When you are laying flagstone, you may get some mortar on the stones. With careful attention to detail, you can easily scrub the mortar off while it is still wet using a wire brush and some warm water. However, sometimes you may miss a little, and then you will have to use some serious cleaners to get the dried mortar off the flagstones. Muriatic acid is one of the strongest cleaners that you can buy for household use. This is what you will need to get dried mortar off of flagstone.
Rinse off the flagstone and the surrounding bushes and foliage. You will be using a very small amount of the acid, so once you have diluted it and then rinsed, it should be OK to simply wash off the area. However, getting the area wet ahead of time will protect the rest of the stone and surrounding greenery from any accidental droplets.
Dilute your acid. Always add acid to water rather than water to acid. Adding water to acid will make the acid "belch" and throw the mixture everywhere. You should use a 10 to 1 ratio for your acid. Start with a very small amount of the mixture because you will need very little.
Brush your muriatic acid dilution onto the dried mortar. Use the long-handled masonry brush to carefully apply the acid only to the areas with dried mortar. The acid will start eating away at the mortar as soon as it is applied. This will dissolve some of the mortar and loosen the rest.
Rinse off the area. As you rinse, scrub off the remaining mortar using a different long-handled masonry brush. At the end of the process, you will need to get rid of both brushes even though the acid will be diluted and probably gone by this time.
Neutralize the acid. Once you have removed all the dried mortar, you can neutralize any remaining acid by rinsing the area with a mixture of one cup of household ammonia and one gallon of water. This will neutralize the acid, but you should still use a great deal of water once you have applied this mixture since it is important to dilute all the chemicals in the runoff. However, you cannot use muriatic acid inside, so you will have to rely on using a great deal of water to dilute the solutions rather than trying to contain the runoff.