According to , Martian colonies will be ideally constructed using primarily locally sourced materials, primarily regolith. The results presented demonstrate the viability of building material composed of compressed simulated Martian soil. It was found that Martian soil simulant Mars-1a could be directly compressed at ambient temperatures into a strong solid without need of additives. This highlights a possible aspect of complete Martian in-situ resource utilisation. It was that the bonding agent was nanoparticulate iron oxide, commonly found in Martian regolith. Furthermore, compacted samples were found to have an unconfined compressive strength of around 50 MPa, roughly double that of typical concrete found on Earth. As such, gas permeability of the compacted samples was found be on the order of 10−16 m2, close to that of solid rock. The compact was found to be suitable for construction, being adaptive to additive manufacturing, where a 3D object is built by adding layer upon layer.
Wan et al. have proposed a method in which simulated Martian soil and molten sulphur were used to produce sulphur-based concrete . Sulphur is abundant on the Martian surface and Martian regolith simulant was found to have a suitable particle size distribution to ensure that the concrete has high strength. The mix for producing Martian Concrete (MC) with optimal strength was found to be 50% sulphur and 50% Martian soil simulant with a maximum aggregate size of 1 mm. Test results showed that optimal MC had an unconfined compressive strength of above 50 MPa. The developed MC was found to be feasible for construction on Mars based on availability of material, recyclability, and ability to withstand the Martian environment amongst other factors.
Concrete production was further investigated in , which examines using magnesite and other magnesium-based compounds commonly found on Mars as building materials. The study showed that magnesium oxide-regolith-based concrete had a compressive strength in excess of 20 MPa, around the lower end for lime-based concrete found on earth. The results show that Mg-based products form a viable method for construction material on Mars with little water required and minimal energy use.
3D Printed Structures
Another construction method being explored is 3D printed colony structures composed of either ice or regolith. Citing new evidence of the potential hazards of perchlorates in the Martian soil, ice-based structures serve as a radiation barrier, absorbing shorter wavelength radiation, while allowing visible light through . While the Martian surface experiences varying temperatures with daily and seasonal cycles, the average temperature is −65˚C , ensuring the structural integrity of the structure is maintained once completed.
The winner of a 2015 NASA competition to produce 3D printed Martian Habitats, Mars Ice House, experimented with scaled ice 3D printing on Earth, as well as small-scale printing robotics, and detailed a process whereby subsurface Martian ice was turned into liquid water which was then printed . The sub-zero Martian surface temperature ensures the liquid water is” instantly turned into ice ”. The plan was described as involving the use of a Mars descent vehicle, a deployable membrane, and semi-autonomous robotic printers to gather and begin deposit subsurface ice prior to arrival of human colonists. The final structure was a 92 m2 translucent habitat which allowed ambient visible light into the habitat. The Mars Ice House design plan also plans to build with regolith .
Other private entities that took part in the NASA challenge looking to build on Mar include Foster + Partners who outlined a concept for a robot-built human habitat based on the use of semi-autonomous 3D printing robots. However, the habitat is built from regolith, meaning that it serves primarily as a radiation shield in addition to providing protection from micrometeorite strikes. The next planned step is the production of construction of complete habitats .
From the above sections, it has been shown that numerous possible regolith-based construction and shielding techniques are viable. Comparing the methods, there are some key criteria which must be taken into account. Firstly, the availability of water is an issue; while large amounts of it are present on Mars, processing the ice deposits to obtain liquid water is costly and time consuming . Furthermore, mining for sulphur is also associated with similar costs, as is the process of mixing evaporant and surfactant used in 3D printing , , . The literature thus points toward compacted regolith as the most viable and low cost given that only compaction equipment is required.
Regolith-based construction techniques have potential to revolutionise the colonisation of Mars. Further studies on these techniques may provide further insight into which techniques are most feasible. The compaction method, as far as the current literature indicates, is the least complex, requiring less maintenance, and thus forms a promising avenue for further research which may extend to deployment systems and testing full scale structures. More research should be made into radiation shielding properties of compacted regolith, in particular types of radiation not discussed in this document, as well as the optimal structural thickness order to evaluate costs.