.5 & 2.0 & 1.7 & 1.9 & 1.8 & 2.1 & 2.0 & 2.0 \\ In total (\%) & 98.1 & 100.0 & 99.0 & 99.6 & 99.3 & 100.0 & 100.0 & 100.0 \\ \hline \end{tabular}
\end{table} 
Table 1: The chemical compositions of typical red muds in China. \begin{table}
\begin{tabular}{c c c c} \hline \multicolumn{2}{c}{**Sintering Process**} & \multicolumn{2}{c}{**Bayer Process**} \\ \hline
**Mineral Compositions** & **Mass Percentage (\%)** & **Mineral Compositions** & **Mass Percentage (\%)** \\ \hline Calcite & 26.0 & Hydrogarnet & 46.1 \\ Calcium orthosilicate & 25.0 & Hematite & 17.0 \\ Hydrated calcium & 15.0 & Perovskite & 13.6 \\ Hydrogarnet & 9.0 & Cancrinite & 12.3 \\ Hydrous ferric oxide & 7.0 & Diaspore & 2.0 \\ Nepheline & 7.0 & Illite & 2.0 \\ Sodium silicate hydrate & 5.0 & \\ Perovskite & 3.0 & \\ \hline \end{tabular}
\end{table} 
Table 2: The mineral compositions (or compounds) of typical red muds in China. As a typical acid or alkali substance, HCl (or NaOH) was selected as the neutralization agent or corrosion solution. The red mud particles (or Shanxi silty soil) were completely immersed in an HCl (or NaOH) solution at a solid-liquid ratio of about 60% in weight and stirred for approximately 5 min and then settled in the solution for 7, 28, and 120 days. For each sample, the weight of the air-dried solid particles was 300 g. The selected concentrations were 1 and 4 mol/L, which correspond to high concentration of acid (i.e., pH \(\leq\) 0) or alkali (i.e., pH \(\geq\) 14). Thus, their limit water content, specific surface area and microstructure were determined.