Well, now that's what I call progress! I wouldn't be too worried about the walls being hollow. In some of the earlier posts it seems he's got a technique of placing rebar between the layers bridging the gap in the wall which would make it pretty sturdy even when hollow. Even filling the shell with regular concrete isn't that big a problem, since well, the printer itself could handle that if needed, just using a more fluid mix and higher pump speed.

What I'm still curious about (and am curious about with all concrete printers, really) is how he plans to put a roof on it.