2021 Volume 75 Issue 1 Pages 247-254
In recent years, a large amount of seabed mineral resources have been found such as hydrothermal deposits, manganese nodules, cobalt rich crusts and rare earth mud exist on the deep seafloor near Japan, and resource development on the deep sea floor has attracted much attention. On the other hand, in order to achieve the Sustainable Development Goals, it is necessary to develop seabed resources in consideration of marine ecosystems.
The authors have proposed eco-friendly mining methods that deep seafloor is covered by cement-based sealing compounds to suppress environmental disturbances associated with the development of seabed resources. Under such methods, cement is hydrated in-situ deep sea. Our previous studies have focused on sea water deterioration of fabricated specimens, and the deterioration and durability of cement-based materials casted and hardened on the deep seafloor have not been clarified. By the casting at deep sea, the materials may be contaminated by seawater and seabed sediments, which can affect the deterioration and durability.
In this work, Portland cement-based sealing compounds were casted in-situ at deep seafloor at a depth of 1890m by using a remotely operated vehicle. The specimens were recovered after 217 days of exposure. The obtained X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy spot analysis data revealed that the significant calcium leaching occurred and carbonates such as hemicarbonate and monocarbonate were precipitated by the reaction with the seafloor sediment.
