The proper place for mortar is between bricks, not on the surface of a brick structure. Ideally, if any mortar gets on the exposed faces of bricks, the builders wipe off the mortar before it sets. However, sometimes a builder may be in a hurry or may simply overlook some wayward mortar, leading to mortar streaks and stains or even blobs of hardened mortar. To remove dried mortar from bricks the most effective treatment is muriatic acid diluted with plain water.
Protect yourself with full safety gear. Muriatic acid is sold in hardware stores, but it is the strongest cleaning agent sold over the counter and is extremely dangerous if not handled carefully. Make sure that you are wearing long sleeves, long pants, and closed-toed shoes in addition to safety goggles and long gloves.
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Add the muriatic acid to water to create the cleaning solution. Always pour the acid into the water; never pour water into acid or add acid to an empty bucket. Follow the dilution directions on the acid container; 10 parts water to 1 part acid is a typical mixture. Use a plastic bucket because the acid will corrode metal.
Fill a second bucket with clean water. You will use this to scrub the mortar.
Brush the acid-water solution onto the unwanted mortared areas of the bricks, using a paintbrush. The mortar will start to fizz and bubble. Let it sit on the mortar for about 5 minutes (or until it stops fizzing), or as directed.
Scrub the mortar with a natural-bristle scrub brush. Dip the brush in the clean water. The mortar should come away from the bricks in crumbles without too much effort. Dip your scrub brush regularly into the clear water to add additional water to the mix for rinsing and dilution of the acid.
Hose down the entire area with a great deal of water. This will dilute the acid sufficiently to render it harmless, but you must use a large volume of water to wash the muriatic acid away entirely or the residue can cause your bricks to weaken over time. Also rinse off the brushes.
Do a final rinse with an ammonia and water combination to neutralize the acid entirely. Use a ratio of 1 cup of ammonia per gallon of water. You can mix it in a plastic bucket and pour it over the work area.
Dispose of the scrub brush and paintbrush in a sealed plastic bag. Do not reuse the brushes.