Solid buoyancy material is an important material for manned/unmanned deep submersibles due to its low density and high compressive strength. Hollow glass microspheres in solid buoyancy materials will gradually damage at a high pressure, leading to an increase in water absorption and thereby reducing the positive buoyancy of the submersible and affecting the safety and usability of manned/unmanned deep submersibles. There is a coupling relationship between water absorption rate and stress of buoyancy materials, which poses challenges in calculating the water absorption rate of buoyancy materials. This paper was to investigate concentric spherical shell cells and analyze the entire water absorption process via adding subroutines by a finite element method (FEM). The distribution, changes, and interrelationships of bulk stress, water absorption rate, and diffusion coefficient of buoyancy materials at different thickness points and durations were analyzed. It is indicated that water absorption rate is linearly related to bulk stress, and the expansion coefficient is related to bulk stress. The results calculated by the finite element analysis are in reasonable agreement with the experimental data. The paper can provide a theoretical guidance for the design and application of solid buoyancy materials at sea depths.

Cement-based materials have a multi-phase and multi-scale structure, and each component forming the structure affects the overall toughness to varying degrees. This review summarized the cracking-resistance and toughening mechanisms of relevant components classified in macroscale, mesoscale and nanoscale to cement-based materials. In the macroscale (such as the cementitious component, aggregate, admixture, fiber, water-binder ratio, curing system, etc.), the influence mechanisms of the physicochemical effects of relevant components on the cracking resistance and toughening characteristics of cement-based materials were analyzed. In the mesoscale (such as microsphere, whisker, capillary crystalline material, etc.), the enhancement effects such as volcanic ash, filling, bridging, bonding, capillary crystallization were described. In the nanoscale (such as nano-SiO₂, nano-CaCO₃, graphene, carbon nanotube, microorganism, nano-polymer, etc.), the mechanisms enhancing and optimizing the microstructure of hydration products, growth mode, and pore structure of hardened paste were introduced. The conjugating toughening effects of cross-scale components such as 'carbon fiber+carbon nanotubes', 'fiber+whisker+graphene' were elucidated. In addition, the shortcomings in the current application/research, and the future development trends for toughening components in different scales were also discussed.

The deep-sea contains an abundant of natural resources and treasures that still remain unrecognized and unexploited. To obtain these resources, developing advanced submersibles that are able to explore the deep-sea becomes a challenge. In the extreme harsh environment such as ultra-high pressure and low temperature in the deep sea, the advanced deep-sea submersibles have to be reliable and stable. In this regard, a power supply system is extremely important for the deep-sea submersibles because its performance directly affects the operation capability, safety and reliability of the deep-sea submersible. Therefore, recent efforts have been devoted to the development of deep-sea power sources. The existing deep-sea power systems have experienced the development stages of lead-acid batteries, silver-zinc batteries, lithium-ion batteries, and high-energy-density solid-state lithium batteries. This review elaborated the main classification of deep-sea power sources. In addition, the challenges and future development trends of batteries for deep-sea submersibles were also discussed. It is expected that this review could provide a reference for the further development of the power supply systems for more reliable and advanced deep-sea submersibles.

Ferroaluminate cement has an effective resistance to seawater erosion due to its advantages in the application of marine engineering. The effect of mixing with seawater and metakaolin on the fluidity, setting time, heat of hydration, electrical resistivity, internal temperature, chemical shrinkage, compressive strength, and hydration products of ferroaluminate cement paste was investigated. The results indicate that the addition of metakaolin reduces the fluidity and shortens the setting time of the cement paste. The chemical shrinkage at 72 h, electrical resistivity and internal temperature peak at 24 h all decrease. However, the heat release rate, chemical shrinkage, electrical resistivity and the formation of ettringite at early age can be accelerated when the content of metakaolin is less than 20%. When the cement is mixed with seawater, the hydration of cement can be accelerated and the fluidity and setting time can be decreased. The formation of the Friedel's salt can occur. The mixing of seawater and appropriate amounts of metakaolin in ferroaluminate cement promotes more ettringite, thus increasing the compressive strength of ferroaluminate cement paste. The compressive strength of different specimens mixed seawater at 180 d is improved by 48.6%-80.2%, compared to that of the sample mixed with fresh water.

