# Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand-Bentonite Mixture
## 1 Introduction  
Bentonite is a kind of soft clay produced by weathering and erosion of volcanic ash with the major mineral composition of montmorillonite [1]. It is a type of clay composed of two tetrahedral and one octahedral flake in a ratio of 2:1. The high plasticity of bentonite makes it an ideal admixture to improve the liquefaction resistance of the sand. Bentonite has distinct natural characteristics due to its special morphology and charge distribution. The bentonite particles are in the shape of thin circular disks with diameters of the order of 0.1 \(\mu\)m. The surface of the thin circular disks is negatively charged, while the edges have positive charges. The total charge of the entire particle is negative [2; 3; 4; 5]. A typical particle has a specific surface area of 600 to 800 and could expand to 1300% of its original volume when exposed to water [6]. The sodium bentonite has a high plasticity index which results in higher swelling and gelation [7]. The swelling characteristic of bentonite would produce great volume inside soil particles. Thus, many microscopic models were proposed to explain the hydration and expansion mechanism of bentonite, such as the Stern-Gouy double-layer model [8; 9], which was one of the first models to attempt to explain the interaction between clay and water at the interface layer [10]. Derjaguin, Landau, Verway and Overbeek (DLVO) explained the impact of clayy suspensions on alarge scale but did not apply to the range less than 20A [11; 12]. Although these theories have been widely used, they cannot well describe the water-soil interface at a smaller scale, nor can they well explain the expansion mechanism [13]. [PERSON] (2005) used the molecular dynamics method to analyze the nA-MT intermolecular expansion behavior of bentonite and hydrated bentonite with different interlayer hydration degrees and revealed the evolution law and mechanism of nA-MT expansion characteristics with the deepening of interlayer hydration degree [14]. Many studies have shown that clay has obvious effects on the mechanical behaviors of sand. Previous results indicated that the anti-liquefaction properties of sand could be improved by adding plastic clay particles [15; 16; 17; 18]; for example, [PERSON] et al. (2008) studied the effects of clay content on liquefaction characteristics of clayy sand and grouped the clayy sands as sand-like or clay-like soils depending on the clay content [19]. [PERSON] et al. (1989) found that there was no direct relationship between clay content and cyclic shear strength of sand and believed that the increase in plasticity index was the reason for the increase in cyclic strength. They linked the cyclic strength with the plasticity index and believed that the higher the plasticity index, the greater the cyclic shear strength [20]. [PERSON] et al. (1991) also concluded a similar conclusion [21]. [PERSON] et al. (1986) reported that the soil would not liquefy when the clay content in the soil was greater than 20% [22]. [PERSON] (1994) pointed out that the plasticity index was not a decisive factor in the cyclic strength of sand mixed with plastic clay [23]. [PERSON] (1999) conducted a series of experiments to explore the effect of changes in clay content from 2% to 37% on the shear strength of Yatesville sand [24]. In fact, the two compositions together affected the behavior of the mixture. [PERSON] et al. (2019) found that sand could reduce the volumetric shrinkage of the bentonite-sand mixtures when its content was larger than 30% [25]. The discrete element method (DEM) was a common and effective method to study the mesoscopic mechanism of rock and soil materials [26]. [PERSON] et al. (2012) conducted a discrete model for simulating shear strength and deformation behavior of rockfill material, in which the particle breakage phenomenon was considered [27]. [PERSON] et al. (2021) studied the mechanical behavior of mudstone in the hollow cylinder torsional shear test through the discrete element method.