Application of multiple solid wastes as subgrade material in expressway subgrade: field test, microcosmic mechanism and sustainability

Based on the concept of sustainable development, these solid wastes, such as soda residue (SR) and phosphate tailing (PT) were used to collaboratively prepare soda residue modified fishpond soil (SRS) and phosphate tailings-soda residue modified fishpond soil (PRS), stabilized by externally adding lime, which were applied as subgrade materials in expressway engineering. Through field tests and comparisons with lime-stabilized fishpond soil (LFS), the feasibility and advantages of them were verified as subgrade materials. Further microstructural analysis using XRD and SEM tests revealed its reaction mechanisms and microstructural characteristics. Additionally, carbon emissions and their economic assessments were conducted. As the curing time increased, the mechanical properties of SRS, PRS, and LFS all improved. After 7 days of curing, the value of CBR f , MR f , deflection, and DCPI of SRS are 71.1 %, 151.2 MPa, 68.2 (0.01 mm), and 1.01 cm/blow, respectively; for PRS, these values are 79.6 %, 164.4 MPa, 59.9 (0.01 mm), and 0.95 cm/blow; and for LFS, the values are 63.6 %, 131.0 MPa, 69.3 (0.01 mm), and 1.09 cm/blow. The road performances of SRS and PRS are slightly superior to those of LFS. XRD and SEM analysis indicate that the reticulated C-S-H and short-columnar AFt in the SRS and PRS systems fill the pores, thereby contributing to the development of strength. Sustainability analysis shows that SRS and PRS are environmentally friendly, low-carbon, and economically advantageous subgrade materials, suitable for application in the subgrade of expressways and highways.