Now Lunarcrete is the best material when you think about it. Its just crushed local rock really. NASA has done a lot of research into this. They are doing tests on the ISS now, and others are getting into it. This company, SQ4D, is paving the way. The process could easily be automated.
This is also a good use of mined material after generating oxygen from it.
Elon Musk, props to you on your twitter tweet about your SpaceX Starship project. That’s a big fuel tank! 9 meters across is a lot of fuel! Easy to include a 3d lunarcrete printing machine and a couple of rover-dozers to feed it. Your doing the hard part, getting it there. The printing tech is mature.