A major problem in efforts to colonize Mars is the sheer difficulty and expense in transporting resources and materials over there in the first place. As a result, the only viable alternative is investing in self-sustaining practices and utilization of Martian natural resources.
A research team led by Gianluca Cusatis at Northwestern University’s Center for Sustainable Engineering of Geological and Infrastructure Materials (SEGIM) has recently published a paper discussing the ability to construct sulfur-based concrete structures on Mars using the planet’s own natural resources. This development represents a major step in continuing efforts to settle on the Red Planet.
While regular concrete is created from water mixed with cement and gravel, the SEGIM team has developed a form of concrete that can be created using materials found exclusively on Mars; sulfur, which is in high abundance on the planet, and Martian soil. The researchers conducted their experiments using simulated soil that has the same chemical and mineral makeup as the original.
“Knowing that Mars has long been considered a ‘sulfur-rich planet’, a new construction material composed of simulated Martian soil and molten sulfur is developed,” states the report.
Sulfur-based concrete has been used as a building material since ancient times. Some of the benefits the report notes are “improved mechanical performance,” including “high compressive & flexural strength, high durability” and “acid & salt water resistant.” Sulfur-based concrete can set “within hours instead of weeks” compared to cement and gravel concrete. Additionally, there are environmental benefits, including a smaller CO₂ footprint, the ability to be readily melted down and recast and the availability of sulfur from gasoline production, the report continues.
In recent years, sulfur-based concrete has been used in infrastructure projects in the United Arab Emirates to build a sewage pipeline and fish reef blocks. With regular concrete fish reefs, the growth and accumulation of algae takes time because of the concrete is alkaline, however, because the sulfur concrete is neutral in alkalinity, algae quickly formed on the reefs, helping the overall aquatic population.
Not only is it a suitable alternative to gravel and cement concrete, but its properties make it especially suited for extraterrestrial applications because there is no need for water to bind the concrete (molten sulfur is used as the replacement). Since the 1990s, NASA researchers have been experimenting with sulfur and soil from the moon to create “lunar concrete.” However, the near-vacuum environment and low temperature of the moon make it difficult to maintain the structure of the concrete. In general, the conditions of the moon are not suitable for human settlement compared with Mars.
“Mars … is not too hot or too cold, and has an atmosphere to protect humans from radiation,” states the report. “Its day/night rhythm is very similar to that on Earth: a Mars day is about 24 hours and 37 minutes. Thus, Mars is the most habitable planet in the solar system after Earth … Due to the dry environment on Mars, sulfur concrete concept is a superior choice for building a human village on the red planet.”
The actual process of constructing the concrete simply involves mixing molten sulfur with the aggregate simulated Martian soil. It is than cast and cooled down until it is ready for use. The remainder of the report details SEGIM’s efforts to find the best ratio of sulfur to soil and particle size in order to construct the most durable concrete, by experimenting with a number of stress tests. In the end, the researchers found the ideal combination was 50 percent of both the soil and the sulfur, with an optimal particle size for the soil no larger than one millimeter.
Cusatis describes the process of using the Martian soil. “Typical sulphur concrete uses sand, which is inert. It’s just filler. In our Martian concrete, the sulphur is not just glue. It reacts with the minerals in the Martian soil. That completely changes the picture.”
The major drawback of sulfur-based concrete, and why it is not used frequently here on Earth, is its low temperature resistance. Sulphur has a melting point of 115.2 degrees Celsius (239.4 degrees Fahrenheit). Cusatis says that this would not be a problem, at least initially, and the team will continue to research ways to fire-proof the concrete.
“You want buildings to be fire resistant, so that could be a vulnerability on Mars,” he said. “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.”