[ted putnam]

You need an acid cleaner to get rid of the rust stains, but any acid will etch limestone.
I would suggest you find an oxalic acid cleaner (often labeled "wood bleach"). Oxalic acid will be milder on the limestone than other acids, but attacks rust by turning it into a water soluble CLEAR chemical (iron oxalate) that will wash off.

I accidentally sprayed iron (lower dose) on a freshly installed fence. It turned the fence grey in less than a minute. I think I bought some of what you are talking about from the hardware store. It was called "Wood Brightener". It worked great(and fast) for what I used it for. I couldn't begin to tell you if it would work on cement or limestone though...

 

[ted putnam]

On wood I used a brush to apply. It was mostly to apply evenly than to actually scrub. Scrubbing was not necessary. I tested a few out of sight spots first and determined that leaving it on there more than a minute(60 seconds) and it did too good of a job (if that makes sense) I had a charged water hose handy for quick rinse. stone/cement will surely take a little longer. I would do like I did and find an out of sight test spot if that's possible to experiment with initially. Sure hope it works for you. Good Luck.