How to clean portland cement off bricks?
There is a product called Sure Klean® that is used nowadays. I am sure there are other brands out there too. Muriatic acid used to be used, at least it was when I was a masons helper, but I think Its availability is regulated. Sure-Klean has it, just in small amounts I guess.

It needs to be diluted and protection worn, it pretty caustic stuff, so read the directions for use, and protect areas you do not want affected