Concrete Duplication

After a specific setup, you can create infinite concrete from one powder. (It requires a command block however).
Set up a repeating command block that is constantly running the command "entitydata @e[type=falling_block] {Time:-32768}" then place concrete powder 1 block above a pool of water. Keep mining the concrete and more will fall down. When you have enough, update the ghost block by right-clicking on the concrete so it appears, then break the concrete powder.