# Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand-Bentonite Mixture
## 2 Materials and Methods
### Test Program  
The effects of bentonite content, hydration time and effective confining pressure on the mechanical properties of the sand-bentonite mixture were investigated by the triaxial test following the ASTM standard [39]. The triaxial test apparatus with strain-controlled is shown in Figure 2. The test system is mainly composed of a pressure chamber, pressure controller and computer. The specimen was installed in the pressure chamber with a membrane, a loading cap, sample base, porous disk and loading piston.  
All specimen preparation was performed in a dry mixing operation. The dry mixing method was pre-evenly mix dry sand and dry bentonite. Then, the sand and bentonite mixture was transferred into the double valve mold through the method of air pluviation. Specimens were prepared and controlled at the same skeleton relative density. Bentonite was treated as a non-solid phase, and the only solid phase was sand. For each specimen, the mass and compacted volume of sand were the same, and the mass of bentonite was different according to the various bentonite content.  
Figure 1: Two types of bentonite used in this study: (**a**) sodium bentonite; (**b**) calcium bentonite.  
After the specimen was prepared, the triaxial tests were carried out with four major steps, which were head saturation for bentonite hydration, back pressure saturation, consolidation drainage and shear test. Time and degree of interaction between bentonite with water directly determine the properties of bentonite itself and its effect on sand. The hydration time of bentonite was adopted in this study to approximately quantify the impact of water on bentonite and bentonite-sand mixture, respectively. The concept of hydration time in this study was referred to the research on the hydration of bentonite by [PERSON] et al., which was the time that bentonite contact with water before the strength tests [40]. The starting time of hydration of bentonite was the completion time of heading saturation. In the process of head saturation, bentonite completed the initial hydration and expansion between sand particles and showed a certain degree of gelation. The ending time of hydration of bentonite was referred to as the time when consolidation was complete, and shear began. Since the time of the back pressure saturation and consolidation drainage was 24 h in this study, the specimens with a hydration time of 24 h were immediately saturated with back pressure after head saturation. The specimens with hydration times of 40 h and 48 h stayed 16 h and 24 h after the completion of head saturation, respectively, before the back pressure saturation and consolidation drainage. The shear test was strain-controlled with a constant shear strain of 0.001 mm/s. The shear rate of 0.001 mm/s can ensure the drainage of water to avoid pore water accumulation. The pore water pressure was noted to be zero during shearing. The shearing is assumed to be completed when the axial strain of the specimen reaches about 8-10%.  
Table 1 lists a total of 15 groups consolidated drained (CD) triaxial tests carried out in this study. The specimens in each group were controlled in the same dimension, which has a diameter of 39.1 mm and a height of 78.2 mm. There were three different types of bentonite contents, which were 0%, 5% and 10%. In the study, the effective confining pressure value varies from 200 kPa, 300 kPa and 400 kPa. In order to evaluate the effect that hydration time of bentonite in the pores of sand particles has on the mechanical properties of sand, this study prepared the sand-bentonite mixture specimens with the hydration time of 24 h, 40 h and 48 h. The relative compactness of all experimental groups was calculated based on the concept of \"skeleton porosity ratio\", that is, the bentonite slurry in saturated specimens was regarded as pore fluid rather than skeleton particles. The solid particles here contain only sand particles.  
Figure 2: The triaxial test apparatus employed in the study.