1. Introduction
National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) announced that they wanted to ensure the possibility of permanent human residence in so-called habitats on the Moon or Mars before 2040
[1][2]. The first manned mission after Apollo 17, Artemis III, is scheduled to take place by 2024 to help implement sustainable lunar exploration
[3]. Human in-space missions (the Moon, Mars, etc.) will require the capability to build structures on site using the local (planet) resources as a potentially more energy-efficient and economically viable alternative to transporting all materials needed for the construction of an outpost from Earth (The Space Launch System, NASA’s new heavy-lift vehicle, delivering more than 25 tons of cargo to the moon is estimated to cost more than USD 2 billion to launch
[4]). Nowadays, one of the most promising materials for that purpose are geopolymer composites
[5].
The geopolymer cement/geopolymer concrete seems to be a reasonable solution for in-space constructions, especially lunar and Martian habitats, because of its advantages, such as
[6][7] attractive mechanical properties (compressive strength: up to 90 MPa, flexural strength: 10–15 MPa at 28 days); high early strength formulation (compressive strength: 20 MPa, flexural strength: 10 MPa after 24 h); fire- and heat-resistant possibilities to applications in different conditions because of their chemical resistance to atmospheric conditions and a variety of acids and salts; simplicity of the application; low shrinkage (<0.05%); and good adherence to such materials as concrete, steel, glass, ceramics, the effectiveness of the manufacturing process, and environmental benefits (low CO
2 emission and energy efficiency during the production process).
2. Materials Possible to Usage for Lunar and Martian Shelters
Building infrastructure on the Moon or Mars is an engineering challenge, but it is a necessary step to develop further space projects. Human in-space missions (the Moon, Mars, etc.) will require the capability to build structures on site using local (planet) resources (so-called INRU) as a potentially more economically viable alternative to transporting all materials needed for the construction of an outpost from Earth
[8]. The cost of transportation of 1 kg of material to the Moon is more than EUR 20,000
[9][10]. Transportation to other planets is even more expensive, and when scaling to account for the infrastructure needed to sustain a lunar or Martian presence the cost becomes ‘astronomical’, not to mention the space required when packing the shuttle
[11][12].
The main challenge in building on the Moon or Mars is the different conditions than in the case of Earth. There are limitations with the use of traditional terrestrial methods used for construction
[11][13]. The basic differences are: the lack of atmosphere that results in pressures near vacuum, low gravity (the Moon at about 1.6 m/s
2 and Mars at 3.721 m/s
2, respectively), high level of galactic cosmic radiation (GCR) and infrequent but very intense solar particle events (SPEs), limitation to access to liquid water, extreme thermal cycling (the Moon from −173 °C to +117 °C and Mars from −140 °C to +21 °C), higher seismic activity for both planets than for Earth, and micrometeoroids
[7][11][13][14]. The potential building materials should match these difficult conditions.
In order to explore extraterrestrial bodies, it will be necessary to develop a cement-like binder. However, the phenomenon of cement solidification in a microgravity (μg) environment is not yet well understood. Several years ago, as part of the Microgravity Investigation of Cement Solidification (MICS) project on the International Space Station (ISS), scientists conducted research on cement solidification in microgravity. During this research, for the first time in space, scientists mixed tricalcium silicate (C
3S) with water, and then made comparisons between cement samples processed on the ground and in microgravity. In their work, the researchers hypothesized that the minimization of transport phenomena (i.e., buoyancy, sedimentation, and thermosolutal convection) caused by gravity would ensure diffusion-controlled crystal growth, resulting in unique microstructures. As a result of their research, the scientists showed that the main differences in μg of hydrated C
3S paste included reduced aspect ratio of portlandite crystals and increased porosity
[15].
Currently, several attempts have been made to construct a technical infrastructure for this kind of facility, especially in the context of lunar shelters
[8][14][16]. These include traditional ordinary Portland cement (OPC)-based concrete with lunar regolith as aggregate, sulfur cement, solar-sintered regolith (basalt), Sorel cement (magnesium chloride-based binder), phosphoric acid binder types of cement, epoxy/polymer-based cement and alkali-activated regolith or ‘geopolymer’ type binders—
Table 1.
Table 1. Types of material possible to use for lunar and Martian shelters.
Concrete-like materials have the highest potential for use in extra-terrestrial construction due to their inherent mechanical properties, resilience, and durability. Therefore, construction materials that use little of those resources, such as geopolymers, while providing sufficient protection against the harsh lunar environment are of interest. Geopolymer cement should provide better radiation protection levels and stability and require significantly less resources in the production process than traditional concrete and other materials presented above
[11][23][24]. Moreover, previous works showed that a shielding thickness of 50 cm (99 g/cm
2) with geopolymer cement should be sufficient for a prolonged crewed lunar mission, with the absorbed dose for a 12-month stay being similar to the annual whole-body radiation worker limit—5 cSv, 5 rem
[25][26]. The same amount of material is sufficient according to the strength and durability requirements for the shielding properties of the geopolymer cement. In general, the geopolymer binder has the following advantages over other concreate-like materials
[11][23][24][27]:
It is also worth noticing that the geopolymerization process should be planned by two different methods using alkali and phosphatic acids. The alkali substances are available on the Moon
[24], whereas phosphoric acid and water are available on Mars
[23][28].
