I've got a large patio which has concrete slabs and looks OK except that the original surface has worn away in places leaving them patchy (see picture). The darker areas look like they have a very thin layer of something hard on top. It resists scraping off and wasn't affected by strong bleach or HCl acid. I'd like to end up with a nice uniform grey colour but I'm not sure what to do.
It's been suggested I might resurface the slabs by using a thin coat of mostly concrete and water mix. I've also thought about painting or staining them but I'm not that keen on either option. Is there anything I can do to help the appearance?
Any help appreciated...