By Kevin Wilcox
Working with a substance that simulates Martian soil, a team at Northwestern University develops a strong sulfur concrete with intriguing possibilities for construction on two worlds.
In an early mission to Mars, the National Aeronautics and Space Administration analyzed the planet’s soil and established its general mineral profile so that research could be conducted on using the regolith as a building material. Timothy Parker, JPL/NASA
January 19, 2016—Civil engineers know the problem well: a project slows down as the team anxiously awaits delivery of a critical steel I beam or precast-concrete girder. But what if that girder was about 38 million mi away from the construction site, delayed by inclement weather on another planet? That's the challenge engineers will face in attempting to construct any kind of permanent structure on Mars with materials from Earth.
The better alternative, of course, is to use the raw materials found on the Red Planet. Toward that end, a research team at Northwestern University focusing on the dry, barren surface of Mars recently made some startling discoveries while examining the feasibility of creating concrete using only the available sulfur as a binder and Martian soil as an aggregate. Not only is such a concrete feasible, it has advantages for construction on both planets.
"Sulfur concrete, per se, is an old concept," says Gianluca Cusatis, Ph.D., M.ASCE, an associate professor in the Department of Civil and Environmental Engineering at Northwestern and the lead researcher on the team. "Sulfur is a by-product of different industrial productions. And so we have here on Earth a number of different products—including one commercial product from Shell Oil—where they take sulfur, melt it, mix it with gravel or sand, and cool it down to make a material."
Sulfur concrete on Earth is primarily used in such applications as sewer lines, in which it has the special advantage of being impervious to chemical reactions. It is typically mixed as 20 percent sulfur and 80 percent aggregate. Any more or less sulfur results in a weaker product.
But there is a significant weakness to traditional sulfur concrete: sulfur melts at approximately 130°C. This research points to the potential to overcome this problem through chemistry.
Sulfur concrete made with sand from Earth has aggregate particles measuring roughly 10 mm, which affects its fire resistance. Courtesy of Gianluca Cusatis/Northwestern University
The university research into how sulfur concrete might be developed for Martian applications was sparked by the interest of Lin Wan, Ph.D., then a research graduate student advised by Cusatis. When Wan approached him about the project, Cusatis remembered earlier research into sulfur concrete for construction of a lunar base. For that application, sulfur has a peculiar drawback.
"In the almost vacuum condition of the moon, the sulfur disappears," Cusatis says. "In the earlier work they showed that when they put [the] material in a vacuum, after a few months—or even earlier—the material is not there anymore because all the sulfur would disappear." The process is known as sublimation, but because the atmospheric pressure on Mars is more similar to Earth, the researchers concluded that sulfur concrete would not be subject to sublimation on Mars.
The researchers purchased a commercially available product that simulates the soil on Mars and is based on a soil analysis conducted by the National Aeronautics and Space Administration (NASA) during an early mission to the Red Planet. The substance mimics the general mineral profile of Martian soil in areas tested by NASA.
The first attempt to make a Martian concrete used the 20 percent/80 percent aggregate sulfur formula that is common on Earth and had been researched for lunar soil applications. That yielded a weak concrete. Wan then conducted a parametric study, testing various ratios, to reveal that the ideal mix of Martian soil and sulfur is actually 50-50; any alteration in the formula produced a weak material. The team used conventional compression tests, fracture tests, and splitting tests on the samples.
"What was surprising, most of all, was the high-strength that we saw. I did not expect that," Cusatis recalls. "I would have been satisfied to have the same strength as regular sulfur concrete, or even lower, to be honest. But the fact is, we got about twice the strength of regular sulfur concrete."
Sulfur concrete made with the Martian simulant features aggregates measuring just 2 to 3 mm, improving its fire resistance. Additionally, the sulfur engages in a chemical reaction with the minerals that helps bind the material together quickly. Courtesy of Gianluca Cusatis/Northwestern University
In fact, the material is even strong than concrete made with cement, he says. "To us that was the most exciting thing we found."
The team then began research into what was driving this unexpected strength. The particle size of the aggregate was one contributing factor. The particles in the Martian concrete were typically 2 to 3 mm compared to aggregate particles about 10 mm in conventional concrete. But that alone didn't account for most of the strength they observed.
"What we realized in the end was that there is a chemical reaction occurring between the sulfur and the minerals in the Martian soil. So that is the main difference," Cusatis says. "Typically, for sulfur concrete, sand is just a filler. It does not react with the sulfur. The sulfur serves as the glue, but there is no reaction.
"In regular concrete you have cement, water, and the aggregate," he adds. "Cement and water react to form the glue. The sand does not react with the paste. So we assumed that would be the case also with Martian soil. But it was not the case. There are chemical reactions going on."
These chemical reactions hold intriguing possibilities for sulfur concrete on earth. As Cusatis explains, sulfur is an unwanted byproduct of many industrial processes and if that could then be used in a beneficial way, the carbon footprint of any project that involves those processes would be significantly reduced. So the next step is to identify the chemical reactions at work and to determine if they can be further advanced to not only improve strength, but fire resistance as well.
The fire resistance of Martian concrete won't be as important an issue on the Red Plant in early building efforts, for which some are examining the concept of using ice for construction. There, solar panels could be used to produce the heat required to melt sulfur.
"We're also looking into is the possibility of using this material for 3-D printing on Mars," Cusatis says. "There is a lot of interest around 3-D printing ...in civil engineering and construction." But the challenge to 3-D printing concrete is the curing time, he explains. "It's not something that you could do in a few hours."
The sulfur concrete that his team is working on is actually quick to harden, however. "The only thing it needs to do is cool down to a temperature at which sulfur is a solid," Cusatis says.
This opens up the intriguing possibility of sending a crew of robots equipped with 3-D printers to Mars before humans arrive on the planet. "There is a lot of research that needs to be done to get there, but as a vision you could think of having robots ...build shelters before the humans get there," Cusatis says.
There are many questions to answer before then, including whether Martian soil is homogeneous or varies. After all, there is not just one soil on Earth As Cusatis explains, but many and varied types, all with unique properties. "To what extent that is the situation on Mars, or not, and how this technology would work with soils different from what we worked on, we don't know," he says.
Still, Cusatis marvels at the many new research questions inspired by Wan's interest in Mars and the work that followed. "All of this came out from the excellence of our students," he says. "One of the exciting things about this project is that definitely, we found many other questions to answer. So there are a lot of opportunities to do research to push this forward. By no means have we solved all the problems."