As one of the most extreme environments, the cryogenic temperature can make materials expose extremely complex and unpredictable potential intrinsic properties. Conducting the relevant research is to reveal the unknown attributes of materials in extreme environments, and to explore their potential applications in the future. This becomes a frontier topic and breakthrough point of scientific barriers in materials science research. The main conclusions are as follows: 1) In cryogenic environments, the mechanical properties of ordinary concrete can deteriorate. Therefore, the cryogenic temperature resistance of concrete structures can be greatly improved via increasing the ratio of stirrups, using prestressed steel bars, adding fibers, and filling high-strength steel pipes with UHPC. 2) The concrete strength is mainly related to its strength grade, temperature, water content, and porosity. As the temperature decreases, the compressive strength, flexural strength, and tensile strength of concrete firstly increase and then decrease, while the bonding strength shows a linear increasing trend. The critical temperature corresponding to the mechanical failure limit of concrete is different. 3) After cryogenic freeze-thaw cycles, the freezing, freeze-thaw and migration of pore water in concrete can destroy the pore structure. The uncoordinated thermal deformation, loose aggregate structure and high water absorption induce cracking in concrete interface transition zone. The C-S-H gel can undergo depolymerization fracture or interlayer ion bonding recombination, leading to interlayer pore collapse and shrinkage cracking. 4) At cryogenic temperature, the strength increase of C-S-H gel and the phase transition of pore water are the internal factors that enhance the concrete performance. During the cold freeze-thaw cycle, based on the volume expansion caused by pore water freezing and its thermodynamic process, the theories of hydrostatic pressure, osmotic pressure, crystallization pressure, micro-ice lens, glue spall and unsaturated poroelasticity are proposed. Although the relevant theories can explain a freeze-thaw failure at low temperatures, they are not fully applicable to explain the failure mechanism of concrete in cryogenic environments.

The existing research are conducted in the related fields, but the preliminary conclusions are only drawn on the macroscopic properties, microstructure, and failure mechanism of concrete. However, the research and application of concrete materials in extreme environments involve multiple levels of physical and chemical reactions, coupled with a superposition effect of multiple external factors, which presents complex and variable characteristics. This has certain technical limitations for human-being to utilize the existing cognition to adapt or modify nature. There are still some key issues that need to be solved in this field, i.e., 1) For ordinary concrete, its extreme temperature-variation resistance is inferior due to the limitation of material properties, and the optimization technology and improvement effect are not effective. Therefore, there is an urgent need to develop new concrete materials with an excellent resistance to cryogenic temperatures and long-term service, and deeply analyze their performance evolution and enhancement mechanism in extreme environments. 2) The existing research on cryogenic concrete mainly focuses on macroscopic mechanical and thermal properties, while there is little research on the in-situ evolution of concrete microstructure characteristics at cryogenic temperatures. Meanwhile, the relevant failure theory is only proposed for ordinary freeze-thaw cycles in a small temperature range, and is not applicable to the deterioration mechanism of concrete in complex and extreme cryogenic environments. 3) From the perspective of technical methods, the temperature-variation effect in cryogenic environments has great challenges to the performance testing of concrete and how to coordinate its thermal coupling effect. Simultaneously, it is particularly momentous to achieve in-situ monitoring of concrete performance in extreme environments. That is because many conventional testing equipment, techniques, and methods for macroscopic or microscopic properties of concrete will no longer be applicable. 4) In cryogenic environments, the macroscopic properties and microstructure of concrete exhibit enhancement and degradation effects, and these effects show different dominant trends in different temperature ranges. It is thus necessary for further research to make clear the dominant effects of the reinforcement or deterioration behavior of cryogenic concrete in different temperature ranges and the internal causes of temperature-performance transition.