In a perfect world, home repairs and construction would be performed with no drips or spills. In the real world, accidents happen. When fixing a loose brick or making repairs near a brick surface, wet cement or mortar might spill over onto the brick. Once it sets, it can be very difficult to remove without damaging the brick. You may need to make several attempts to clean the brick before the cement is completely removed.
Spray the brick with water from a spray bottle until it is thoroughly wet, or have a helper direct water onto the brick from a garden hose. Lubricating the brick will offer some protection against damage as you are working to remove the cement.
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Hold the chisel so that it is nearly parallel to the brick. Tap the chisel lightly but sharply with the hammer to chip away at the dried cement. Take your time and don't try to remove large pieces all at once. The more precise you are, the less likely the brick will be damaged.
Put on eye protection and gloves.
Mix together a solution of one part muriatic acid to 10 parts water. While the brick is still wet, apply the diluted muriatic acid. Bubbles will form as the acid reacts with the cement.
Wait about 10 minutes to give the muriatic acid time to work and then scrub the brick with a scrub brush. Repeat the process of applying the acid and scrubbing several times until all the cement is dissolved.
Rinse the brick well with several gallons of clear water to remove all of the acid.