# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 1 Introduction  
Red mud (bauxite residue) is a highly alkaline ultrafine-grained waste material generated during the industrial production of aluminum oxide from bauxite [1; 2; 3]. China is now a world-leading producer of aluminum oxide, accounting for approximately 30% of the total production in the world. In 2019, aluminum oxide production exceeded 70 million tons in China. This production will give rise to the formation of a large quantity of hazardous substances (e.g., heavy metal ions and radioactive elements) in the byproducts (e.g., solid waste materials and leachate), causing serious damage to the environment and human health [4; 5]. The disposal of red mud requires a large storage space, and the infiltration of various chemical elements into soils may cause soil salinization and groundwater pollution [6]. Importantly, long-term exposure to these hazardous substances can have devastating consequences on human health. Thus, there is an urgent need for the safe disposal of red mud in China to minimize its negative impacts. Red mud can be utilized for various applications [7; 8; 9]. Some useful materials, such as valuable metals, can be recovered from red mud that otherwise would be disposed of as waste [10; 11]. Red mud can also be used as raw materials for the production of bricks [12; 13], concrete [14], admixtures for subgrade and high-performance concrete, calcium-siliconcompound fertilizer, microporous calcium silicate heat insulation products, ceramic composite materials, thermal insulation and refractory materials [15].  
The main pollutants of red mud include heavy metal ions, fluoride, radioactive substances, and sodium and aluminum ions. The extremely high pH of red mud (\(>\)12) can be attributed to the presence of a large quantity of highly alkaline chemicals, making it corrosive to various biological and metallic materials (e.g., aluminum and steel materials) and siliceous materials (e.g., glass) [14]. There are considerable concerns about the potential leaching of red mud leachate into groundwater, which can cause an increase in pollutants and the pH value of groundwater [16]. In this regard, it is of practical importance to gain a better understanding of the changes in the physicochemical and mechanical properties of soils subjected to red mud pollution [4; 17]. Neutralization of red mud with acids (HCl, HNO\({}_{3}\) and H\({}_{2}\)SO\({}_{4}\)) before storage can be an effective way to make red mud environmentally benign [18]. [PERSON] et al. [7] investigated the effectiveness of bauxite neutralization using atmospheric CO\({}_{2}\) and whether neutralization could be accelerated when Ca\({}^{2+}\) sources were supplied in solid (flue gas desulfurization gypsum) and aqueous (CaCl\({}_{2}\) solution) forms. Obviously, neutralization of red mud can cause changes in the physicochemical, microstructural, and mechanical properties of red mud. From this perspective, red mud can be utilized to improve the cementation of acid soils, which provides a promising approach to improve their microstructure [9; 18]. [PERSON] et al. [19] investigated the effects of flow velocity and the concentration of red mud particles and OH\({}^{-}\) ions on the penetration of red mud filtrate with very fine particles in a porous medium. In recent years, red mud has started to be considered as a potential valuable resource instead of waste, and it has been used for the remediation of soils contaminated by high concentrations of metals/metalloids [9].  
In this study, we investigated the changes in the physicochemical and microstructural properties of typical red mud in China subjected to acidic and alkaline treatments, which were compared with those of a typical silty soil. This study can provide some insights into the safe disposal of red mud, which has good engineering significance for soil restoration.