There are some kinds of sludge, like quicksand, that you sink into. There are some kinds of sludge, like custard, you can walk on. And there are some kinds of sludge that just get stronger the more you stand on them.
The man of steel is impossible. Let's not pretend otherwise. The guy has X-Ray vision. He has super strength. He has supercold breath. He goddamn flies. There is no way he could do all that. But if there were, here's how they would work — according to what we currently know about the laws of physics.
The harder they're hit, the stronger they are. They stop bullets and let people walk on custard. They're non-newtonian fluids.