Masonry Magazine February 1997 Page. 18
These steps will help keep masonry clean. Unfortunately, when it comes to cleaning the inevitable stains, there is no one-size-fits-all method. The specific cleaning procedure required will differ by the material and circumstance involved.
Test Panel
Before you begin cleaning, test the effects of your materials on the wall surface. A sample panel approximately four feet wide and six feet high will provide a good test surface. On one end, clean an area 16 inches wide. Using a different treatment, clean a comparable area on the other end. This process will establish the effect of each treatment on the materials, and give you a measure of quality control. This preview of what your cleaning materials will do to your masonry is crucial. Remember: if you burn the mortar on the construction itself, there's no undoing it.
Clay Masonry
Whenever cleaning clay masonry with anything stronger than mild detergent, be sure to protect other nearby masonry materials, as well as glass, wood, or metal surfaces. If brick is used in combination with cement masonry, glazed units or stone, then these other masonry materials must be protected to prevent damage. For example, the acid in proprietary materials can etch the surface of concrete masonry, so when cleaning brick with a proprietary material, protect the adjoining concrete masonry below the brick courses.
Before using a proprietary cleaner, it's important to saturate the wall below the cleaning area so that the runoff doesn't cause staining. Also rinse thoroughly afterwards. Note: Don't scrimp on the water. Although soaking can lead to efflorescence, this is a stain that can be removed, whereas acid burning is permanent.
To begin the cleaning process, first remove large particles of fresh mortar with wooden paddles, nonmetallic scrapers, or chisels. For removing small splatters, stains, or the residue from larger pieces, brush the surface with a medium-soft fiber-bristle brush.
Mud, dirt, or soil can be washed away with a simple detergent solution made by dissolving 1/2 cup dry measure trisodium phosphate and 1/2 cup dry measure laundry detergent in one gallon of clean water. One word of caution: avoid detergents containing soluble salts. They can contribute to efflorescence.
Before removing mortar, let it set and cure for approximately seven days. Mortar that cures too long is harder and more expensive to remove than freshly cured mortar. You've probably heard that hydrochloric acid, commonly called muriatic acid, is suitable for cleaning unglazed clay masonry. Forget that. It may be true--in theory-- but acid shouldn't be used in the real world. Theoretically, acid cleans by dissolving mortar stains. Then the resulting acid-water-dirt solution is washed away, and the masonry surface is clean.
But all too often, the cleaning solution is improperly mixed: the acid isn't sufficiently diluted with water. It's critical that acid is mixed in a one-in twenty, or five percent solution of acid-to-water.
What often happens is that the solution is mixed too strong. When this occurs, the acid will burn the mortar's surface, giving it a sandy appearance. The acid can also eat down into the mortar, leading to a premature need for tuckpointing.
A related problem with acid cleaners is so-called "white scum," a residue following an acid cleaning. Like burning, "white scum" is also caused by an insufficient amount of water, this time during the acid wash. The white residue is chemically altered cement that will have to be cleaned with a proprietary cleaner.
Given the exacting demands of muriatic acid, the safest course of action is to simply use a proprietary cleaner. Yes, proprietary cleaners cost more. But they come premixed, and virtually eliminate the risk of acid burn and its related problems.