The geopolymer cements were developed at the University of Delaware, with the aim of providing a good substitute for conventional cement.
“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 researcher Norman Wagner.
One requirement for extraterrestrial construction materials is that they must be durable enough for the vertical launch pads needed to protect manmade rockets from swirling rocks, dust and other debris during lift-off or landing. Most conventional construction materials, such as ordinary cement, are not suitable under space conditions.
First, the researchers mixed various simulated soils with sodium silicate, a solvent with a high pH. This dissolves the clay, freeing the aluminium and silicon inside to react with other materials and form new structures.
They then cast the geopolymer mixture into ice cube-like moulds and waited for the reaction to occur. After seven days, they measured each cube’s size and weight, then crushed them to understand how the materials behave 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. “On Earth, we were able to make materials in little cubes that had the compressive strength necessary to do the job.”
In space it might be a different matter, so the team subjected the samples to simulated environments, including vacuum and low and high temperatures.
Under vacuum, some of the samples formed cement, while others were only partially successful. Overall, the geopolymer cement’s compressive strength decreased under vacuum, compared to geopolymer cubes cured at room temperature and pressure.
“There's going to be a trade-off between whether we need to cast these materials in a pressurised environment to ensure the reaction forms the strongest material, or whether can we get away with forming them under vacuum, the normal environment on the Moon or Mars, and achieve something that's good enough,” said chemical engineer Jennifer Mills.
Under temperatures of about -80ºC, the geopolymer materials did not react at all.
“This tells us that we might need to use some sort of accelerant to achieve the strength we see at room temperature,” said Mills. “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 about 600ºC, the researchers found that every Moon-like sample got stronger. They also became more brittle, however.
“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 may play an important role in material strength. Smaller particles increase the available surface area, making them easier to react and potentially leading to greater overall material strength.
Two of Wagner’s graduate students are exploring ways to use geopolymer cements to 3D-print houses and to activate geopolymer materials using microwave technology. Microwave heating could accelerate geopolymer curing, providing a way for terrestrial builders — or astronauts — to cure geopolymer concrete in a targeted way.
Other projects have previously explored other ways of forming concrete for lunar bases. Researchers from Norway, Spain, the Netherlands and Italy suggested using the urea from astronaut urine as a plasticiser, making the mixture more pliable before it hardens.
The work was published in Advances in Engineering.
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