It goes without saying that any humans living for extended periods of time on Mars, the Moon, or any other far-off world would have to be entirely self-sufficient. This means they will need to have the necessary means to make their own food, fuel, and even air right then and there, on-site. It all starts with a home, though, and building a permanent settlement so far away from Earth is perhaps the most challenging task of them all.
Think about it. It costs around $2,700 per kilogram to launch stuff into space with a SpaceX Falcon, the rocket now regularly used to supply the International Space Station. That's still dirt cheap compared to the $55,000 per kilogram it used to cost in the space shuttle days. The cost of getting a kilogram of material to Mars is orders of magnitude more expensive though. It's not clear how much it costs exactly, but it cost Mars Curiosity about $2.5 billion to land an 889-kilogram rover on Mars, for a per-kilogram cost of approximately $2.78 million.
Even if you had all the money in the world, the free space for a payload on any rocket is limited. It goes without saying that we can't haul any type of construction materials to Mars or even the Moon.
Geologists and chemists at the University of Delaware have a bright idea though: use the natural resources already present at the destination.
That may sound like an obvious thing to do, but turning Martian dirt into a reliable and working construction material is anything but straightforward.
One major requirement for any off-world settlement is durability and strength. Concrete is great for this, but the recipe requires cement, which cannot be carried through space.
To solve this problem, researchers led by professor Norman Wagner of the University of Delaware turned to geopolymer chemistry. Geopolymers are inorganic polymers consisting of aluminosilicate minerals that form solid ceramic-like materials at near ambient temperatures. Here on Earth, they're typically found in clays virtually everywhere in the world.
When these geopolymers are mixed with an alkaloid solvent, such as sodium silicate, the clay is dissolved, freeing the aluminum and silicon to react with other substances and form new materials -- and this includes cement.
Soils on Mars or the Moon also contain these common clays but "this is not a trivial thing," Wagner said. "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.”
Off-world cement
For their new study, the scientists mixed simulated Martian and lunar soil with sodium silicate, then cast the liquid mixture in cube-shaped molds. After a week, the material from each cubic mold was removed, measured, and weighed, before they had their structure tested to see how much load they could withstand. The materials were also subjected to various environmental factors they would encounter in space, including vacuum, as well as high and low temperatures.
“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,” Wagner said. “At least on Earth, we were able to make materials in little cubes that had the compressive strength necessary to do the job.”
The researchers reported the successful conversion of one Martian regolith simulant and three lunar regolith simulants to geopolymer binders. The tests showed that the geopolymer cement had a poor compressive strength when formed under vacuum. Meanwhile, at temperatures of -80 degrees Celsius or less, the geopolymer didn't undergo a chemical reaction at all. This shows that astronauts would have to cast these materials in a pressurized environment and the geopolymer would have to be heated.
Understanding what affects the strength of materials sourced from other worlds is critical to maintaining a viable colony. This is why the researchers plan on improving their recipe while noting the importance of exploring the topsoil materials on Mars and the Moon in greater depth because every bit of information matters.
There are a lot of things that could go wrong, but the plan itself sounds good. Rather than packing sacks of cement to send to Mars, which would be both foolish and futile, astronauts could just take the solvents with them. The quantity they would need to build a decent-sized settlement can very well fit the payload range of a Mars-bound rocket.
And if all else fails, at least we have the chance to make better concrete here on Earth. Geopolymers need less water than traditional cement to make because the water itself is not consumed by the reaction. This means it can be recovered and reused. Researchers at the University of Delaware are already busy building 3D-printed homes using geopolymer cement, which they plan on activating using microwave technology.
The findings appeared in the journal Advances in Space Research.
