Repairing a break in threaded water pipe joint buried in wall
Wow, that is a big problem. 2 possible fixes come to mind: 


Buy some "x-pando" pipe joint compound. You can "google" the term to see where you can purchase it. I used it for years to seal bad pipe threads and for situations similar to yours. It is a powder you mix, put on the threads like pipe dope, screw the fitting in and wait till it sets up; follow directions on the can. As it sets up it will expand slightly. 
If that does not work go to a pipe shop, take a 6 inch long brass pipe nipple and have them add 2 or 3 extra threads to that nipple. Use x-pando again and the extra threads may allow you to insert the nipple farther into the joint past the crack. If neither of these ideas work call a plumber or get out your demolition tools. 


One last thing, after studying the picture, that gray material may not be concrete. It may be dried plumbers putty. Get a small chisel, old screw driver or some sharp tool, and a hammer and chip at it. If it falls apart it may be dried plumbers putty and not concrete. If it's not concrete you may be in luck to just replace the faucet more easily.