3. Raw Materials Used for Manufacturing Lunar and Martian Habitats
It is rather obvious that it is impossible to have access to the proper amount of materials to prepare samples and building elements from the testing lunar and Martian constructions. Because of that, the preparation of the lunar and Martian regolith simulant is an important part of each piece of research
[1]. The preparation of the simulant is not a trivial task, due not only to the possible changes in the material in different areas of the moon or Mars and limited access to this data but also because a large number of features have to be considered in this characteristic
[10][30]. The most important are: chemical and mineralogical composition (
Table 2), physical properties, mechanical properties, and morphology. However, there is a possibility to find simulants ready for lunar and Martian soil, but they do not always fulfill all requirements and have a very high price. Because of that, many authors decided to design their own compositions
[1].
Table 2. Comparison of chemical and mineralogical composition for the most popular simulants for lunar and Martian regolith.
To achieve the proper composition, researchers present different approaches. The most popular is using the ready base, such as fly ash or volcanic tuff or other raw materials, and supplementing it with proper metal oxides
[28][30][34]. Currently, the main problem with the chemical composition of regolith simulants is iron. For example, in JSC-1A the total iron content is reported as being made up of 76% Fe
2O
3 (i.e., 9.8% of the total JSC-1A mass); however, in practice, on the extraterrestrial system post this element is expected to be in the form of FeO (the iron on the moon surface is considered to be at the lower oxidation state, since oxidation takes place in the presence of moisture and oxygen, both being scarce on the moon)
[8][35].
It is worth noting that some previous investigations show that a small amount of additives can significantly improve material properties, for example, supplementing the simulant with aluminum sources could improve compressive strength by 100.8% and reduce alkali content, resulting in significantly reducing the mass of materials transported from the earth for the construction of lunar infrastructure and saving space transportation costs
[7][30].
A second important element is the mineralogical composition. The major components of lunar regolith are glasses; fragments of rocks and minerals, mainly consisting of silicates, such as olivine, pyroxene, and plagioclase; and non-silicates, such as ilmenite
[32][36][37]. This composition is also quite similar to many terrestrial volcanic ashes
[38][39]. The expected minerals in the planned composition are plagioclase and pyroxene and a small amount of ilmenite and olivine. The other research suggests that the following minerals could also appear in regolith simulants: anorthite, albite, enstatite, feldspar potassian, alkali basalt, orthopyroxene, orthoclase, wollastonite, estatite, ferrosilite magnesian, fayalite, titanomagnetite, and forsterite
[25][38][39]. In the case of the Martian regolith, the olivine and pyroxene and secondary alteration minerals are mainly expected
[33]. Moreover, a certain amount of plagioclase and minor Ti magnetite, Carich pyroxene, olivine, glassy, and ferric oxide particles can be present
[33].
The physical composition of materials will include properties, such as density, particle size, and particle size distribution. This distribution is not always relevant in commercially available simulants to previous research of lunar and Martian soils. The analysis of samples that came from the moon shows that the particle size distribution generally follows the log–normal curve with mean values in the range of 45–100 µm
[40]. The ‘Dust’ fraction of lunar regolith includes particles smaller than 10 µm, of which approximately 90% (by weight) are particles smaller than 1 µm, although some particles can even be as small as 10 nm
[40][41]. Particles with dimensions larger than 0.25 mm (250 µm) constitute only 10% (by weight) of a regolith. Meanwhile, lunar regolith simulant—JSC-1A—is a basaltic powder, JSC-1A has particles <1 mm and ranges in sizes from approximately 10 to 50 µm
[42].
The next important issue is the particle’s morphology. The lunar and Martian regolith contains morphological forms that do not exist on Earth; these are spherical lunar chondrules (with dimensions from a few microns to 0.5 mm) formed as a result of meteorite falls and the sudden melting of lunar rocks. The particles are predominantly angular with smooth facets
[1][41]. The fragmentation of the lunar regolith is the result of mechanical impacts but also thermal stress caused by high daily temperature differences and erosion caused, among others, by ionizing radiation of the solar wind and galactic cosmic radiation
[1][43]. Meanwhile, the commercially available regolith simulants have sharp shapes because they are received by crushing and milling
[1]. The granulometric composition of most simulants differs from that of the lunar regolith. They mainly correspond to coarser fractions, since obtaining very fine particles is associated with difficult and expensive technology
[1].
The design of the regolith and other features should be taken into consideration. One of them is adhesion. There is proof that lunar regolith is characterized by very strong adhesion to various surfaces. This is a negative property in the context of functioning on the surface of the Moon and can cause (together with combination with the high abrasiveness) damages to machinery mechanisms and research equipment, including optics. However, in the context of adhesion of the binder to aggregate in cementitious materials, this feature seems to be very important and desirable
[1]. Another important element to design a regolith’s simulant is a method of production that could have a significant influence on the material’s properties
[44][45][46].
Another challenge is to find more effective technologies for production in extraterrestrial conditions. Some investigations show the possibility of obtaining oxygen by reduction techniques from the regolith
[47]. The by-product of this process could be used as a raw material for shelters; however, it has been not taken into account in the current investigation, nor has composition for the regolith’s simulants.