c5?! in the QGD

White should only play c5 in QGD lines if it's with tempo and White is ready to break with e4.

The most common example is in the Slav Defense.  Often times, White will hit b7 at some point with Qb3, to which Black's most common response is ...Qb6.  It is only at this point that White should play c5!  If c7 is not covered by White's Dark-Squared Bishop, then Black's best move is usually ...Qc7 after he has induced the c5 push, and then it's a question of who gets the pawn break first?  White with e4, or Black with e5.  In addition, White does not want to have an unprotected Rook on a1.  Protected is fine, Rook anywhere else is fine, but an unprotected Rook on a1 leads to ...b6, b4 a5 ideas, breaking up White's Queenside.  If c7 is covered, or if Black Voluntarily trades Queens, the resulting pawn structure is better for White.  He'll advance b4, and if Black advances a6 and has an unprotected Rook on a8, b5 is still possible.  If he plays a6 and moves the Rook away from the a-file, sacrifices on a6 are possible where White owns the a-file and has a nasty pawn storm going on the Queenside, along with an attack on the weak c6-pawn, as compensation for the sacrificed material, whether that be the Bishop or Rook that's sacrificed.

Check out this game that I had this past Sunday morning in the 4th round of a 5-round tournament, where I sacrifice a Rook for the Queenside play:

http://www.chess.com/groups/forumview/rook-sac-in-the-slav-brilliancy-or-bust

In almost all other circumstances, c5 by White is a weak move as it takes all pressure off of d5, and makes e5 an easier pawn break as White can't answer e5 with cxd5 to weaken the d5-pawn.

What happens if White plays an early c5 on move 3?

I replied Nf6 but got crushed.