It's possible to get some mortar on the stones when you're laying flagstone no matter how careful you are. You can easily scrub the mortar off while it's still wet if you notice it in time using a wire brush and some warm water. If you may miss a little, you'll have to use some serious cleaners to get the dried mortar off the flagstones. One of the strongest cleaners that you can buy for household use, muriatic acid is what you'll need to get dried mortar off of flagstone.
Things You'll Need
Clothing that covers arms and legs
Plastic buckets
Garden hose
Several masonry brushes
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How to Clean Dried Mortar
Warning
Muriatic acid is very dangerous. You must not let it come into contact with your skin or eyes. Should it splash on you, remove that clothing immediately. Always wear acid-resistant gloves and safety eyewear when dealing with this extremely strong household cleaner.
Step 1: Prepare the Area
Rinse off the flagstone and the surrounding bushes, foliage, and soil. You'll be using a very small amount of the acid, so once you've diluted the acid and rinsed the area, it should be safe 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.
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Step 2: Prepare the Muriatic Acid
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 of water to acid for your muriatic acid cleaning solution. Start with a very small amount of the mixture because you will need very little.
Step 3: Apply the Diluted Acid
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's applied. This will dissolve some of the mortar and loosen the rest.
Step 4: Rinse the Flagstones
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.
Step 5: Neutralize the Muriatic Acid
Neutralize any remaining acid by rinsing the area with a mixture of one cup of household ammonia and one gallon of water. Rinse the flagstone patio with lots of water once you've applied this mixture since it's important to dilute all the chemicals in the runoff. Rinse the flagstones quickly after you're done cleaning to prevent damaging your flagstones. You'll have to rely on using a great deal of water to dilute the solutions rather than trying to contain the runoff.