Efflorescence
Efflorescence is a chemical phenomenon that affects concrete, brick, and other forms of masonry found in many homes and offices. Efflorescence is the loss of water from a hydrated (water saturated) or dissolved salt (mineral compound) due to the material's exposure to air and low humidity. The result is a crystallization of the material. An example of efflorescence is the formation of table salt crystals when a solution of table salt is left standing in open air. Efflorescence can cause cosmetic and structural problems if not treated properly.
Frustrated or concerned because a white, powdery substance mars the look of a concrete surface, or interferes with decorative surface adhesion? The white powder is probably efflorescence, leached minerals from concrete.
There are two types of efflorescence.
Primary efflorescence routinely occurs during the curing of concrete, firestop mortars between walls, and other cementitious materials. Heat is generated during the curing process, and the heat drives moisture to the surface of the material. The water carries with it salts that are normally bound to the other components of the material to the surface of the material, forming a fluffy white substance called efflorescence. Primary efflorescence in this context is often referred to as "saltpetering". Primary efflorescence can usually be wiped off. Since primary efflorescence brings out salts that are not essential to the structural integrity of the material, it's a cosmetic concern only. Secondary efflorescence does not occur during the curing process. Rather, it is usually caused by external concrete "poisons" such as chlorides. A very common example of secondary efflorescence occurs in concrete bridges, garage floors, driveways, sidewalks, and other concrete structures that are exposed to road salt and similar chemicals that contain chlorine. Salt solutions get on concrete and penetrate it. The chlorine in salt solutions dissolve cement stone, weakening the structure. Often, one will see stalactites of dissolved cement hanging from horizontal surfaces. The dissolved cement leaves voids in the structure that can lead to collapse and crumbling.I've also seen efflorescence form on concrete floors and walls inside buildings. Floor tiles can pop off when efflorescence forms on the concrete, interfering with the adhesive. The same can happen with concrete walls. Generally this happens when there are water problems -- high water tables, poor drainage, or even broken water lines -- and the concrete absorbs the water which dissolves the salts within the concrete. Consider efflorescence a warning sign you need to tend to water issues before they become a structural deal.
Protecting against harmful efflorescence is a matter of sealing porous concrete, brick, tile, and other materials with a hydrophobic sealer product. Manufacturers of such sealers include Aldon Chemical and V-SEAL. Be advised, however, that in climates subject to freeze/thaw cycles, anti-efflorescence sealers may lead to structural damage from cracking.
Efflorescence of both types can be removed using dilute phosphoric acid. After application, the acid solution should be neutralized and removed with a mild detergent solution. This treatment will remove the crystals of efflorescence from surfaces, but it does not cure the root cause of efflorescence. Efflorescence will appear again if the material is not protected against penetration by salt solutions.
Rebar, the steel rods often used to reinforce concrete, are also subject to corrosion caused by the formation of efflorescence. The most common solution to this problem is to coat the rebar with an epoxy compound that seals out salt solutions. Another solution involves the application of a slight electrical current, which also protects against rusting. One may also use stainless steel rebar, although this product can be quite expensive.
Certain cements are more resistant to chlorides than others. The choice of cement can greatly affect the likelihood of efflorescence formation. Homeowners should consult their contractors about efflorescence-resistant concrete products.
