Concrete is a durable building material that takes special materials to dissolve once it's hardened into place. Phosphoric acid and trisodium phosphate are the main compounds used to dissolve concrete leftover from masonry work. Sometimes, that doesn't do the trick. If that's the case, use muriatic acid, an industrial grade of hydrochloric acid -- but only after washing away the other types of acids and donning all the right safety equipment.
Don your safety goggles, gloves and long apron. Even weak acids can harm your skin and eyes.
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Open a box of baking soda and bring a hose to where you're working. You may need these to neutralize an accidental spill of acid. Spray all foliage nearby with water and a powder of limestone -- in case you accidentally spray them with the acid.
Spread a plastic tarp over areas like carpet or grass where you don't want the acids to go. Open your windows, if you're cleaning the concrete inside. You'll need the extra ventilation.
Spread your phosphoric acid or trisodium phosphate over the concrete area you want to clean first, using a mixture with water that's recommended by the individual manufacturer. Use a brush to scrub the acid into the concrete. These are weaker acids that just might work to dissolve a sufficient amount of concrete. After these acids have soaked into the concrete for about one minute, scrape the concrete you're trying to dissolve with a sharp putty knife.
Rinse the area with water and inspect it. Decide whether you'll need a tougher acid.
Mix your muriatic acid with water in a durable -- and expendable -- plastic bucket. A common mixture is 10 parts water to one part muriatic acid, but follow the directions on the bottle you've purchased. Never pour water into the acid; always pour the acid into the water to avoid potential damage from the acid from splattering.
Gently pour the acid mixture over the concrete you want to dissolve. Let it soak for at least one minute, scrubbing the acid into the concrete with a brush. After a minute, try to scrape up the concrete you're trying to dissolve.
Rinse the acid and dissolved concrete with water. It should be a fine powder called calcium carbonate.