When building a stone wall or other stone projects, it is almost impossible to avoid getting mortar on the stones where you don't want it. If you try cleaning off the mortar while it is still wet, it just smears on the stones. An easy solution is to let the mortar dry and, at the end of the project, clean the mortar off the wall with muriatic acid. You can apply these same techniques to most masonry projects involving cement products.
Muriatic acid is also called hydrochloric acid, spirits of salt, or acidum salis. You can buy muriatic acid at hardware stores and home centers. The acid must be mixed with water to reduce its strength so it does not damage the stone or other materials. But even when diluted, an acid-water solution must be used very carefully to prevent damage to materials and potential injury. Always use muriatic acid in a safe manner.
Equipment / Tools
- Garden hose
- Safety goggles
- Acid-resistant long rubber gloves
- All-plastic bucket
- Stiff-bristled plastic brush
- Muriatic acid (standard concentration)
- Household ammonia (as needed)
Wet the Stones
Use a garden hose to fully saturate the stone. Be sure to get any stone that may come in contact with the acid solution. This is a critical step to prevent damage. Applying acid to dry stones can stain them a yellowish-green color.
Dilute the Acid With Water
Put on safety goggles and long rubber gloves. Fill a clean, all-plastic bucket with 1 gallon of water. Carefully pour 1 cup of muriatic acid into the water.
Always pour acid into water. Never pour water into acid, as this creates a dangerous chemical reaction that can cause the solution to bubble and splash out of the bucket.
Apply the Acid Solution
Brush the diluted acid solution onto the stone, using a stiff-bristled plastic brush. Allow the solution to sit for a few moments. You know the muriatic acid is working if you can see it bubble as it reacts with the cement.
Scrub the Stones
Use the brush to scrub the cement from the stone. Work quickly so that the acid does not remain on the stone for an extended period.
Rinse the Stone
Thoroughly rinse the stone with clean water to remove the acid solution. If there are surrounding materials that may be damaged by the acid, rinse the area with a solution of 1 cup household ammonia and 1 gallon of water to neutralize the acid.
If the first application of acid does not remove the cement, repeat the application with a slightly stronger acid solution, such as 1 part acid to 10 parts water.
Tips for Cleaning Stone With Muriatic Acid
It's best to clean the stone as soon as possible after the mortar or other cement-based material has sufficiently cured. Cement becomes more difficult to remove over time. However, make sure the mortar between the stones has cured before you clean the stones. If the mortar is too soft, the acid will eat into that, too.
Do not let the muriatic acid sit too long on the stones. Keep an eye on the cement that you want to keep, and make sure it is not deteriorating. Also make sure that the stones remain wet so that they do not become stained by the acid.