The European Space Agency and London-based architecture firm Foster are exploring the possibility of constructing buildings on the Moon that are 3D printed using lunar soil and inflatable domes. While the soil itself wouldn't be durable enough to be used in structures, scientists have mixed simulated lunar soil with magnesium oxide to produce a more robust solution.
The structures themselves would first be inflated using a type of fabric to create a dome-shaped building. Then, the modified soil would be laid on top of the dome to make the building more structurally sound. Plus, that added layer would protect inhabitants from meteorites, gamma rays, and extreme temperatures.
The ESA has already created a 1.5-ton block of simulated lunar soil using 3D printing methods, and it's made from a mixture of terrestrial basaltic rock, magnesium oxide, and a "binding salt." Of course, the extreme temperatures and the zero-gravity state on the Moon could affect the chemical reactions with the modified soil, but testing out the printer in a vacuum proved that the concept could easily work.
Proposed locations for where lunar bases would be built are on either poles of the Moon, where temperatures are more accurate to Earth's climate. It's not exactly set on when the bases on the Moon will begin construction, but numerous companies are already lining up to help out with the massive undertaking, and Deep Space Industries (DSI) is already planning to have a capable 3D printer built by 2020.