Cement Trucks and Newton's Laws

Think about a cement truck. The part that actually holds the cement is egg-shaped, right? The pointy side of the egg is up, and the fat part is down. If you're having trouble imagining this, here:

That part of the truck is called the mixing drum.

Okay, we all know, since we asked a lot of annoying questions when we were children, that the cement is in the drum, and the drum spins to mix the concrete. But here's the puzzler: The spout of the drum is the highest part. So how does the concrete get from down in the bottom of the drum and out of the spout at the top?

I figure, on the inside of the drum, there's a variation of an Archimedes Screw. What's that, you ask? Essentially, it's a spiral that works like a screw. Turn it one way and nothing happens. Turn it the other way, and whatever liquid is inside is forced to move up the screw.
Now if I knew that cement trucks spun one way to mix and the other way to unload, I know I'd know I was right. But I don't.

So feel free to tell me.