Martian concrete could be tough stuff

It remains to be seen whether human settlements on Mars will ever be feasible, or whether they will forever stay in the realm of dreams and science fiction. If Martian cities are to become reality, though, an important first question is what they’ll be made of. Ferrying building materials all the way from Earth would be extremely expensive. Most likely, at least some of the components of future Martian structures would have to be sourced locally.

Concrete—in use since Roman times and still the most prevalent construction material on Earth—offers an appealing way to transform loose sand and gravel into a sturdy solid. And researchers at Northwestern University are exploring what concrete might look like if made out of the Martian regolith.

In its most common terrestrial formulation, concrete is held together with a binder made from cement powder and water. But cement is made mostly from limestone—a mineral not present on Mars—and Martian water is rare and precious. In 2016 Gianluca Cusatis and colleagues turned their attention to a different form of concrete bound together by sulfur, of which Mars has plenty.

Recipes for sulfur concrete date back to the early 20th century, and the material has been produced in moderate quantities on Earth, mostly as a way of getting rid of the excess sulfur left over from oil refining. But mixing up Martian concrete will take more than choosing the right binder—it’s also necessary to check that the physical and chemical interactions between the binder and the Martian regolith endow the concrete with the right material properties.

No samples of regolith—or anything else—have ever been brought from Mars back to Earth. Fortunately, remote measurements by Mars landers have provided enough data on Martian mineralogy that it’s now possible to concoct realistic Martian soil simulants that mimic the regolith in both particle size and composition.

Using the soil simulants, Cusatis and coauthors found that the ideal Martian concrete needs about twice as much sulfur as terrestrial sulfur concrete does. And now his colleague Ange-Therese Akono has taken the investigation a step further by studying the microstructure and mechanical responses of simulated Martian concrete.

Akono made three batches of concrete—one of them shown in the figure—using three soil simulants formulated based on the regolith from different parts of Mars. To measure the material properties and failure mechanisms, she poked and scratched the concrete chunks with tiny diamond probes.

Of particular interest were her measurements of fracture toughness, or how well the concretes resisted crack propagation. One of the samples underperformed somewhat, in part because the concrete was full of pores where cracks could get started and in part because the mineral grains themselves were brittle. But the other two sailed through the test with flying colors, with fracture toughnesses even higher than that of cement-based terrestrial concrete. (A.-T. Akono, Mech. Res. Commun., 2022, doi:10.1016/j.mechrescom.2022.104013 .)