The Planetary Science Institute’s Associate Research Scientist Nicholas Castle is interested in questions about outer space that could lead to improvements for life on Earth. Recently, he wondered what materials would be most viable for construction on Mars.
“If we’re going to live somewhere else in the Solar System, then we can’t just import structures from Earth. Delivery of raw materials would be expensive, and wouldn’t be sustainable if a lot of people end up living there,” he said. “Ideally, we’ll build with what’s locally available.”
Most structures on Earth today are built with concrete – a mixture of 10% cement, 10% water and 80% ground up rocks. Luckily, most of concrete’s ingredients can be found almost anywhere in the Solar System, but cement, which is a calcium-silicon binder that makes concrete stick together, is a problem. On Earth, limestone is the typical source of the calcium, while clay provides silicon.
“You can get the silicon from all kinds of other places and many kinds of rock. The problem on Mars is sourcing the calcium,” Castle said. “To understand why there’s not readily available limestone on Mars, we should talk about how we get limestone on Earth.”
Generally, there are two ways nature makes calcite, the main mineral in limestone. One is the chemical weathering of silicate rocks, for example the volcanic rock basalt. Dissolved carbon dioxide in the water reacts with the rock, breaking it down and extracting calcium among other things. The dissolved calcium can react with the dissolved carbon dioxide and precipitate calcite.
This is typically how calcite is formed on other worlds, Castle said, but the largest deposits on Earth formed from shells and marine skeletons as massive limestone deposits.
“Unfortunately, there’s a lack of shelled ocean life on other worlds,” Castle said. “So, I did some digging and found different research groups that were pushing different alternatives to calcium, but they all had their challenges.”
For example, some explored alternatives such as phosphates (which are scarce on Mars) and sulfur (which is, well, stinky).
“The most viable, in my opinion, was a magnesium-silicon binder,” Castle said, which was developed by Allan Scott at the University of Canterbury, in New Zealand. Magnesium, which is commonly found in basalt, mixed with silicon, produces a binder that behaves like cement, but even stronger. “You could get a bag of the magnesium-silicon binder, knock into it with a shovel, dump that into your cement mixer, dump in your gravel and water, and end up with stuff that will pour and then harden just like concrete, which was one of the main goals of the research group at Canterbury.”
What’s more, it is four to 20 times stronger than traditional concrete.
Here’s where the story piques Castle’s interest: This technology has the potential to radically change life here on Earth.
The vast majority of built structures rely on cement, and its manufacture is responsible for 10-15% of global carbon dioxide emissions. This magnesium-based alternative, sourced from basalt, could cut these emissions.
What’s even more exciting, according to Castle, magnesium carbonate is ideal for long-term carbon sequestration. If we can work out how to efficiently manufacture it from basalt, imitating what nature does in silicate weathering but on an industrial timescale, then it accelerates our ability to draw down carbon dioxide from the atmosphere while also cutting emissions.
“One of the reasons that I do planetary research is because again and again we see how asking questions about how we might do things in space ends up solving problems we have right here on Earth. This is one of my favorite examples, but there are countless others, and I love showing them to people to demonstrate why space research is so valuable.”
