# Influence of Five Additives on No Loading Swelling Potential of Red Clay
## 3 Results and Discussion
### Direct Shear Test  
Through the above tests, it was found that, when cement or steel slag is used as a single additive, 9% cement content and 15% steel slag content are more suitable for reducing the swelling potential of red clay. Therefore, we selected the specimens with 9% cement  
Figure 4: SEM micrographs at 100 \(\upmu\)m of (**a**)-red clay, (**b**)-15% steel slag-modified specimen at 7 days curing, (**c**)-15% steel slag-modified specimen at 28 days curing, (**d**)-9% cement-modified specimen at 28 days curing. content cured for 7 and 28 days, and the specimens with 15% steel slag content cured for 0, 7 and 28 days for the direct shear test. The obtained cohesive strength and internal friction angle were compared with those of the red clay specimen to verify the influence of the above treatment scheme, which can effectively reduce the swelling potential of red clay, on the strength of the specimens. By comparing the values of cohesive strength and internal friction angle obtained from the direct shear test given in Figure 5, it can be seen that the addition of steel slag leads to the loss of cohesion of the clay. When the specimen is not cured, the cohesive strength of the modified clay containing 15% steel slag is reduced to 5.37 kPa, which is 75% lower than that of red clay. After a period of curing, the cohesive strength of the steel slag modified clay is restored. When the curing time is 28 days, the cohesive strength of the steel slag modified clay can be restored to 94% of the cohesive strength of the red clay before adding steel slag. The cohesive strength of the cement-modified specimens can be greatly improved after a certain curing time. After curing for 7 days, the cohesive strength of the modified clay with 9% cement content is 43% higher than that of the red clay. After curing for 28 days, the cohesive strength of the modified clay with 9% cement content is 82% higher than that of the red clay. Regardless of whether the additive is cement or steel slag, and the curing age is long or short, no particularly significant effect on the internal friction angle \(\varphi\) can be observed. Based on the XRF test results and SEM images, the ability of steel slag hydration to generate CAH and CSH colloids is far lower than that of cement, so the cementation and filling effect of steel slag on red clay particles is not obvious because the quantity of reactants for the pozzolanic reaction in steel slag is less than that in cement. When steel slag is added to the red clay, most of the calcium oxide in steel slag first reacts with the water in the clay. Then, CSH and CAH colloids are gradually formed, which takes a period of time. The above reaction process leads to a significant reduction in the water content of the clay in a short period after the steel slag is added to the clay. The decrease in water content in red clay will result in two problems. The first is the decrease in water tension between clay particles. The second is that part of the cohesion of clay depends on the hydrogen bond, and the water plays the role of the connecting node between the two clay slices [21]. The decrease in water leads to the loss of the bond strength between the clay particles, which relies on the hydrogen bond. This effect is more obvious in illite and montmorillonite, which are more hydrophilic. Previous experiments show that the shear strength of illite and montmorillonite will decrease with the decrease in water content when the clay's water content is lower than a certain level [6]. At the same time, because there is not enough CSH colloid in the clay to provide cementation between particles, the cohesion of the clay will be lost to a large extent. Then, with the increase in curing time, the pozzolanic reaction in steel slag is carried out gradually, and a part of the newly generated CSH colloid provides the adhesive force between some soil particles, which shows that the cohesion of the clay modified by steel slag measured by the direct shear test is partially restored after a period of curing.