# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 2 Material and Methods  
### Components and Physicochemical Properties of Red Mud in China  
The chemical composition of red mud depends largely on the nature of the bauxite ore, while the mineral composition is also related to the techniques used for the production of aluminum oxide [5; 8; 10]. Over 95% of alumina produced globally is produced through the Bayer process. In China, aluminum oxide is mainly produced by an alternative sintering process or a combination of sintering and Bayer processes for low-grade diasporic bauxite that is rich in aluminum and silicon. However, more red mud would be generated during the sintering process than during the Bayer process. The survey of several aluminum oxide manufacturers in Guizhou, Shanxi, Shandong and Henan provinces shows that red mud is composed mainly of aluminum and ferric oxides (Table 1). Table 1 also shows that the red mud produced by the combined method has a low content of alkali components but a high content of calcium oxide. In China, the red mud produced by the Bayer method is rich in ferric and aluminum oxides (Table 2), which are lower in ferric and aluminum oxides than those produced in other countries by the same method. Table 2 shows that there is a significant difference in the main mineral components of red mud produced by the sintering process and the Bayer process in China, which is from a statistical result based on the Chinese literature [3; 4; 5; 6; 7]. The red mud is composed of 5% particles of \(d>0.075\) mm, 90% particles of \(d=0.075\)-0.005 mm, and 5% particles of \(d<0.005\) mm [3; 9; 20]. The physical properties of typical red mud in China are as follows--specific gravity \(G=2.70\)-2.89, void ratio \(e=2.53\)-2.95, water content \(w=82.3\)-105.9%, liquid limit \(w_{\rm L}=71\)-100%, plastic limit \(w_{\rm P}=44.5\)-81%, and plasticity index \(I_{\rm P}\)=10-30, which are all higher than those of the silty clay and silt; natural unit weight \(\gamma=14.2\)-15.1 kN/m\({}^{3}\) and dry unit weight \(\gamma_{\rm d}=6.6\)-8.11 kN/m\({}^{3}\), which are both lower than those of the silty clay and silt; the liquid limit index \(I_{\rm L}=0.92\)-3.37, indicating a liquid plastic state; and the saturation degree \(S_{\rm r}=91.1\)-99.6%, indicating almost complete saturation.  
The cation exchange capacity is generally high and varies substantially in red mud [9; 21]. In China, the cation exchange capacity of red mud can reach a maximum of 578.1 meq/kg and a minimum of 207.9 meq/kg, and it normally falls in the range of 250-300 meq/kg, which is higher than that of kaolin and illite but lower than that of montmorillonite. Clearly, the cation exchange capacity of red mud is not stable, and thus, its physicochemical properties are likely to change by the external environment. In addition, the specific surface area of red mud is large, with a maximum of 186.9 m\({}^{2}\)/g, and varies substantially. The specific surface area can be indicative of the dispersion and lattice structure of red mud, which is closely related to the high alkalinity and particular microstructure of red mud.