If you think building a house on Earth is expensive, try building a space station on the moon. That’s what NASA hopes to be doing in 2020, as part of its plan to return to the moon – this time with a four-astronaut team that will live on the lunar surface for seven days or longer. Beyond the physical difficulty of getting building materials to the moon, NASA estimates the cost of transporting them there will run about $50,000 per pound. Imagine the price tag, then, for taking enough water to the moon to mix concrete in the traditional way. Hence NASA’s interest in waterless concrete, a product developed by Houssam Toutanji, chair of the civil and environmental engineering department at the University of Alabama in Huntsville.
Toutanji reports the details of making waterless concrete in the Oct. 2008 edition of Civil Engineering magazine. An Oct. 17 NewScientist.com article highlights that report, explaining that moon dust is the primary aggregate and that purified sulfur, derived from lunar soil, is used as the waterless concrete’s binder.
“You want the sulfur to be in a liquid or semi-liquid form to work as a binding agent,” the article quotes Toutanji as saying. This requires heating it to a temperature between 130°C to 140°C, because the sulfur “is generally expected to melt at about 119°C and to stiffen above 148°C,” the scientist states. In another article on The Future of Things website, Toutanji reveals that waterless concrete – sometimes known as sulfur concrete – “usually contains 12 percent to 22 percent sulfur by mass and 78 percent to 88 percent aggregate by mass.” He notes that the sulfur “can contain about five percent plasticizers and the aggregate can include both coarse and fine particles.” He also says fiberglass can be mixed into the concrete as a reinforcement “to improve its tensile and flexural strength.” Fiberglass, Toutanji points out, “can be produced directly from the lunar soil or from byproducts obtained in extracting such metals as aluminum and titanium” from the moon’s surface. Once the aggregate and binder are mixed together, the UA professor says, the resulting medium is ready to pour or mold. Unlike conventional concrete, where “you have to wait seven days, in extreme cases even 28 days to get maximum strength,” Toutanji claims his waterless concrete “hardens like a rock” within an hour.
Testing process: Richard Grugel, a geological engineer at NASA’s Marshall Space Flight Center in Huntsville, has assisted Toutanji in preparing and testing the new sans-aqua product. The two men report simulating lunar soil by adding “35 grams of purified sulfur to every 100 grams of dust” and casting the resulting mix into a number of small cubes measuring about five centimeters on each side. The men say they then exposed the cubes to 50 cycles of severe temperature changes, freezing the cubes at -27°C and bringing them back to room temperature, over and over again. According to the pair, the waterless concrete was able to “withstand compressive pressures of 17 megapascals, or roughly 170 times atmospheric pressure.” Toutanji says, if the material is reinforced with silica, which can also be produced from moon dust, the compressive pressure can be raised to about 20 megapascals.
Competing formula: As it turns out, more than one scientist has a recipe for waterless concrete. NewScientist.com reports that Peter Chen of NASA’s Goddard Space Flight Center in Greenbelt, Md., has developed his own formulation utilizing epoxy as a binder. While admitting his formulation would require transporting epoxy to the moon, Chen says – once there – epoxy concrete would be easier to make than Toutanji’s sulfur-based product. He notes that, in addition to scoops and mixers, Toutanji’s concrete would also require a “power source to bake sulfur out of lunar soil and melt the concrete mixture.” Toutanji concedes that Chen’s remarks are true. He believes, however, that the energy cost would still be cheaper than transporting epoxy, although he says he hasn’t had a chance to gather supporting data yet.