Looks like flatfloor's gotcha covered here. I could go into a long and boring commentary about what might have caused your slab to be the way it is, but you'd soon realize that I was just guessing, which is the best I could do. And, hell, y'all can do that yourownselfs.
But your question was what can you do about it now, and the best answer is to use one of the many products out there like the one Jim's (flatfloor) recommending.
Unless you get serious about re-surfacing the whole thing, then our resident chemist, Kemguru, has a home remedy that you can try. The formula and method have been described here somewhere, but I don't think it ever found its way into the Liberry, which is where it should be.
Let us know if you decide to do something other than stabilize the existing surface.
