# Performance Study of Black Shale Modified Soil for Road Use Based on Eshelby-Mori-Tanaka Theory
## 4 Study on Modified Soil Mix Proportions with Black Shale
### Analysis of Results
#### 4.2.3 Analysis of the Optimal Ratio for Black Shale-Modified Soil  
Considering the economic feasibility of engineering construction and analyzing the relationship among strength, cement content, and black shale admixture, the standard for selecting the optimal ratio of materials is to meet the strength requirements of highway subgrade specifications. The principle is to consume as much black shale as possible, minimize the use of cement, and reduce construction costs. Table 2 shows that some composite materials (Y4S25, Y4S30, and Y5S30) can meet the requirements of high-grade highway subcases for medium-duty and light traffic and the requirements of grade two and below highway base for medium-duty and light traffic. However, the cement consumption is too high, making it impractical. When the cement content is 9%, the strength of the modified soil decreases with the increase in black shale admixture (as shown in Figure 10). However, with the black shale admixture increasing from 55% to 70%, the strength does not significantly decrease and meets the relevant requirements. Comprehensive analysis indicates that the mass ratio corresponding to composite material Y7S30 (M\({}_{\text{shale}}\):M\({}_{\text{clay}}\):M\({}_{\text{cement}}\) = 70:21:9) is optimal. (1) Physical and Mechanical Properties of the Optimal Modified Soil Y7S30  
The results of the strength test and water absorption test for the Y7S30 specimens during the 7-day, 14-day, and 28-day curing periods are shown in Figure 11. It can be  
Figure 10: (**a**) Relationship between unconfined compressive strength of black shale modified soil and cement content. (**b**) Black shale modified soil with different cement content. clearly observed that the total water absorption of the sample increases with the increase in curing age, while the rate of water absorption decreases. The 14-day strength is 1.22 times of the 7-day strength, and the 28-day strength is 1.05 times of the 14-day strength, reaching 1.66 MPa. The notable increase in material strength is attributed to the hydration reaction of cement with water, binding the surrounding substances into a solid structure. This reaction requires a substantial amount of water, and as the curing period increases, the hydration reaction gradually reaches its limit, leading to a stabilization of the strength of the modified soil. Figure 12 illustrates the strength and mass loss of Y7S30-type specimens after 7-day curing and different \"drying and soaking\" cycles. The specimens in indoor conditions show almost no mass loss, and the strength gradually increases, although at a slower rate compared to the specimens under the same age-standard curing conditions. The specimens under immersion conditions exhibit minimal mass loss in the first 10 days, stabilizing afterward, and their strength shows an increasing-decreasing trend with 10 days as the turning point. The early strength is higher than that in indoor conditions, indicating that early immersion promotes more extensive hydration of cement, thereby enhancing strength. However, prolonged immersion weakens strength as water gradually infiltrates the interior. Figure 11: (**a**) Relationship between curing age and strength. (**b**) Relationship between curing age and mass increase rate of specimen after water absorption. For specimens subjected to \"drying and soaking\" cycles, significant mass loss occurs, increasing over time. The first 10 cycles have little impact on strength, but after 10 cycles, the effect becomes pronounced. It is evident that repeated \"drying and soaking\" environments progressively damage the specimens from the surface to the interior. Additionally, there is slight mass loss before water exposure, attributed to the shedding of debris from the specimen's surface and edges. Through indoor simulations of three different environments, it is observed that the most detrimental environment for the material is the \"drying and soaking\" cycle, followed by immersion and indoor conditions.