Nanomaterials on Earth and Beyond Series
Concrete is one of the most ubiquitous materials on earth and holds the promise to be a cornerstone for our expansion beyond. More than 10 billion tons of it are produced every year for everything from major infrastructure projects like bridges, tunnels, dams, and entire under-city sewage and utility structures to homes, stadiums, and skyscrapers. It is a mixture of cement, sand coarse aggregate and water. The mechanical property of concrete arises from a phenomenon that occurs at the micro and nano scale i.e. interlinking of minute crystal needles of calcium silicate hydrates during the hardening process (17). Nanoscale binders can modify the structure of concrete material and improve its properties including bulk density, mechanical performance, volume stability, durability and sustainability of concrete (17).
Traditional concrete has nanoscale pores in them. Adding silicon dioxide nanoparticles and filling the pores makes the concrete harder and denser, resulting in a new material sometimes called “nanocrete”. Nano-silica also improves strength, resistance to water penetration, and helps control calcium leaching (18). Steel fibers added to concrete improves their tensile strength; and the concrete attains a steel-like compressive strength of over 200 N/mm2 (for comparison standard concrete blocks have compressive strength < 10N/mm2). Polymer additives such as artificial resins help liquefy and stabilize the cement suspension, generally used to develop self-compacting concrete (17). These concretes are referred to as Ultra high performance/ high-strength concrete and have found applications in lightweight and delicate constructions such as bridges. An example is the Gärtnerplatzbrücke (19) over the Fulda River in Kassel (Germany), the first larger bridge in Germany (2007) to use ultra high performance concrete for the prefabricated elements.
Small amounts (1% by weight) of CNT’s added to portland cement phase and water can improve the mechanical properties of cement. The high defect concentration on the surface of oxidized multi-walled Nanotubes (MWNT’s) lead to a better linkage between the nano structures and binder, showing an improvement in strength of over 5 times when the nanocrete is compressed and over 3 times in its ability to withstand deformation, compared to the samples without the reinforcement (9).
One barrier to space colonization may be the cost of shipping materials for construction in space. It is estimated that the minimum cost to carry a pound of material to moon is ~$50,000 (20). Nanotechnology materials used to create earth-based Ultra high performance/ high-strength nanocrete might be a solution here, as they can be combined with space-based raw materials for construction beyond earth. NASA scientists have developed a new type of lunar concrete by mixing carbon nanotubes with moon dust and epoxies. As a first demonstration they built a 12-inch-wide (30-centimer-wide) bowl-shaped object that could be coated with aluminum and transformed into a mirror (21). Once the material has undergone reliability studies and shows that the epoxies can withstand the radiations in space, the ubiquitous supply of moon dust, low weight of carbon nanotubes, and epoxies will pave the way to rapid construction growth on the lunar surface.