A new proposal for 3D printing lunar habitats has been unveiled by Tomas Rousek, Katarina Eriksson and Dr. Ondrej Doule of the International Space University, and this one looks like it just might work.
Previous proposals involved shipping 3D supplies from Mother Earth, but that obviously requires more energy and expense. The SinterHab proposal uses freely available sunlight to produce electricity that then is converted into microwaves that melt lunar soil.
Lunar soil is rich in nano-sized iron particles, which melt at high temperatures and form the soil into a durable ceramic-like substance. Once you can do that, it's only a matter of designing combinations of structural elements and assembling them into a functioning habitat - which the "space architects" have already done.
Their concept is rigid sintered structures enclosing "deployable membranes" that hold in air and moisture. The rigid structure not only holds the membrane, but also provides protection against micrometeorites and solar or cosmic radiation.
The process of building is accomplished by a "Sinterator", which appears to be a mobile robot equipped with microwave apparatus.
Will this become a reality? Maybe not, but perhaps some variation of this proposal will once we return the moon in coming years.