Pozzolan is the 'secret ingredient' in Roman cement. It turns out they knew and used this (according to the source) in the construction of dams in the 1930s. Roman cement is also known as hydraulic cement.
https://duckduckgo.com/?q=pozzolan&t=ffsb&ia=web
The largest unreinforced dome in the world is
still the Pantheon.
Even with spacecrete process a box-like shoe on the end of an arm could replace all the hand labor It could have vibrating finger built right into it at the trailing edge of the shoe.
In the video at 36:00 he talks about welded tuff, which is a rock that that is form from ash that precipitates out of the atmosphere while it is still white hot. This suggests that a 3D printer head that is an oxy-acetelyne torch that is fed powdered ash. It is really hard stuff.