Regolith-based magnesium oxychloride composites doped by graphene: Novel high-performance building materials for lunar constructions
Graphical abstract
Introduction
In the past, material transport from Earth to the Earth orbit was extremely expensive. The typical price of the transport to the low Earth orbit (LEO) was approximately 8,000 USD/kg, while it reached approximately 45,000 USD/kg for the geosynchronous Earth orbit (GEO). In the recent years, the advances of private space agencies such as SpaceX pushed the prices dramatically down (LEO 4,000 USD/kg, GEO 13,000 USD/kg) due to the reusability of the first stages of rockets [1], [2], [3]. After the finalization of the Starship project (BFR) the transportation of large amount of material and a large count of astronauts to space for an available price will be possible [4]. It comes without saying that the Moon offers itself as the first target for extra-terrestrial colonization. After the first stage of colonization, there will be a need to build the basic lunar infrastructure rapidly, simply and safely. In order to fulfill these three conditions, it is necessary to use lunar raw materials.
The lunar landscape is quite heterogeneous. In general, the main areas are highlands and lunar maria, where the content of the lunar mineral regolith is different [5], [6]. On the other hand, the chemical composition of the surface is quite similar, as was confirmed by the analysis of samples collected during Apollo and Luna missions [7], [8], [9], [10], [11]. It is almost impossible to obtain the samples from the Moon in large scales, nevertheless, there are various regolith simulants with almost identical chemical and phase composition and also, with very similar particle size distribution [12], [13], [14]. These simulants are sufficient for the experimental work on Earth.
Magnesium oxychlorides (MOCs), which were discovered in 1867 by French engineer Stanislas Sorel, are non-hydraulic binders. Chemically they can be described as various compounds of the MgO–MgCl2–H2O system [15] differing in the stoichiometric ratio between these three constituents. The four most common phases are Phase 3 (3 Mg(OH)2.MgCl2·8H2O), Phase 5 (5 Mg(OH)2.MgCl2·8H2O), Phase 2 (2 Mg(OH)2.MgCl2·4H2O) and Phase 9 (9 Mg(OH)2.MgCl2·4H2O). Phase 3 and Phase 5 are formed at ambient temperature and have been previously described in the literature [16], [17], [18], [19]. In general, MOCs have very specific properties, which make them unique in some ways. The most interesting ones are fire resistance, low thermal conductivity, resistance to abrasion and their mechanical properties, such as compressive and flexural strength [20], [21], [22], [23], [24], [25], [26].
Despite the sufficient mechanical properties of MOC, it is possible to further improve their performance using various additives or nano-additives [27], [28]. It was previously shown that the addition of carbon-based nanomaterials significantly improves not only mechanical properties but also the lifetime of MOC based materials. Graphite oxide revealed a large influence on the flexural strength, while graphene helped to improve the compressive strength [29]. Due to the limited stability of graphite oxide or graphene oxide regarding the conditions on the lunar surface, we decided to apply graphene only [30], [31].
The main disadvantage of MOC on Earth is in the CO2-capture ability and its poor water resistance [32], [33], [34]. In the CO2 sequestration process, the secondary phases such as chlorartinite and brucite are formed depending on the amount of water present in the environment [35], [36], [37]. On the other hand, there is almost no atmosphere on the Moon. The lunar environment contains neither CO2 nor water and the pressure on the lunar surface is 0.3 nPa [38], [39]. The most abundant elements in the Moon atmosphere are noble gases. The temperatures on the surface of the Moon are between −170 °C and 130 °C depending on the location [40], [41], [42], [43]. The high-temperature stability of MOC makes them suitable for such conditions.
In our contribution we focused on the development of novel composite materials with a high amount of regolith (lunar soil) that can be used on the moon surface. Use of local raw-materials is very important in order to reduce the amount of transported raw materials from the Earth as much as possible. The developed composites can be used not only for landing paths and bases for the starting ramps but also for roads and other facilities. The regolith in the prepared composites should not react with the magnesium oxychloride matrix hence its role would be as a filler. On the other hand, addition of small amount of graphene should significantly improve mechanical properties of composites. The main aim is to develop high-performance composite material that will be stable for long-term and will contain as much lunar raw-material as possible.
Section snippets
Materials
For all experiments, the following chemicals were used: MgCl2·6H2O (>99%, Penta, Czech Republic) and MgO (>98%, Penta, Czech Republic). Deionized water (16.8 MΩ) was used for all syntheses. The graphene nanoplatelets (obtained from Alfa Aesar, Czech Republic) had a declared surface area 500 m2/g. The purity was analyzed by XRF. The results showed 99.9 wt% purity with only small traces of S, Si and Fe. The measured surface area (BET) of graphene was 672.7 m2∙g−1, which is even more than the
Results and discussion
In this contribution, we prepared regolith-based magnesium oxychloride composites doped by graphene. We used a commercial regolith simulant with similar phase and chemical composition as the ‘real’ regolith for the synthesis of composites. The prepared composites were then thermally treated (from −58 °C to 150 °C) to simulate conditions on the lunar surface. In the next step, these thermally treated samples were characterized in detail and compared to the un-treated composites. Photographs of
Conclusion
In this paper, we developed novel building materials for lunar constructions based on regolith, graphene and magnesium oxychloride cement. These materials are an alternative to regolith-based laser-sintered materials. The prepared samples were characterized in detail with the focus on the phase and chemical composition, microstructure and mechanical properties. The prepared materials were subjected to the repeated heating and cooling cycles in order to reproduce the conditions on the Moon.
CRediT authorship contribution statement
Anna-Marie Lauermannová: Writing - original draft, Data curation, Investigation. Ivana Faltysová: Data curation, Investigation. Michal Lojka: Data curation, Investigation. Filip Antončík: Data curation, Investigation. David Sedmidubský: Data curation, Investigation. Zbyšek Pavlík: Writing - original draft, Supervision, Methodology. Milena Pavlíková: Writing - original draft, Supervision, Investigation. Martina Záleská: Data curation, Investigation. Adam Pivák: Data curation, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Czech Science Foundation, grant number 20-01866S. This work was also supported from the grant of Specific university research – grant MSMT no. 20-SVV/2020. Authors also would like to thank Melissa Roth and Vince Roux from Off Planet Research, LLC for help with the selection of the regolith simulant.
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