Mechanisms of solidification and subsequent embrittlement of dephosphorization slag used in a subtidal zone as an alternative to sea sand and prevention of solidification by adding dredged soil

In the recent years, steelmaking slag is attracting attention as suitable materials for restoration of estuary ecosystems. However, there is concern about solidification when the material is applied to create seagrass beds. In this study, dephosphorization slag (DePS) was immersed into seawater for 10 months to examine the solidification processes and its mechanisms to control the strength of solidification. The hypothesis in this study is that solidification could be alleviated by adding dredged soil to the DePS. After 5 months of immersion, the shear strength of the DePS increased from 1.8 to 5.0 kN/m²; however, its shear strength decreased significantly to 3 kN/m² after 10 months. Furthermore, after 5 months, reddish color was observed on the surface of the DePS, whereas the color of the surface of the DePS turned black at 7 months under reducing condition with covering by mud. To validate the results, we carried out an additional study, in which the DePS was immersed in seawater, and the solidified DePS was subsequently treated with Na₂S; the increase and decrease of the shear stress of DePS were reproduced. The solidified DePS before and after exposure to reducing conditions was also analyzed using a combination of microanalysis with an electron probe and Mössbauer spectroscopy. These analyses showed that the solidification was caused by the formation of bridges that composed of iron oxyhydroxides, whereas the subsequent embrittlement of the solidified DePS was attributed to changing in the chemical species of iron.