A Study on Mechanical Properties of Modified Soil–Cement Mixed with Ferronickel Slag Powder under Dry–Wet Cycles in Marine Environments

Soft soil foundations in marine environments are under coupling actions of seawater erosion and dry–wet cycles due to tides. Ferronickel slag is a solid waste produced in the smelting of ferronickel alloys. To recycle industrial solid waste and conserve energy, ferronickel slag is partially substituted for cement to solidify the soft soil foundations in marine environments. Unconfined compression tests were conducted for soil–cement mixed with ferronickel slag of various proportions to investigate its apparent erosion characteristics and mechanical characteristics under dry–wet cycles. In the tests, the corresponding numbers of cycles were set to 0, 6, 12, and 18. To further investigate the microscopic action mechanism of ferronickel slag on soil–cement, a nuclear magnetic resonance (NMR) device was utilized to analyze the microstructure of the soil–cement. According to the testing results, the unconfined compressive strength of soil–cement first increased and then decreased when the number of cycles of seawater erosion increased. With other conditions being the same, the addition of ferronickel slag can improve soil–cement strength, and changes in soil–cement strength were more significant than that with no ferronickel slag mixed. Moreover, the optimal amount of admixture was proved to be 45%. As the number of dry–wet cycles increased, the mass of soil–cement first increased and then decreased. With the same number of dry–wet cycles, soil–cement mixed with ferronickel slag had a smaller mass loss rate than that with no ferronickel slag added. After six dry–wet cycles, apparent erosion of soil–cement becomes increasingly serious, including the absence of edges and corners, deformation of surfaces, and even spalling and cracking. The NMR analysis revealed that dry–wet cycles can promote the evolution of small pores into larger ones within the soil–cement, thereby increasing the number of larger pores, leading to an increase in porosity, a decrease in the compactness of the soil–cement, and a reduction in strength.