When building a shelter on Mars, the best way is going to be using materials that can be found on location. This is because building materials would vastly increase the payload the rocket would need to carry and the cost of getting it to Mars.
For NASA's 3D-printed Mars habitat challenge, contestants came up with hypothetical solutions. But a team led by associate professor Gianluca Cusatis at Northwestern University's McCormick School of Engineering decided to come up with something a little more practical. They actually experimented and created a type of concrete made from Martian materials.
Like Earth concrete, the Martian version is made from a gravel aggregate and a binding agent. However, rather than the gravel, cement and water used on Earth, the team used Martian soil as simulated by NASA as an aggregate, and molten sulphur, which exists in abundance on Mars, as a binding agent.
There is a precedent for this. In the '90s, concrete made from molten sulphur was proposed for construction of concrete on the moon, and it has been used on Earth in recent decades in limited applications.
Sulphur concrete has some significant advantages. It only takes 2 to 3 hours to fully set, unlike Earth concrete, which takes 28 hours, which means it could work very well with construction-sized 3D printers. It's also highly corrosion-resistant, and the material developed by Cusatis' team was more than twice as strong as typical sulphur concretes, which the team attributed to the fine particles of the Martian soil. When adjusted for Martian gravity, it would be as strong as concrete used for Earth skyscrapers.
"Typical sulphur concrete uses sand, which is inert. It's just filler," Cusatis said. "In our Martian concrete, the sulphur is not just glue. It reacts with the minerals in the Martian soil. That completely changes the picture."
But sulphur concrete is not without limitation, and there's a reason it's not used more widely on Earth. Sulphur has a melting point of 115.2 degrees Celsius (239.4 degrees Fahrenheit), so it's highly susceptible to heat and fire. This, Cusatis said, would not be a concern initially.
"You want buildings to be fire resistant, so that could be a vulnerability on Mars," he explained. "But for the first settlements, fire won't be the problem. The problems will be having secure shelters and durable buildings that can survive meteorite impacts."
The team will, however, be looking into ways to fire-proof the concrete, as well as trying to find out more about the mysterious chemical reaction that reinforces the strength of the material.
The paper has been submitted to the journal Construction and Building Materials, and is available to read on arXiv.