Sustained space exploration will require infrastructure that currently does not exist: buildings, housing, rocket landing pads.
So where do you turn for building materials when they’re too big to fit in your carry-on and there’s no Home Depot in the space?
“If we’re going to live and work on another planet like Mars or the Moon, we have to make concrete. But we can’t take bags of concrete with us, we have to use local resources,” said Norman Wagner, Chair Unidel Robert L. Pigford of Chemical and Biomolecular Engineering at the University of Delaware.
Researchers are exploring ways to use clay-like topsoil materials from the moon or Mars as a base for extraterrestrial cement. Success will require a binder to glue the alien starting materials together through chemistry. One of the requirements for this unusual construction material is that it must be durable enough for the vertical launch pads needed to protect man-made rockets from whirling rocks, dust and other debris during take-off or landing. Most conventional building materials, such as ordinary cement, are not suitable for spatial conditions.
Wagner and his colleagues at UD are working on this problem and have succeeded in converting simulated lunar and Martian soils into geopolymer cement, which is considered a good substitute for conventional cement. The research team also created a framework to compare different types of geopolymer cements and their characteristics and reported the results in Advances in space research. The work was highlighted recently in Advances in Engineering.
Geopolymers are inorganic polymers formed from aluminosilicate minerals found in common clays everywhere from Newark, White Clay Creek to Delaware to Africa. When mixed with a high pH solvent, such as sodium silicate, the clay can be dissolved, releasing the aluminum and silicon within to react with other materials and form new structures, like cement.
Soils on the Moon and Mars also contain common clays.
This led Maria Katzarova, a former associate scientist and member of Wagner’s lab at UD, to wonder if it was possible to activate simulated lunar and Martian soils to become concrete-like building materials using the geopolymer chemistry. She pitched the idea to NASA and secured funding through the Delaware Space Grant Consortium to try with the help and expertise of Jennifer Mills, then a PhD student at UD, who studied Earth geopolymers for her thesis. of doctorate. The researchers systematically prepared geopolymer binders from a variety of known simulated soils in the exact same way and compared the performance of the materials, which had never been done before.
“It’s not a trivial thing. You can’t just say give me some old clay, and I’ll make it work. There are parameters, chemistry that you have to worry about,” said Wagner.
The researchers mixed various simulated soils with sodium silicate, then poured the geopolymer mixture into ice-cube molds and waited for the reaction to occur. After seven days, they measured the size and weight of each cube, then ground it to understand how the material behaves under load. Specifically, they wanted to know if slight differences in chemistry between the simulated soils affected the strength of the material.
“When a rocket lifts off, there’s a lot of weight pressing down on the landing strip and the concrete has to hold, so the compressive strength of the material becomes an important measure,” Wagner said. “At least on Earth, we were able to make materials in small cubes that had the compressive strength to do the job.”
The researchers also calculated how much ground material the astronauts would need to take with them to build a landing strip on the surface of the moon or Mars. The estimated amount turns out to be well within the payload range of a rocket, ranging from hundreds to thousands of kilograms.
Simulation of spatial conditions
The research team also subjected the samples to different environments found in space, including vacuum and low and high temperatures. What they found was instructive.
Under vacuum, some of the material samples formed cement, while others were only partially successful. However, overall, the compressive strength of geopolymer cement decreased under vacuum, compared to geopolymer cubes hardened at room temperature and pressure. This raises new considerations depending on the destination of the material.
“There’s going to be a trade-off between whether we have 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 doctorate in chemical engineering at UD in May 2022 and now works at Dow Chemical Company.
Meanwhile, at low temperatures of around -80 degrees Celsius, the geopolymer materials did not react at all.
“This tells us that we might need to use some sort of accelerator 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 trigger the reaction for certain applications or certain environments.”
At high temperatures, around 600 degrees Celsius, the researchers found that each moon-like sample grew stronger. This was not surprising, Mills said, given how the kinetics were hampered at low temperatures. The research team also found changes in the physical nature of geopolymer cement when heated.
“The geopolymer bricks became much more brittle when we heated them, shattering instead of compressing or breaking in half,” Mills said. “That could be important if the material is going to be subjected to any kind of external pressure.”
Based on their findings, the researchers said chemical composition and particle size can play an important role in the strength of materials. For example, smaller particles increase the available surface area, which makes them easier to react and potentially leads to greater overall strength of the material. Another possible factor is the amount of aluminosilicate contained in the starting materials, which can be difficult to estimate when the added solutions may also contain small concentrations of these materials and contribute to the performance of the materials.
What does all this mean?
Well, Amazon doesn’t offer two-day delivery to space, so design the right formulation of starting materials to take matters into your own hands. It’s also important to understand what affects the strength of materials, as astronauts will source topsoil materials from different places on planets – and possibly even different planets.
These results can also be used to make geopolymer cements on Earth that are better for the environment and can come from a wider variety of local materials. Geopolymer cements also require less water than is needed to make traditional cement because the water itself is not consumed in the reaction. Instead, the water can be harvested and reused, a plus in water-limited environments, from arid landscapes to outer space.
Now two of Wagner’s current graduate students are exploring ways to use geopolymer cements to 3D print houses and activate geopolymer materials using microwave technology. The work is a collaborative project, funded by the National Science Foundation, with researchers from Northeastern and Georgetown universities. Similar to the microwaves you use to heat up your morning coffee, microwave heating can speed up the curing of geopolymers and may one day provide a way for earth builders – or astronauts – to harden geopolymer concrete in a targeted way. .