What exactly have you figured out about this process? A 3D printer for concrete isn't a very complex machine; you need a source of liquid concrete, a pump that can push it through some tubing through a nozzle, and a way to move the nozzle in 3 directions as programmed. If you've got a pump like that, all you need is to be able to turn it on or off. The amount of concrete it will dispense will depend on the pump and the tubing; it should come out at a reasonably constant rate. If you haven't calculated it, you can measure it after constructing your machine. Ideally, your printer would be programmed to turn off the pump when it comes to a stopping point as specified in the toolpath.

Andrew Werby
www.computersculpture.com

I don't know of any pre-made "extruders" for concrete - I doubt they even exist, although I suppose you could invent one. Just be sure to clean it out thoroughly after using it.