To investigate the bond performance between seawater sea-sand alkali activated concrete (SSASC) and carbon fiber-reinforced polymer (CFRP)/glass fiber-reinforced polymer (GFRP) bars, center pull-out tests were carried out at different FRP bars, FRP bars diameters, bond lengths, concrete strengths, and concrete curing ages. Based on the statistical analysis of existing experimental data, the bonding performance between alkali activated concrete (ASC) and FRP bars is significantly better than that of ordinary Portland concrete (OPC). The bond strength can be predicted by the corresponding empirical model. The results show that the bond strength decreases with the increase of FRP bars' diameter and bond length, and there is no significant change as the diameter changes. Concrete ASC30 is mostly destroyed in pull-out failure, while concrete ASC60 is mostly destroyed in splitting/reinforcement fracture failure, and the concrete with a higher strength also has a higher bond strength. The bond strength increases with the increase of curing age. The bond strength between OPC and FRP bars is less than that of ASC. In addition, the bond strength predicted by the empirical model is in reasonable agreement with the experimental data.

Effect of cement mixture containing bagasse ash (SCBA) and slag (BFS) on the corrosion properties of steel in sea-sand mortar was investigated. The electrochemical impedance spectra, open circuit potential, and corrosion current density of steel were determined, and the chloride ions content, pH value and hydration products were measured. The results show that an increase in bagasse ash content leads to an increase in the corrosion rate of steel and [Cl^-]/[OH^-] value. However, an increase in slag content can reduce the corrosion rate of steel, and cause that the [Cl^-]/[OH^-] value initially increases and slightly decreases after 28 d. The addition of 15% bagasse ash and 15% slag has a positive synergistic effect, effectively increasing the resistance of mortar protective layer and charge transfer resistance, improving the stability of the passivation film, and reducing the corrosion rate of steel in the mortar by 31.86%, compared to the reference group. The optimum corrosion resistance of the steel can be obtained. It is indicated that the use of a cement mixture containing bagasse ash and slag can effectively improve the corrosion resistance of steel in sea-sand mortar.

The deterioration of the durability of marine concrete and the degree of corrosion of internal steel bars in the tidal, splash and atmospheric zones are often serious in salt spray wetting and drying cycle. The diffusion of chloride in cement-based materials was investigated at different parameters (i.e., mix ratio, dry-wet ratio, and exposure time) by a cycle test of salt spray wetting and drying and a test method of layered grinding and potentiometric titration. The results show that the deposition and transport of chloride ions in mortar are primarily affected by salt spray wetting-drying cycle, dry-wet ratio, water-binder ratio, and exposure period. The influence of water-binder ratio is dominant, while the influence of dry-wet ratio is slight. The maximum concentration of chloride C_max and the effective chloride diffusion coefficient D_eff increase as water-binder ratio improves. C_max firstly increases and subsequently declines as fly ash amount and frequency of dry-wet cycles increase. D_eff decreases as exposure time increases. D_eff firstly increases and subsequently reduces as fly ash content increases. Low water-binder ratio and use of fly ash can effectively postpone chloride erosion, thereby enhancing the durability of structures.

A CaO-MgO-Al₂O₃-SiO₂ amorphous ceramic material was fabricated by a high-temperature melting method, and calcium magnesium aluminum silicate ceramic coating was prepared on 45# steel substrate via high-speed grinding and plasma spraying. The phase composition, microhardness, and microstructure of monolithic ceramics and coatings were analyzed, and the bonding strength of the coatings was tested through tensile experiments. The corrosion resistance and mechanism of the coating were investigated through salt solution immersion corrosion experiments. The results indicate that the porosity of the calcium magnesium aluminum silicate ceramic coating is 7.93%±3.27%, without an obvious layered structure. The microhardness value of the coating is 6.63 GPa, which is only 3.89% lower than that of monolithic materials. The non-porous area has the microstructure and mechanical properties of monolithic ceramic materials. The bonding strength of the coating is (16.25±2.11) MPa, and the fracture occurs at the interface between the coating and the metal transition layer, which is equivalent to the bonding performance of other plasma spraying ceramic coatings. The corrosion rate of the coated sample is 0.102 6 g·m^-2·h^-1, which is reduced by 12.3 times, compared to the substrate sample, indicating a good corrosion resistance through a 1 000 h-immersion-corrosion-experiment. Based on the analysis of the corrosion cross-section, the dense structure of the coating plays a mechanical isolation role in corrosion. The dissolution rate of corrosion products accumulated on the pores and crack surfaces of the coating is balanced with the rate of corrosion liquid penetrating into the substrate, reducing the corrosion.

As a special cement-based material, low pH value cement-based materials are mainly used in the solidification engineering of medium and low radioactive waste, heavy metal pollutants and the engineering of high-level radioactive waste repositories. This review analyzed the main influencing factors and changes of pore solution pH value of blended Portland cement, magnesium silicate cement and magnesium phosphate cement. and summarized the current research work on the flowability, compressive strength, durability and solidification performance of low pH value cement-based materials. In addition, the application of low pH value cement-based materials in the solidification of low or medium radioactive waste and heavy metal contaminant and high-level radioactive repository engineering was also discussed.