The European Space Agency is contemplating 3D printed moon-bases:
By using the Moon's loose rocks (regolith) as a base for concrete, robots based on Monolite's D-Shape 3-D printer will be able to build up a structure that uses as many local materials as possible. The idea is that with a shell made of moon rocks to act as a shield against micro-meteors and similar hazards, the living quarters for moon colonists could be inflatable envelopes protected by these shells.
3-D printing concrete in a vacuum is very, very different from printing it on earth. The teams have been experimenting with simulated moon rock material in vacuum chambers to find methods of construction that work. The problem being that concrete relies on applying liquids and unprotected liquids boil away when there's no atmosphere. They discovered that by inserting the 3-D printer's nozzle underneath the regolith, capillary forces kept enough liquid in place long enough to set properly.
This is also the premise of a novella I'm writing for Neal Stephenson/Arizona State University's Heiroglyphyics project. Nice to see reality clipping along!
3-D Printed Buildings Coming Soon to a Moon Near You [Tim Maly/Wired]