If humanity ever decides it is time to move to another planet, people will need to construct infrastructure there, in the form of buildings, housing, and rocket landing pads. This represents a considerable engineering challenge as current construction materials on Earth, such as cement, are far too heavy to include in the payload of a space rocket. And we don’t even know whether they would work under the temperature and pressure conditions experienced extraterrestrially. Future residents on other planets will most likely have to use locally available materials with which to build their infrastructure.
This has led researchers to explore ways of using clay-like topsoils from the moon and Mars as the basis for space cement. There is lots of soil available on these bodies, but this will have to be bound together to form concrete by using a binder that works under extraterrestrial conditions. One requirement for such an out-of-this-world construction material is that it must be durable enough for the vertical launch pads needed to protect man-made rockets from swirling rocks, dust and other debris during liftoff or landing. Most conventional construction materials, such as ordinary cement, are not suitable under space conditions.
“If we’re going to live and work on another planet like Mars or the Moon, we need to make concrete. But we can’t take bags of concrete with us – we need to use local resources,” said Norman Wagner of the University of Delaware.
The researchers are exploring ways to use clay-like topsoil materials from the moon or Mars as the basis for extraterrestrial cement that can be used as a binder to form concrete for building purposes. Maria Katzarova, a former associate scientist and member of Wagner’s lab at UD, wondered whether it would be possible to activate simulated moon and Martian soils to become concrete-like building materials using geopolymer chemistry.
Geopolymers are inorganic polymers formed from aluminosilicate minerals found in common clays everywhere on Earth, as well as in extraterrestrial soils such as those found on the moon and Mars. When mixed with a solvent that has a high pH, such as sodium silicate, the clay dissolves out, freeing the aluminum and silicon inside to react with other materials and act as a binding agent, in the same way that cement does on Earth.
Katzarova proposed the idea to NASA and obtained funding via the Delaware Space Grant Consortium to test out this idea, with the help and expertise of then-UD doctoral student Jennifer Mills, who studied terrestrial geopolymers for her doctoral dissertation. The researchers systematically prepared geopolymer binders from a variety of simulated lunar and Martian soils and then compared the materials’ characteristics and performance, which hadn’t been done before. Their findings were reported in the journal Advances in Space Research, earlier this year, and have also been highlighted recently in the journal Advances in Engineering.
“This is not a trivial thing. You can’t just say give me any old clay, and I’ll make it work. There are metrics to it, chemistry that you have to worry about,” Wagner said.
The researchers mixed various simulated soils with sodium silicate then cast the geopolymer mixture into ice-cube-like molds and waited for the reaction to occur. They found that three simulated lunar soil samples and one Martian soil sample were successfully converted to geopolymer binders using this method. After seven days, they measured each cube’s size and weight, then crushed it to understand how the material behaves under load. Specifically, they wanted to know if slight differences in chemistry between simulated soils affected the material’s strength.
“When a rocket takes off there’s a lot of weight pushing down on the landing pad and the concrete needs to hold, so the material’s compressive strength becomes an important metric,” said Wagner. “At least on Earth, we were able to make materials in little cubes that had the compressive strength necessary to do the job.”
The research team also subjected the samples to environmental conditions that are commonly present in space, including vacuum and low and high temperatures. They found that, in a vacuum, some of the material samples did form cement, while others were only partially successful. Overall, the geopolymer cement’s compressive strength decreased when it was formed under vacuum conditions, in comparison with the geopolymer cubes cured at room temperature and pressure.
“There’s going to be a tradeoff between whether we need to cast these materials in a pressurized environment to ensure the reaction forms the strongest material or whether we can get away with forming them under vacuum, the normal environment on the moon or Mars, and achieve something that’s good enough,” said Mills, who earned her doctoral degree in chemical engineering at UD in May 2022 and now works at Dow Chemical Company.
Temperature also played a significant role in determining the characteristics of the geopolymer cement. At low temperatures of about -80oC, the geopolymer materials didn’t react at all, and did not set.
“This tells us that we might need to use some sort of accelerant to achieve the strength we see at room temperature,” Mills said. “Maybe the geopolymer needs to be heated, or maybe we need to add something else to the mix to kickstart the reaction for certain applications or environments.”
At high temperatures of around 600oC, all the samples using moon soils were stronger than at room temperature. This was not surprising, Mills said, given that the reactions were so sluggish at low temperatures. The research team also saw changes in the physical nature of the geopolymer cement under conditions of increased temperature.
“The geopolymer bricks became much more brittle when we heated them up, shattering as opposed to becoming compressed or breaking in two,” said Mills. “That could be important if the material is going to be subjected to any type of external pressure.”
Based on their results, the researchers said that chemical composition and particle size of the soils may play an important role in the strength of these materials. For example, smaller particles increase the available surface area for chemical reactions, making the soils easier to react and potentially leading to greater overall material strength.
Two of Wagner’s current graduate students are also exploring ways to use geopolymer cements to 3D-print houses and to activate geopolymer materials using microwave technology. The work is a collaborative project, funded by the National Science Foundation, with researchers at Northeastern and Georgetown Universities. Similar to the microwaves you use to reheat your morning coffee, microwave heating can accelerate geopolymer curing and may one day provide a way for terrestrial builders – or astronauts – to cure geopolymer concrete in a targeted way.