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.

Ferronickel slag is an industrial waste, ranking as the fourth largest smelting slag in China, with an annual discharge of about 30 million tons. However, only 8% to 10% of this slag is utilized, leaving a substantial amount of ferronickel slag unutilized. This not only pollutes the environment, but also results in a significant waste of resources. Recycling ferronickel slag wastes allows for the optimum use of resources, which benefits initiatives in ferronickel slag powder production and environmental protection. In light of this, ferronickel slag powder (FSP) and mineral powder (MP) are mixed here to achieve the optimal proportion when incorporated into soil-cement (the mixture is simplified as FSMP) for application in subgrade reinforcement. In the subgrade soil of airport runways, in fact, it often endures the impact of loads, therefore, to study the influence of added ferronickel slag powder on the mechanical behavior of soil-cement after impact compression, the Split-Hopkinson pressure bar (SHPB) testing was conducted on a series of soil-cement samples with varying FSMP replacement ratios (0, 10%, 20%, 30%, 40%, 50%, and 60%). The failure mode, compression after impact (CAI) strength, and dynamic stress–strain relationship of the soil-cement samples were studied. The results indicate that, as the FSMP replacement ratio increases from 0 to 60%, the rate of peak stress growth at 60-day age are 11.39%, 12.08%, 12.57%, 16.11%, −16.83%, and −13.20% respectively. This implies that the dynamic stress first increases as FSMP increases and then decreases after the peak is reached at 40% FSMP replacement. Moreover, as the curing ages, the peak dynamic stress gradually increases. The rate of this increase is significantly higher between the curing age of 7d and 28d than between 28d and M60d. The addition of a suitable amount of ferronickel slag powder can enhance the impact resistance of soil-cement. These findings can serve as a foundation for understanding the application of soil-cement in engineering under impact.