I would suggest just using a power washer on them. If you don't have one, the electric ones are fairly inexpensive and you will later find lots of things to wash with it.

Sue

You might try checking with a concrete contractor and see what they might suggest or be able to do.

I found this on line too at the link below...It tells how to clean concrete before it can be stained.

Clean the concrete by scraping, applying solvents or stripping agents, sanding, and grinding�"these are the principal removal methods. To pick up contaminants more easily, use stripping agents that will mix with water. If you choose to grind the surface, avoid making grinding marks that will reflect through the colored finish by using either a cup grinding head with a fine-grit (diamond or black abrasive) or diamond pad. Use a light touch, laying the cup flat on the concrete and moving it in small-diameter circles until the blemish disappears.

I'd think that should remove your stains.

Here is a link that might be useful: Cleaning concrete before staining

I second the power washer. It is great for cleaning decks, patio stones, driveways, siding and almost anything outside that gets dirty. The bonus is you use nothing but water to clean with. You can buy one for around $200 or rent one. We have gotten great use out of ours and consider the money well spent.

Our patio slabs were a mess when we bought ours. 5 years since being laid, the weather and my many flower pots had left them discolored--some were black and moss was growing on them. I tried different cleaners and a stiff push broom, but the never came quite clean. An hour with the pressure washer and they looked like new.

every mechanic I've ever been around gets lots of stains on their concrete floors and my BIL operates a radiator shop. I have seen one of two products every time.

1. Super Clean. I first got it to clean up the gas pumps at the gas station I worked at (Boss' request/money). we bought it at Auto Zone. (it is a purple liquid that was made for cleaning shop floors and imitations don't work nearly as well). apply, scrub with a broom let sit, scrub some more if you want, rinse. If at all possible keep it away from flowers. Will cause rubber soled shoes to start deteriorating in six months time, so use old shoes.

2. Carborator cleaner. (can be bought at any auto parts place and there is 2-dozen brands). Comes in an aerosol can like WD-40. also keep it away from flowers if possible and don't use it around a flame.

Thanks for the suggestions! I'm going get a power washer and see how that does. The walkway runs along a flower border, and I did notice that the muriatic acid has killed the grass where I tried it.

The nice thing about a power washer - besides working well - it really takes very little water to accomplish the job. We use it on the garage floor without flooding the place. And a hearty "yes" for Paver Cleaner. An old broom may be helpful to have handy for a little extra scrubbing (if necessary), and a plastic rain coat and pants to keep your clothing from getting completely soaked from splash-back, and a pair of rubber/plastic gardening shoes. It's on our to-do list, but not just yet - more snow in the forecast today....

-Grainlady

5 years ago when we moved into this house we rented a power washer to clean the concrete pavers and only a very narrow stream of water came out which meant it took 30 minutes to clean one paver. It also cracked the concrete. We desperately need to clean this patio with concrete paving stones, what do we need to do different?
We are totally new to home ownership and are at a loss.

Tanjacarroll: gas power washers often can get to a very high pressure. Most also have different tips (measured in angles). Sounds like you got a pinpoint, 0 degree tip, which was way too concentrated. Try a 25 degree tip on lower pressure and you should have more success and less damage.