Outdoor fireplace

I have concrete outdoor fireplace that I would like to cover with mosaic (outside only).
I have loads of bathroom tiles leftovers that I was planning to use but now somebody told me I can't use them. I need heat resistant tiles or everything with crack and fell off.
I already have fire-resistant mortar that I intend to use to stick them on and also adhesive.
I was advised to use some sort of fire resistant adhesive that is stretchable so that when whole thing warms and cools it will not crack.

So can I use bathroom tiles?
I am pretty sure I can but still want to check with all of you professionals just in case since I plan to build it up through summer use also junks of mirrors.

thank you