I agree with 'bc'. To add, don't use the brick for any more than a decorative or facade use. The simple weight of them on top of each other may 'crush' them.
Normal brick has a somewhat 'hard' exterior and relatively soft interior. IF and when you sandblast, you'll most likely destroy that protective layer. (It's very, VERY thin.) This means that your 'new' exterior face will be soft to begin with. That's why if you stack too many of them, they may crumble under weight. They'd be GREAT for bordering and decorative looks. I'd think again about using them around a fireplace. Perhaps a local mason can talk to you about options on cleaning them and what you could use them for.