Certain arches or domes, maybe a lining for a tunnel. A tower if it’s not very windy. Really just all the stuff the Romans built, since that’s what they were working with, and their volcanic ash-based cement was somewhat weaker than modern cement.
It would be pretty hard to print between rebar. You’d need a crazy multi-axis head, and at that point it’s probably cheaper to just build a form. If they can achieve some significant strength with long fibers, which seems likely, you could spool that into the stream of concrete, but just concrete is already an actively researched problem. Printing one big form in foam or plastic and then filling it could be considered. The manual equivalent certainly makes a great building, especially in harsh climates where insulation is a concern.
Do you really think you could build a tower without tensile reinforcement? The hoop stress on the base of a cylindrical tower is no joke, especially when made from something as dense as concrete…
I will plead not a professional engineer on the one. The Tower of Pisa and it’s less leaning cousins are thing, although Wikipedia informs me they were actually medieval and made of joined masonry rather than cast concrete, despite appearances. That’s the main reason I brought it up.
