Efflorescence is a crystalline deposit of water-soluble salts (usually white) on the surface of masonry. All masonry materials are susceptible to efflorescence. In a chemical analysis, water-soluble salts consisting of only a few tenths of one percent is sufficient to cause efflorescence on a masonry surface. The amount and character of salts of the deposits can vary widely, depending the nature of the soluble materials and the atmospheric conditions.
Temperature, humidity, and wind affect efflorescence. In the summer, even after long rainy periods, moisture evaporates quickly and leaves small amounts of salt or efflorescence. Efflorescence is usually more common in the winter when the slow evaporation rate causes salts to migrate to the surface.
Efflorescence on new construction is referred to as ‘new building bloom’. New building bloom is generally no cause for concern, as it will weather off within a few months. Persistent efflorescence in masonry walls and chimneys generally means that excess moisture is entering the system and (if not remedied) is a precursor to more serious damage.
Efflorescence-producing salts are usually sulfates of sodium, potassium, magnesium, calcium, and occasionally iron and/or carbonates of sodium, potassium, and calcium. Over twenty different compounds have been identified as crystalline deposits on masonry walls. In mortar and concrete, the hydrated cement contains some calcium hydroxide (soluble) — an inevitable product of the reaction between cement and water. When this calcium hydroxide is brought to the surface and combines with carbon dioxide in the atmosphere, it forms calcium carbonate, which appears as a whitish deposit. While the most common color is white, some minerals, such as vanadium, molybdenum, and magnesium compounds, may produce a greenish deposit, commonly referred to as ‘green stain’. ‘Brown stain’ may result from manganese compounds.
Cause and Conditions
Three conditions must exist before efflorescence will occur: water-soluble salts must be present somewhere in the wall, sufficient moisture must render the salts into a soluble solution, and a path must exist for the moisture to migrate to the surface. If any of these are not met, efflorescence cannot occur.
Little can be done about the mineral make up of masonry or about the path soluble salts may travel. Solutions to these problems should focus on eliminating the sources of moisture. In chimneys there are three major sources of moisture. The primary source of moisture is rainwater; the problem is often compounded by cracks in the crown, mortar joints, or masonry units. Poorly bonded or improperly filled mortar joints and faulty flashing are common sources of rainwater penetration.
Two other sources are condensation and groundwater. Water vapor, a natural byproduct of the combustion process, can often migrate through a system and condense within the wall, especially in improperly vented high-efficiency gas furnaces in masonry chimneys and in masonry chimneys with large chase areas. During heavy rains the water table may coincide with the ground surface. If the fireplace or chimney foundation does not have an adequate moisture barrier, moisture can be wicked up through the masonry by way of capillary suction. Determine the cause of the moisture and take corrective measures to keep water out of the chimney.
Because moisture causes efflorescence, it is generally best to remove efflorescence by dry methods such as brushing, vacuuming or light sand blasting. If dry removal methods are unsatisfactory, it may be necessary to wash the surface with a diluted muriatic acid solution — generally twelve parts water to one part commercially available muriatic acid. (Caution: acid resistant gloves, splash goggles, and other protective clothing should be worn when using any chemical solution. All precautions on label should be observed). For integrally colored concrete masonry units or mortar, a more diluted solution (15:1) may be necessary in order to prevent surface etching that may reveal the aggregate and change colors and textures.
Before using any chemical to clean masonry, test it in a small, inconspicuous area to guarantee no adverse effects. When using any chemical cleaning compounds, flood the surface with clean water to prevent the chemical from being absorbed deeply in the masonry work and causing damage. Application should be to a small area, not more than three or four square feet at a time. Wait about five minutes before scouring off the salt with a stiff, non-metallic brush. Immediately and thoroughly flush with clean water to remove all acid. Since an acid treatment may slightly change the appearance of treated areas, it is generally best to wash the entire chimney to avoid discoloration or mottling. Green stains, which more commonly occur on buff or gray brick from, or brown salts should never be treated with an acid. Acids will react with these compounds and produce an insoluble brown stain or salt that is extremely difficult to remove.
To remove green stain, dampen masonry with clean water, then wash in same manner as above with a solution of one part sodium hydroxide crystals (lye) and ten parts water and thoroughly rinse with water. If chemical and water washing must be used, do so in the summer when the water will evaporate quickly and not cause additional efflorescence formations.
Efflorescence that is not the result of new building bloom is often a visible sign of excessive moisture in a chimney system. Chimney professionals should examine the chimney system closely and offer the appropriate corrective measures. Early detection and prevention of moisture sources that cause efflorescence can save homeowners hundreds or even thousands of dollars in future repairs.