Research on the Material Performance and Forming Process of Lunar Regolith Simulant

Establishing a safe and sustainable lunar base on the moon is the foundation for human exploration and development activities. Due to the high transportation costs of the Earth Moon space, a large amount of construction materials are difficult to obtain from Earth. Therefore, how to utilize lunar resources for in-situ construction on the lunar surface has become the key challenge for the construction and operation of lunar base. Faced with special lunar simulant materials and extreme lunar environments, lunar construction faces a series of problems and challenges. This article conducted research on lunar regolith simulant and forming process, providing guidance for lunar surface construction.


Introduction
Since the 21st century, various countries have successively conducted research on lunar landing and landing on the moon.NASA proposed the "Artemis" plan in 2019, with plans to build a lunar base by 2028.ESA has launched the "Dawn Project", aiming to build a lunar base as a frontline space exploration base by 2030 to promote human landing on Mars.Russia plans to build a comprehensive base on the moon between 2036 and 2040, and to build a strategic observatory around the base for radio astronomy, cosmic ray research, and other missions.Since the Chang'e-5 spacecraft brought back to the real lunar regolith from the moon in 2020, China's Chang'e lunar exploration project has completed the three-step plan of "orbiting, landing, and returning", entering the stage of "exploration, construction, and use".It is planned to build a basic unmanned lunar research station by 2030, improving the lunar science and resource application capabilities.Domestic and foreign scholars have analyzed the demand for in-situ lunar resources during the entire construction process of lunar research stations, and have demonstrated it from aspects such as equipment, materials, and energy requirements [1][2][3] .In recent years, with the increasing demand for lunar development and the deepening of research on lunar surface construction, low-cost construction plans mainly based on in-situ resources and supplemented by upstream resources have gradually become a consensus.Major aerospace countries have proposed plans for lunar base construction, and lunar regolith has developed from covering soil to various application forms such as lunar regolith blocks, lunar regolith bags, and in-situ hardened sites.The structural materials for lunar surface construction have entered a rapid development period [4- 7] .

Research progress on lunar regolith simulant
The development of in-situ lunar resource construction requires first addressing the demand for raw materials.From the US Soviet lunar exploration to the Chang'e program, researchers have established a systematic understanding of the material properties and engineering performance of lunar regolith through in-situ testing, lunar samples, and remote sensing data, including internal friction angle, cohesion, elemental composition, mineral composition, particle size distribution, low magnification morphology characteristics, and structural characteristics of lunar regolith.This has led to a systematic understanding of petrography, mineralogy, lunar evolution history, lunar regolith formation, and space weathering.Based on the understanding of the characteristics of real lunar regolith, research institutions in various countries use earth materials to prepare lunar simulant.At present, more than 60 kinds of lunar simulant have been developed, including MLS-1, JSC-1 in the United States, FJS-1 in Japan, and CAS-1, NAO-1 in the Chinese Academy of Sciences, CUG-1 in the China University of Geosciences, TJ-1 in Tongji University, JLU in Jilin University, QH-E in Tsinghua University The NEU-1 of Northeastern University and CUMT-1 of China University of Mining and Technology are shown in Table 1 [2][3][4] .We independently developed the concrete slurry of "lunar simulant+viscous silicate+additive" and prepared lunar simulant bricks with C30 strength, meeting the performance requirements of lunar protective structures and other related materials.

Figure1. 3D printing of arched lunar regolith brick
Bonding and solidification use bonding materials to bond lunar regolith particles to form strength.Cementitious materials mainly include polymer materials, sulfur, biomaterials, etc.The strength of components made by these methods varies with the type and content of additives.

Bonding+sintering solidification
Bonding+sintering solidification: Firstly, the lunar regolith particles are preliminarily formed using bonding materials, heated for solidification, and finally subjected to high-temperature sintering for final formation.The component strength obtained by this method is very high, but the proportion of additives is high, and the process flow is complex.In this type of method, the process of using photosensitive resin for photocuring forming has obvious advantages in forming high-precision and high-strength parts.

Sintering and melting solidification
Sintering and melting solidification are the processes of heating up the lunar regolith to a local component or overall melting state, and the lunar regolith particles form metallurgical connections.
Sintering technology includes direct heating sintering, microwave sintering, etc. Melting technology includes solar melting, laser melting, and complete melting.This method generally does not use additives and relies entirely on the metallurgical bonding between lunar regolith particles to form strength.Figure2.Laser sintering formed sample of Lunar regolith simulant and its microstructure

Conclusion
Overall, the forming technology of lunar regolith components is not yet mature, mainly manifested in the following aspects: (1) When existing studies mention the properties of lunar regolith, they generally only consider the composition, and some studies change the particle size distribution of lunar regolith by selecting fine powder through screening, without considering the influence of other characteristics such as particle shape on the formation.
(2) Except for sintering and melting solidification, other forming processes require a high content of additives and are not suitable for large-scale construction.
(3) The forming dimensions of components are small, mostly in the order of a few millimeters to tens of millimeters.For processes that require heating and pressurization of components, the performance of large-sized components significantly decreases due to uneven heating/cooling and insufficient pressurization.
In summary, the research on the formation and solidification of lunar regolith is still in the comprehensive exploration stage.China has accumulated a good research foundation in lunar simulant, component forming, and performance evaluation, but there are still a series of bottleneck problems that need to be solved urgently.Lunar regolith is a widely distributed in-situ resource on the lunar surface, and the development of high-performance lunar regolith solidification forming process with high utilization efficiency of plateau resources is an inevitable trend for in-situ construction of the lunar base.

Table 1 .
Lunar simulant both at home and abroad reactions, and autoclaved hydrothermal reactions.This method requires the addition of a certain amount of curing agent, with a forming temperature generally not exceeding 200℃.The compressive strength of lunar regolith components is mostly in the tens of megapascals, and increases with the increase of curing agent content.

Table 2 .
Representative Research Achievements in Chemical Reaction Solidification

Table 3 .
Representative research results of bonding+sintering solidification

Table 4 .
Representative Research Results on Sintering or Melting SolidificationBeijing Satellite Manufacturing Factory Co., Ltd.adopts a "KUKA robotic arm+self-made powder feeding and laser head" experimental device, and uses laser sintering forming technology to manufacture highdensity lunar simulant cube and frame structures.The microstructure of the components is shown in the figure.