Masonry Magazine February 1997 Page. 20
Green Stains on Brick
these stains are usually caused by washing brick with acid solutions. Preventing green stains caused by vanadium is important because efforts to remove them may turn them into brown insoluble deposits. To minimize the occurrence of green stains: store brick off the ground and keep it covered; never use acid solutions to clean light-colored brick; follow recommended cleaning procedures of the brick manufacturer.
Should green staining appear after brick has been cleaned with an acid solution, flush the brick with water immediately following the acid wash. Then wash and spray the brick with a solution of potassium- or sodium hydroxide, consisting of one-half pound hydroxide/one quart of water, (two pounds per gallon). Allow this solution to remain in place for two or three days. Finally, use a hose to wash off the white salt remaining on the brick work from the hydroxide.
Various proprietary materials have been developed to remove green stain. Their efficiency on a particular wall should be determined by test. Never attempt to remove green stain with acid.
Removing "Brown Stain" (Manganese Salts)
During firing in the manufacturing process of some brick, manganese coloring agents experience several chemical changes. This results in compounds that are not soluble in water, but are soluble in weak acid solutions. Since brick can take up acid by absorption, such weak acid solutions can prevail in brick washed with hydrochloric acid, resulting in brown stain.
To minimize this problem, do not use any acidic solutions on tan, brown, black or gray brick. If brown stain appears, perhaps from acid rain, choose one of the special proprietary cleaning compounds available for cleaning manganese. Be sure to test first for effectiveness and follow the advice of the brick manufacturer.
Cleaning new masonry is still very much an art developed through trial-and-error. It requires a combination of information, good judgment and patience. Given the diverse nature of the procedures and problems, it is safe to conclude that nothing is quite as effective as keeping materials clean during construction. Like the old cliche says: "An ounce of prevention is a heck of a lot cheaper and safer than a pound of cure.
References
1. "Cleaning Brick Masonry." Technical Note 20, Revision H. Brick Institute of America. New 1990.
2. "Cleaning New Masonry, The Magazine of Masonry Construction. March 1990; 125-127.
3. "Removal of Stains from Concrete Masonry." NCMA-Tek 45. National Concrete Masonry Association.
Photos courtesy of Pro So Co, Inc., manufacturers of Sure Kleen masonry cleaning products.
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