Removing concrete stains on slabs?

[A.Lee]

I laid a patio last week. All went well, but the owner insisted that he finsihed the grouting between the slabs. OK, no problem.Leave him to it. The slabs have a yellow tinge to them, and the owner has now told me he has grey concrete stains on them from over spill when he grouted, and will I go round and have a look. So, whats the best way of getting the light discolouring off of these slabs? Ta Alan.

[Andy Hall]

You don't say what the slabs are. Concrete, limestone, slate,.....??

If it's material that is not soluble in acid (at least quickly), then brick acid could be a solution. It should be tried on a scrap piece first.