Increasing interest in space travel and extraterrestrial resources has focused attention on building habitable dwellings on our closest planetary neighbor, the Moon. Establishing a long-term human presence on the forbidding lunar landscape will require far more materials than can be transported from the Earth. Scientists and engineers are, therefore, investigating the utilization of in-situ materials for lunar construction. This project actually has a name, In-Situ Resource Utilization (ISRU), and is seen as an essential strategy to solve the logistical challenges and significant costs associated with extraterrestrial construction.
The search begins with lunar regolith, the Moon’s surface layer, which is composed of loose rock and minerals. This omnipresent resource is an obvious candidate for making cementitious building materials, and NASA has announced plans to construct lunar concrete structures on the Moon by 2034. The agency’s so-called Artemis project has narrowed its focus from over a dozen possible ingredients to five promising candidates: geopolymers, sulfa-aluminate, magnesium oxy-sulfate, and sulfur binders. Geopolymers made from lunar aluminosilicates—a common regolith compound—are viewed as particularly promising.
Northwestern University researchers are collaborating with robotics company ICON Technology, Inc. to evaluate the mineral composition of moon dust to determine optimal methods of transforming lunar soil into construction materials. The aspiration is to devise an on-site additive manufacturing technology in which excavated soil is melted to create a uniform feedstock in a 3D printing process. Although moon dust appears homogeneous, its local composition varies widely. According to Northwestern mineralogist Steven Jacobsen, “different minerals in lunar dirt melt at different rates, [and] the 3D-printing process is very sensitive to changes in mineralogy.”
Engineering faculty at the University of Delaware are exploring a process to create geopolymer concrete bricks from this feedstock. The researchers are simulating the process here on Earth using similar soils incorporating sodium silicate, casting modular units, and then subjecting them to mechanical tests. The goal is to create precast units with the ultra-high compressive strength required to withstand the extreme pressures encountered at launching pad sites. One challenge concerns the variability of success in forming cement in a vacuum and at low temperatures. To improve the process, the team is investigating potential catalysts that can kickstart the chemical reaction.
Also anticipating the viability of “lunarcrete,” Middle Tennessee State University researchers are developing techniques to create structural building components from regolith. The application-based focus recognizes that lunar construction will comprise not only dwellings but also infrastructure like roadways, landing pads, blast shields, and berms. According to the MTSU team, regolith has a smaller, more uniform particle size than the sand and aggregate components of Earth-based concrete. Additionally, Moon-based contractors will not be able to rely upon steel rebar, so other reinforcing methods will be required. The MTSU researchers have cast multiple batches of structural beams using simulated soil for testing. “I call them moon beams,” said Kelly Strong, former MTSU School of Concrete and Construction Management director. “It’s not quite the moon beams you grew up singing about, but we have moon beams in our freeze chamber in our lab.”
Scientists at the European Space Agency have created modular bricks using moon dust—or something very similar. The researchers pulverized fragments of a 4.5 billion-year-old meteorite to make a powder suitable for 3D printing. Their shape mimics the form and interlocking capabilities of LEGO bricks, demonstrating the blocks’ potential for mortarless load-bearing construction. The iconic form is sufficiently recognizable that LEGO has exhibited the bricks in some of its stores. “It's no secret that real-world scientists and engineers sometimes try out ideas with LEGO bricks,” said ESA Branding and Partnerships Head Emmet Fletcher. “ESA's space bricks are a great way to inspire young people and show them how play and the power of the imagination have an important role in space science, too."
One reason geopolymer cement is an attractive option for lunar concrete concerns its water use. Geopolymers need less water than traditional cement, and the water may be recovered and reused as it is not consumed during the chemical reaction. Nevertheless, using any volume of water on the Moon will be heavily scrutinized due to its extreme scarcity. For this reason, Louisiana State University scientists have begun developing waterless lunar cement. The researchers use a sulfuric compound, raising its temperature to the melting point to form a high-performing binder for 3D-printable concrete. Although the quantity of sulfur in lunar regolith is still unknown, early indications suggest that a higher sulfur concentration exists near the lunar poles (NASA intends to construct its first Moon base at the South Pole). Presuming there are sufficient quantities of sulfur, the waterless cement’s remaining obstacle is the amount of heat required for its processing.
Although the quest to develop lunarcrete presents significant challenges and will doubtless require time to perfect, the search is yielding benefits for both Moon- and Earth-based construction. The extreme lunar environment, which includes dramatic temperature swings, microgravity, and radiation, necessitates the utilization of only the most robust materials. Developing new building products that can withstand such a harsh environment also has advantages for our planet.
The lessons uncovered about alternative feedstocks can also present new opportunities. For example, the LSU researchers have noted that some areas in the Middle East contain high quantities of sulfur. Thus, their sulfur-based waterless concrete could be a preferred candidate for in-situ casting in this arid region. “This [technology] is great for people out there working on another planet who don’t have a lot of support,” said Philip Metzger, a physicist at the University of Central Florida. “But there are already plenty of analogues to that here on Earth.”
