Concrete in SPACE!

Concrete is awesome stuff.  If there was one thing I wish I had studied more as an undergraduate, it was concrete. Now concrete is used everywhere here on earth, but is it a suitable material for construction on the Moon? A relatively new study looks at that possibility and that paper is summarized in the New Scientist. http://www.newscientist.com/article/dn14977-astronauts-could-mix-diy-concrete-for-cheap-moon-base.html

Back when I was an undergraduate at the South Dakota School of Mines & Technology, concrete was a big deal. The Civil Engineering department students made a canoe out of concrete and raced it in a national championship – winning more than one of the years I was there. We even proposed a project to NASA’s Reduced Gravity Student Flight Opportunities Program (RGSFOP) where students would fly in the Vomet comet KC-135 for the purpose of mixing high strength, fast cure epoxy to determine whether the concrete mixture would have a different chemistry while curing in Zero-G. But it was always clear to me that Concrete was a somewhat ambiguous substance to most people, subject to misunderstanding and generally eschewed.

The first major misconception is the difference between concrete and cement. The terms are generally used synonymously by the public, but there is a major difference. Concrete is loosely defined as a mixture of cement, aggregate, and sometimes, water that are mixed to form a hard, strong building material. The official civil engineering definition (found on CivilEngineeringterms.com, http://www.civilengineeringterms.com/civil-engg-construction-and-graphics/definition-of-concrete-concrete-history-and-strength-concrete-popularity/) is a little too strict, mentioning specific types of cement, including Portland Cement, the most commonly used cement. If that was, in fact, the case, the Concrete Canoe competition would not be possible as the canoes are built from epoxy as the cement and styrofoam pellets as the aggregate. So, by the loose term we could also consider jello with fruit cocktail as a very weak, tasty concrete.

The other major misconception is that concrete is not a sophisticated material. While in its simplest form, concrete was a technology accessible to ancient civilizations – I remember visiting Egypt and all the buildings on the outskirts of Cairo were poured concrete with steel rebar sticking from the roof (the buildings were unfinished for tax purposes). Heck, even I am excited about someday building my own concrete counters into my house – and if I can do it… Modern day concretes are stronger in compression than, I think, any other building material, and creative designs allow for concrete to be used in long spans and towering structures alike. The concrete canoe floats – how is that not sophisticated?

So that being said, is concrete a suitable building material on the moon? Well there are some plusses and some minuses.

The Plusses

Concrete is only really strong in compression. Thanks to modern day building designs, spans can be built by steel reinforcement that brings a long span normally in tension to compression. The significantly reduced gravity of the moon means that concrete can cover much larger open distances with reinforcement than on earth. Since the material gains strength by adding thickness, extra thick walls are also consistent with the need to add particle shielding for any human habitat on or near the surface.

Finally, there is some interesting ideas about how raw materials found on the moon’s surface could be manipulated to make concrete using the loose description we talked about above. In this paper, elemental sulfur could be used as the cement to bind raw aggregate regolith from the moon’s surface to make a kind of concrete. The process is simple, heat up sulfide rich rocks to extract the raw sulfur. Melt the sulfur at it’s low melting point (somewhat above 113 degrees C) and mix it with the aggregate. This mixture of sulfur and moon rock would behave something like asphalt at temperatures that are comfortable to people.

It seems possible that this could be done and at a large cost savings compared to bringing in steel or other prefab modules. If all the materials are present, the only thing an astronaut construction crew needs to do is generate energy (easily done from harvesting solar energy or nuclear reactors) and bring the manufacturing equipment to do this work. Scouting out suitable locations that have the correct dirt is also important, but feasible even today. After some period of time, the cost of building from processed materials onsite can be much less than the cost of shipping in every pound of building materials.

The Minuses

How feasible is bringing the infrastructure to do this kind of work on the Moon? It took twice as long and a hundred times more than anticipated to build the International Space Station. The equipment to create concrete on the moon must be even more sophisticated and reliable than anything we have sent to space to date. At this point, the space industry has expertise in building modules that fit together. I might agree that a new technique would be required to have real outposts on the Moon, but I think that many of the first outposts will focus on the task oriented prefabbed habitats that are similar to the ISS.

How does concrete behave in a vacuum in reduced gravity? This is a question that I do not think has been answered. While I think the paper that this story is based on was well done, I suspect that it was not done in temperatures or the kind of vacuum that exists on the moon. I am certain it was not done under reduced gravity conditions that could have a significant effect on the strength of the material. Even the simplest idea of how do you handle an asphalt-like substance on the moon makes me wonder if it is possible?

I think this is a fantastically interesting topic. If travel to the moon becomes routine within my lifetime, I hope to hear more about building with concrete.