What's the issue with rinsing? Concrete is very alkaline and will
neutralize any HCl residue if you scrub it in and then sponge it up.

I'm not sure exactly....the directions are a bit vague about the
penalties for not rinsing. Perhaps it is remnants of the chemical
reaction that need to be cleaned out, rather than the chemical itself.
I do have this mental images of it eating through my foundation,
though. ;-)