# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 5 Microstructure of Red Mud under Acidic and Alkaline Conditions  
Primary and secondary mineral components of common soils (e.g., silt and clay) are rarely present in red mud. Figure 4a clearly shows that Shanxi red mud particles are composed of andradite (hydrated, Ca\({}_{3}\)(Fe\({}_{0.87}\)Al\({}_{0.13}\))\({}_{2}\)(SiO\({}_{4}\))\({}_{1.65}\)(OH)\({}_{5.4}\), 27.7%), tridirpite 2H substructure (SiO\({}_{2}\), 6.0%), coesite (SiO\({}_{2}\), 26.3%), sodium aluminum silicate carbonate (Na\({}_{8}\)(AlSiO\({}_{4}\))\({}_{6}\)(CO\({}_{3}\))\({}_{1.09}\), 17.6%), calcite (Ca(CO\({}_{3}\)), 18.0%) and hematite alpha (Fe\({}_{2}\)O\({}_{3}\), 4.4%), as determined by X-ray diffraction (XRD, D8 Advance Mode, Bruker, Karlsruhe, Germany). Figure 4b shows that the Shandong red mud particles are composed of aluminum titanium oxide (Al\({}_{2}\)TiO\({}_{5}\), 17.6%), titanium aluminum oxide (Ti\({}_{2.79}\)Al\({}_{1.21}\)O\({}_{0.024}\), 2.4%), aluminum iron oxide (FeAlO\({}_{3}\), 17.9%), hydrohematite (Fe\({}_{2}\)O\({}_{3}\)\(\times\)H\({}_{2}\)O, 23.9%), macaulayite (Fe\({}_{24}\)Si\({}_{4}\)O\({}_{43}\)(OH)\({}_{2}\), 23.2%) and histinerite (Fe\({}_{2}\)Si\({}_{2}\)O\({}_{7}\)\(\times\)H\({}_{2}\)O, 14.9%). Figures 5-7 show the changes in microstructure (SEM images) of the Shanxi silty soil, Shanxi red mud and Shandong red mud immersed in a 4 mol/l HCl or NaOH solution for 120 days, including the untreated samples. The results show that the two red muds are honeycombs with particles (Figures 6 and 7) that are much smaller (<10 \(\upmu\)m) than that of silty soil (Figure 5). For silty soil, even if an acid (HCl) or alkali (NaOH) solution is added, the crystal structures of their mineral particles are still relatively complete. Red mud particles are poorly crystallized and contain amorphous substances in dispersed and disordered states. The fine particles of red mud form a solid mass with very few large pores and low hydraulic conductivity. Obviously, very few plants can survive on red mud due to its low infiltration rate and high water-holding capacity. Red mud is generated after a series of complex physical and chemical processes, especially after dissolution in a highly alkaline NaOH solution. Thus, the structures of primary and secondary mineral components in red mud can be greatly destroyed, indicating that the water-holding capacity is not a good indicator of the water-holding capacity. Figure 4: X-ray diffraction (XRD) patterns of the solid particles—(**a**) Shanxi red mud, and (**b**) Shandong red mud. red mud no longer has the physicochemical and mechanical characteristics of common silty soil or clay soil. For this reason, the interactions between the solid particles and pore water cannot be directly explained by the exchange between the bound water and free water. Thus, physical parameters such as the plasticity index for the classification of silty soil and clay soil no longer apply to red mud, and it is necessary to establish more relevant standards for the classification of red mud based on its particular physicochemical and microstructural properties. Figure 5 shows that there is a dramatic change in the microstructure of the Shanxi silty soil immersed in the HCl or NaOH solution. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [22], the addition of acidic substances into soils makes it easier for fine solid particles in soils to form a loose suspended state (Figure 5a), while addition of alkaline substances makes it easier for finer solid particles to be attached to the surface of larger particles (Figure 5c). Under alkaline conditions, the zeta potentials of some materials, such as quartz sands and kaolinite particles, tend to increase with increasing pH value [23]. The potential energy is the sum of the repulsion and the attraction components, indicating an increase in the thickness of the adsorption layer [24].