White Dust on Brick Fireplace
Are stains on brick and mortar a sign of water damage?
For the past year or so, the brick surround of my fireplace (below the fireplace opening, underneath a stone hearth) has been producing a white-ish dust. The fireplace is on the first floor, over a crawlspace—both the crawlspace and 1 ft. of the first floor flooded during Hurricane Sandy in 2012. We had the entire first floor torn up and refinished and all the necessary remediation in the crawlspace at the time. It appears that the white substance is “growing” on the mortar. What is causing the white dust, and what, if anything, can I do about it?
—SHARON STREGER, Fairfield, Conn.
John Carroll, a North Carolina mason and author of The complete Visual Guide to Building a House, responds: The white stains on your brick hearth were, and may still be, caused by excessive moisture in the masonry. When brick masonry gets saturated, water-soluble compounds in the bricks and mortar dissolve. Then, as the surface of the masonry dries, these compounds are left behind in the form of white powder or stains.
This process, called efflorescence, comes in two forms. Water-soluble efflorescence is caused by the leaching of dissolved salts to the surface. This type of efflorescence is powdery and can be washed away with clean water and a brush.
Water-insoluble efflorescence, often called calcium carbonate stains, is caused by the dissolution and leaching of calcium hydroxide to the surface. calcium hydroxide does not dissolve as easily as salt and only occurs in the presence of heavy, prolonged saturation, such as occurred in your flood. When it reacts to the carbon dioxide in the air, the calcium hydroxide hardens and becomes a water-insoluble calcium carbonate stain. It can only be removed with masonry cleaning solutions. calcium hydroxide is present in both Portland cement and lime, so the calcium carbonate stains usually appear at the mortar joints.
To determine if the stains are water-soluble, try washing them with clean water and a brush. If this doesn’t put a dent in the stains, you probably have calcium carbonate stains and will need to use a chemical masonry cleaner.
Efflorescence is more than an aesthetic issue, however. If the stains are increasing in number and size, they indicate that you have an ongoing water problem. Given the location of the stains, I would guess that the source of the water is a wet crawlspace.
Photo: Samantha Maver
From FineHomebuilding #288