You’d probably want to put it somewhere it isn’t going to get moved a lot. There are concrete additives you can mix in to help with the strength of the finished material as well.
Here’s a link to a genuinely fascinating dive into concrete: https://m.youtube.com/playlist?list=PLTZM4MrZKfW90PdaBFt70BLTbz1bTF6Mn
I also figured that if you made your QR code so that the black sections were slightly raised, making it a working QR code could be as easy as dipping it in white paint, letting it dry, then painting the tops of the raised sections black.




There’s a possible option to engrave instructions on the back, but I get what you mean