While you are laying stone in concrete, it is very important to sponge the stone off. It can be a pain, but the results are worth it.
The rule is that the concrete stays in the joints, not on the stone.
After you have laid the flagstone pad in concete and have sponged it off, you then need to wash the pad down with a masonry cleaner.
-----IMPORTANT!!------- Check with your local Building Materials Store about the detergent you should use on the stone you have done. We live and work in Colorado and have used 600 Detergent (See Graphic Below) on Arizona, Colorado, and Pennsylvania Stone. The 600 Detergent maybe too strong for the stone you are using......

If you do use 600 Detergent, make sure you READ the directions. It is very stong stuff. One whiff of that and you'll feel like your lungs are on fire.
We use about a 10 parts water to 1 part 600 Detergent ratio.
We mix it in a 5 gallon bucket and brush it on with a long-handled brush.

After brushing it on, we wait about two minutes and then wash it off VERY WELL with water. Most of the time we use a hose, but if you don't have access to a hose, make sure you have a lot of buckets of water.
If you let the detergent stay on too long, it will burn the stone.
The flagstone cleaning and washing should get rid of most of the sand too. There might be a little left over. Just sweep that off and your flagstone pad will look GREAT!!
If it is a fresh laid pad, wait two weeks before you seal it.
If you have any comments or questions about flagstone cleaning
or anything else, feel free to contact us here.
Or you might want to call Prosoco directly at 1-800-255-4255.
They're very friendly and know a lot more about cleaning stone than we
do.
If you need to know how to install flagstone click here Flagstone
Cleaning and Installation
If You Need Help With Cleaning Black or Green Mold Off of Your
Flagstone, CLICK HERE!!
Does Your Stone Have White Stuff All Over It? That's Called
Efflorescence! Here's a Video to Help You Get It Out.