ASK OHJ

For the past year or so, the brick surround of my fireplace (below the opening, underneath a stone hearth) has been producing a white-ish dust.

- 

WHITE DUST ON BRICK?

The fireplace is over a crawlspace; both the crawlspace and one foot of the first floor flooded during Hurricane Sandy, in 2012. We had the entire first floor torn up and refinished and did the necessary remediation in the crawlspace. 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 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 is powdery and can be washed away with clean water and a brush.

Water-insoluble efflorescence, or calcium-carbonate staining, is caused by the dissolution and leaching of calcium hydroxide to the surface. Calcium hydroxide does not dissolve as easily as salt and occurs only in the presence of heavy, prolonged saturation, such as a flood. When it reacts to the carbon dioxide in the air, the calcium hydroxide hardens and becomes a water-insoluble calcium-carbonate stain, removable only with masonry cleaning solutions. Calcium hydroxide is present in both portland cement and lime, so the calcium carbonate stains usually appear at mortar joints.