There is growing interest in the goal of sending humans to Mars. Various space agencies have begun to study the numerous problems such a mission would present, not least of which is protecting humans during the journey.
But once humans arrive on the red planet, they will require high quality buildings in which to live and work. They can take certain structures with them but this can only be a temporary solution. The first colonizers will quickly have to find a way to build structures using the planet’s own resources. But how?
Today we get an answer thanks to the work of Lin Wan and pals at Northwestern University. These guys have worked out how to make Martian concrete using materials that are widely available on Mars. And, crucially this concrete can be formed without using water, which will be a precious resource on the red planet.
The key material in a Martian construction boom will be sulphur, says the Northwestern team. The basic idea is to heat sulphur to about 240 °C so that it becomes liquid, mix it with Martian soil, which acts as an aggregate, and then let it cool. The sulphur solidifies, binding the aggregate and creating concrete. Voila—Martian concrete.
Of course, the idea of using sulphur to bind aggregates is far from new. Engineers have been experimenting with this kind of material for at least a century and initially found that sulphur-based concrete had its fair share of problems.
For a start, as sulphur cools, it solidifies to form monoclinic sulphur and then transforms into orthorhombic sulphur, the stable allotrope at lower temperatures. But it also shrinks during this process and this shrinking creates cavities and sets up stresses that severely weaken the material.
What’s more, in the 1970s, materials scientists studied the possibility of using sulphur concrete to build lunar bases on the moon. They quickly discovered that in a vacuum, sulphur sublimates—it turns from a solid directly into a gas. So any sulphur concrete on the moon would quickly disappear into the ether.
So an important question is whether sulphur concrete can be made strong enough and durable enough to be a useful on Mars.
To find out, Wan and co made some. They used simulated Martian soil consisting mainly of silicon dioxide and aluminium oxide with other components such as iron oxide, titanium dioxide, and so on. They also tested various different sizes of particles in this aggregate.
The tests were straightforward. Having mixed the aggregate with different percentages of molten sulphur and allowed the samples to cool into blocks, they measured the physical properties of the resulting materials, such as their compressive strength and failure mechanisms. They also chemically analyzed the mix and simulated its behavior.
The results make for interesting reading. It turns out that using an aggregate of smaller particles reduces the formation of voids, which significantly increases the strength of the material. “The best mix for producing Martian concrete is 50 percent sulphur and 50 percent Martian soil with maximum aggregate size of 1 mm,” they say.
And it is strong stuff, reaching a compressive strength in excess of 50 MPa, particularly if it is compressed during curing to reduce the formation of voids. This strength is also partly a result of the chemical bonds that sulphur makes with the Martian soil. By comparison, residential building standards on Earth require concrete with a compressive strength of about 20 MPa.
Wan and co also say that the atmospheric conditions on Mars are suitable for this stuff. “Both the atmospheric pressure and temperature range on Mars are adequate for hosting sulphur concrete structures,” they say.
And there’re are other advantages. Martian concrete can be recycled by heating it, so that the sulphur melts. So it can be re-used repeatedly. It is also fast-setting, relatively easy to handle and extremely cheap compared to materials brought from Earth.
That’s interesting work. It means that the first permanent structures on Mars should be straightforward to make. All we need now are a new generation of Martian architects to design buildings made of Martian concrete that will be suitable structures for humans to live and work in.
Time will be on their side. The first human missions to Mars are a good few years away yet. But it’s just possible that the first humans to live in these structures have already been born and are currently growing up in rather different buildings on Earth.
Ref: arxiv.org/abs/1512.05461: A Novel Material for In Situ Construction on Mars: Experiments and Numerical Simulations