# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 3 Physicochemical Properties of Red Mud under Acidic and Alkaline Conditions  
### Variation in Limit Water Content  
Figure 2 shows the liquid and plastic limits of the Shanxi silty soil, Shanxi red mud and Shandong red mud immersed in the different concentrations of the HCl or NaOH solution for 7 days, where the concentrations of the HCl solution are indicated by negative values and those of the NaOH solution are indicated by positive values for the sake of comparison. Each value was the average of three repeated test results. In Figure 2, \"0 mol/L\" on the abscissa denotes no acid or alkali added. It is clear that red mud generally has a higher liquid limit and plastic limit but a lower plasticity index (\(I_{\rm P}\) = 4.8-18.7) than silt (\(I_{\rm P}\) = 4.5-31.3) under either acidic or alkaline conditions. As the concentration of the NaOH solution increased, the liquid limit and plastic limit of the Shanxi silt soil tend to increase, whereas that of red mud remained almost unchanged. This outcome can be attributed to the high ionic strength and pH value of pore water in red mud (Table 1), resulting in strong physicochemical interactions between the solid particles and pore water. Thus, the addition of a small quantity of the HCl or NaOH solution can have a negligible effect on the red mud. It is no longer acceptable to classify red mud according to the standards obtained based on common soils with low ion concentrations and neutral pH values.  
Figure 3a shows that as the concentration of the NaOH solution increases, the plasticity index of the Shanxi silty soil increases significantly from \(I_{\rm P}\) = 13.7 before immersion to \(I_{\rm P}\) = 25 after immersion in a 4 mol/l NaOH solution for 7 d. However, the opposite trend is observed with increasing concentration of the HCl solution, and its plasticity index reaches a minimum of \(I_{\rm P}\) = 4.9 for the Shanxi silty soil immersed in a 4 mol/l HCl solution for 7 days. It is important to note that this increasing or decreasing trend becomes more pronounced with the increase in immersion time from 7 to 120 days. The liquid  
Figure 1: Digital photographs of the raw samples—(a) Shanxi red mud and (b) Shandong red mud.  
limit and plastic limit of soils are closely related to the ion concentration and pH value of pore water. It can be concluded that the addition of the HCl solution can result in a decrease in the liquid limit, plastic limit and plasticity index, whereas the opposite trend is observed for the addition of the NaOH solution.  
Figure 3b shows that for highly alkaline Shanxi red mud, the addition of the NaOH solution within 7 d has no significant effects on the plasticity index, but the addition of the HCl solution can result in a decrease in the plasticity index due to neutralization action. However, the plasticity index of the Shanxi red mud immersed in the NaOH solution also tends to increase over time (e.g., 120 days). Figure 3c shows that the changes in the plasticity index of the Shandong red mud are in good agreement with those for the Shanxi red mud, as shown in Figure 3b.  
Figure 2: The limit water content as a function of the HCl or NaOH solution concentration—(**a**) plastic limit, and (**b**) liquid limit.