# Study on Strength and Microstructure of Red Clay Reinforced by F1 Ionic Soil Stabilizer
## 1 Introduction  
Red clay is a carbonate weathering product rich in strongly hydrophilic clay minerals such as montmorillonite, illite, and kaolinite [1]. It has several undesirable engineering properties such as swelling and softening in water, shrinkage and dry cracking upon drying, poor water stability, low strength, and large deformation [2; 3; 4]. When directly used as an engineering filler in structures, it often causes issues such as uneven foundation settling, subgrade collapse, and slope instability [5; 6]. Red clay could threaten the service performance and life cycle of highways, railways, reservoirs, and other engineering structures in areas with a red clay subsurface; therefore, research into red clay reinforcement and augmentation is very important to engineers. Line, cement, and other traditional binding materials are widely used in red clay reinforcement because of their low cost, abundance, and availability in bulk quantities. [PERSON], [PERSON], and [PERSON] studied the physical and mechanical properties of lime-modified red clay and found that traditional cementitious materials can significantly improve the physical properties and strength of red clay [7; 8; 9]. [PERSON] studied the influence of particle size on the bearing ratio of lime-modified red clay and found that its strength and water stability could be significantly improved given careful control of its particle size [10]. [PERSON], [PERSON], and [PERSON] stabilized red clay with cement and found that a reasonable amount of cement could significantly improve its hydraulic properties and mechanical strength [11; 12; 13]. While conventional binding agents are proven to work, they come with a high carbon footprint and emission. As an alternative, [PERSON] used sand and gravel to improve the specialengineering properties of red clay [14]. [PERSON] found that a composite curing agent can significantly improve red clay's water stability and bearing capacity [15]. [PERSON] studied the mechanism of red clay reinforcement by calcium carbonate nanoparticles from the perspective of the colloidal chemistry of mineral particles [16; 17]. [PERSON] studied the mechanical properties and microstructure of modified red clay at different pH levels and found that acidity and alkalinity can significantly affect the degree of intergranular interaction in red clay [18]. [PERSON] augmented red clay with rubber particles from waste tires and increased shear strength [19]. Ionic soil stabilizers are a new industrial product that require a lower dosage, cost less, reinforce the soil well, and work across various soil varieties. [PERSON], [PERSON], and [PERSON] found that ionic soil stabilizers can reduce the plasticity index and improve the compactness and strength of red clay [20; 21; 22]. [PERSON] found that SS299 soil stabilizer can significantly improve the strength of red clay [23]. [PERSON] and [PERSON] found that ionic soil stabilizers can change the type and concentration of ions in soil with excess pore water, thereby reducing the thickness of water film and disrupting the electrical double layer through a series of physicochemical reactions including ion exchange, adsorption, and encapsulation [24; 25]. [PERSON] found that the reinforcing effect of ionic soil stabilizers is influenced by the soil's mineral composition [26]. [PERSON] used F1 ionic soil stabilizer to reinforce loess and found that F1 can reduce the thickness of water film and significantly increase the dry density and compressive strength of loess [27; 28]. Since the 20 th century, researchers have intensively studied the reinforcement of expansive soil, loess, and other special soils with ionic soil stabilizers. Despite the long history, the research effort has not left the experimental exploration stage, and the concept has not seen wide commercial adoption. Therefore, studying the engineering characteristics and strengthening mechanisms of red clay reinforcement is a top priority.