Efflorescence is drying or crystallization of salts on a porous surface. Either dissolved salt migrates to the surface or a hydrate loses its water of crystallization. The term efflorescence means “to flower out” in French and refers to the way the coating flowers out onto a surface. The opposite of efflorescence is deliquescence.
Efflorescence is important in chemistry and construction. It’s not just a cosmetic problem, since it weakens structures by either blocking pores in the material or degrading its composition.
Here are examples of efflorescence, how to identify it, and how to prevent and remove it.
Primary and Secondary Efflorescence
Primary efflorescence occurs when concrete or mortar cures and drives out water and salts not bound to the material. In chemistry, primary efflorescence happens when a hydrate gives up its water in air or a saturated solution spontaneously crystallizes and loses its water.
In either case, water evaporates, typically leaving behind a fluffy white deposit. Since the salts are not part of the cement, a primary efflorescence deposit is an aesthetic concern rather than a structural one. However, adding fatty acid mixtures to the batch coats particles and controls the problem. Otherwise, brushing away the deposit results the issue.
Secondary efflorescence results from external influences, such as dissolved road salt, leaks, or seasonal humidity. Here, salt dissolved in water invades the porous material. Secondary efflorescence potentially damages structures.
What happens is water carries dissolved salts through pores via capillary action. According to the International Association of Certified Home Inspectors, building materials draw water from as far as six miles away. One way water penetration damages structures is by dissolving substances. But, osmotic pressure causes significant problems, too. Salt concentration is higher where efflorescence forms, drawing water toward. The pressure of water flow can be immense, exceeding building material strength and turning pores into cracks.
Here are efflorescence examples, both in chemistry and in construction:
- Copper sulfate pentahydrate crystals (CuSO4.5H2O) start out translucent blue, but develop a powdery gray surface as they lose their water of crystallization and form anhydrous copper(II) sulfate.
- A 5M NaCl droplet spontaneously crystallizes at 45% relative humidity at room temperature, forming a salt crystal cube. The water evaporates into vapor. Salt crystals found along the coast of the Dead Sea form this way.
- Gypsum (CaSO4.2H2O) stored in a dry environment gives up its water, leaving its anhydrite, CaSO4.
- The white discoloration visible on many dams is secondary efflorescence.
- Deposits on the surface of freshly cured concrete are primary efflorescence.
- Fluffy white or gray crystals or powder on brick are indicative of secondary efflorescence. This is common in basements or exterior walls.
How to Identify Efflorescence
For a homeowner, the biggest challenge is distinguishing efflorescence from stains or mold.
How to Tell Efflorescence and Stains Apart
Distinguish between efflorescence and stains using color and texture:
- Efflorescence is white, gray, or yellowish. Stains tend to be reddish brown or other colors.
- Efflorescence has a texture, while stains are flat.
How to Tell Efflorescence and Mold Apart
Efflorescence and mold occur on similar surfaces and often look a lot alike. So, how do you tell them apart?
- Efflorescence crumbles into a powder when pinched. Mold does not.
- The deposits from efflorescence dissolve in water. Mold does not dissolve in water.
- Usually, efflorescence is more common on inorganic surfaces (brick, mortar, stucco), while mold forms on organic surface (wood, cloth). However, there are exceptions, so this is only a guideline.
- Both efflorescence and mold occur in white, brown, or yellow. However, any other color almost certainly indicates mold. Mold comes in black, pink, and purple, while efflorescence does not.
Efflorescence prevention involves protecting building materials during construction and planning its prevention.
- Store bricks on their pallet and cover them overnight or in inclement weather.
- Place polyethylene sheeting under concrete slabs.
- Place capillary breaks between layers of building materials.
- Don’t let sprinklers sprays concrete, brick, natural stone, or stucco and keep foliage from touching surfaces.
- Make sure landscaping lets water drain away from buildings.
- If appropriate, apply a hydrophobic sealant so surfaces. Note, some sealants are problematic in areas with freeze/thaw cycles.
- Check for damage from weather events, leaky pipes, or faulty drains.
- Use overhangs and flashings to minimize water contact with walls.
- Where appropriate, use admixtures in grout.
How to Remove Efflorescence
Choosing a dry, warm day for removing efflorescence. There are a few ways of removing deposits:
- Brush away efflorescence.
- Use a pressure washer and blast away deposits. Make sure the surface dries afterward or else crystals may reappear.
- Apply dilute vinegar. Vinegar contains acetic acid, so it dissolves most deposits. Ideally, neutralize the vinegar afterward using dilute detergent or baking soda. Rinse with water.
- Commercial removal typically uses dilute phosphoric acid or muriatic acid (hydrochloric acid), followed by dilute detergent and a rinse using water. Strong acids are not appropriate for all surfaces.
The best time to apply a repellent or coating is either before efflorescence occurs or after the cleaned surface dries.
- Gromicko, N.; Gromicko, B. “Efflorescence for Inspectors”. International Association of Certified Home Inspectors (InterNACHI).
- IUPAC (1997). “Efflorescence”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
- Smith, G.K. (2016). “Calcite straw stalactites growing from concrete structures“. Cave and Karst Science 43(1): 4-10.