Futurology

Yup, it needs to dry fast enough for the next layer, but not so fast it makes a clog, and it needs to flow in just the right way you can predictably extrude a layer on. And be cheap-ish.

Of course, you still have to add a lot of non-structural components, like the other user said, and if it needs to take significant loads you also need to add reinforcement somehow, since concrete is weak under tension.