Roman concrete is pretty impressive-they made a type of concrete (pozzolana) which is tronger than modern concrete. Only, when you need a lot of it (such as pouring the dome of the Pantheon), how do you mix it? did they have an army of slaves doing this? or did they have some kind of crude mechanical concrete mixer?
Why, specifically, would you need a “big pour” other than for reasons of economy? Many very large concrete structures are a succession of small pours.
Info here on how they constructed the dome of the Pantheon. (Scroll down)