By understanding the contributing factors, efflorescence can be minimized.
By Sue McCraven
Efflorescence as a chemical phenomenon presents its bright side in many venues. At night, the eerie greenish efflorescent glow of ocean phytoplankton is almost magical. In another realm, botanical efflorescence thrills avid gardeners as blossoms reach their peak of floral beauty. Intellectually, efflorescence can mean the zenith of artistic or mathematical genius. In our natural world, the surprising bloom of efflorescence is a welcome event.
But efflorescence also has a dark side: It has a long history of uninvited appearances in the whitish, crystalline stains that mar the finish aesthetics of masonry, concrete, precast block, architectural precast surfaces and decorative tiles. Whenever efflorescence shows up on a finished construction surface, it is unanimously deemed both unsightly and problematic: unsightly for the owner and problematic for the builder, architect and concrete producer.
Despite numerous publications and research papers on efflorescence over the years, questions about its cause, prevention and remedies continue to be posed by architects and engineers in the construction industry. Questions persist despite the fact that the whitish crystal growth of surface efflorescence is only a visually unpleasant and wholly aesthetic concern – it does not in any way adversely affect concrete’s strength or durability. Further, many industry sources indicate that efflorescence on precast structures will disappear over time with exposure to CO2 (carbon dioxide) and weathering. So why does efflorescence remain a chronic issue among those who build and design with concrete?
What certainly hasn’t changed is efflorescence itself, which has been hanging around in the same ugly guise on man-made buildings for centuries. Significant, positive changes have occurred, however, in precast concrete aesthetics. New cement technologies, cutting-edge designs, integral colors, exposed aggregates, outstanding craftsmanship and production quality controls have changed the face of precast. Today, precast concrete is widely considered a versatile and sophisticated player, able to compete effectively in the building product market with more expensive alternative materials. Precast can easily replicate the high-end look of masonry, marble, cut stone and even exotic and sculpted motifs in both physical beauty and durability. Expectations for precast finish quality are high among architects, engineers and owners.
Fortunately, efflorescence is not generally a concern for most precast products that are made under plant-controlled procedures that ensure consistently high quality, high strength and low porosity. Precast efflorescence does occasionally occur, and while dry-cast products (segmental retaining walls, precast tile and block) are more prone to this problem because of their higher cement content, producers need to know everything they can about efflorescence and how to prevent it. In this article, NPCA invited industry professionals to share what they have learned about efflorescence.
EarthStone Products’ efflorescence story
EarthStone Products LLC (www.earthstoneonline.com) of Fort Collins, Colo., produces incredibly realistic simulated stone landscape and architectural precast, from lightweight stone veneers for building exteriors and fireplace surrounds to patio paving stones and wall and column caps.
Tad Couture, president of EarthStone Products, explains his experience with efflorescence: “While we have never experienced concerns with efflorescence in our line of veneer products, we occasionally saw isolated incidences of efflorescence on our patio stones. In some instances, efflorescence would appear at our facility after periods of excessive rain or, particularly, snow. But more often we would hear of efflorescence in finished applications.”
A necessary ingredient for efflorescence is a source of water, and patio pavers are particularly prone to efflorescence because they are installed directly on the ground. Sources of water may include excessive precipitation, irrigation, inadequate drainage or proximity to other water sources.
“During our investigation of these incidences, we researched all factors that contribute to efflorescence,” says Couture. While there may not be a single remedy for eliminating efflorescence, a close examination of all materials and practices may reveal opportunities for significant product improvement and thus a reduced likelihood of efflorescence. “We began with a look at our concrete mix ingredients including water, aggregates and cement as possible sources of salts and sulfates. We changed to a low-alkali supplier for our cement.”
The significant improvements ultimately came in the formulation of the mix design and curing processes, explains Couture. “We reduced the water/cement ratio with the use of water-reducing admixtures and modified our sand/aggregate ratios to create a denser, less permeable finished product. It should be mentioned that our experience in dealing with this issue has led us to target water reduction as opposed to cement addition to achieve lower water to cement ratios. The addition of cement leads to the formation of more calcium hydroxide, a potential source of efflorescence.”
As previously mentioned, excessive rain or snow greatly increases the occurrence of efflorescence particularly in concrete that is not fully cured. Incomplete curing of the concrete due to moisture loss or cold temperatures results in unhydrated cement, which can also be a great source of compounds that contribute to efflorescence. In response, EarthStone Products now adheres to strict control of the curing process. Finished products are kept in a temperature-controlled environment and product is covered to prevent moisture loss until nearly all cement is hydrated and the product is fully cured.
“The complete hydration of the cement in our products has taken us a long way toward eliminating efflorescence concerns, but we have taken one more step to ward off efflorescence,” says Couture. “For additional protection against efflorescence, as well as providing other benefits, we have specified the application of a breathable, penetrating sealer as part of our standard installation instructions. A penetrating sealer blocks the capillaries in the concrete to minimize the migration of water into and ultimately out of
our paving stones, thus reducing another source of water, the key component in the formation of efflorescence.”
Available chemistries to reduce efflorescence
BASF’s industry manager for construction materials, Kenneth Kruse, explains the difference between primary and secondary efflorescence. Primary efflorescence on concrete “is seen at early age after curing, usually within 72 hours,” says Kruse. Secondary efflorescence can occur either in the yard or in the field application. Water is always the common denominator for both types of efflorescence.
“Carbon dioxide and water are always present in concrete, and there will always be slightly soluble salts, especially calcium hydroxide,” Kruse explains. “When soluble salts reach the surface, they can react with carbon dioxide to form the insoluble efflorescence that is visible on the surface when the water evaporates. The challenge, for both primary and secondary efflorescence prevention, is to reduce the transport of salts from the interior of the concrete to the visible surface. That is typically done through the use of admixtures.”
There are three types of integral water-repellent/efflorescence-control admixtures for concrete. One type of water-repellent admixture uses combinations of reactive powders to reduce the porosity/permeability of concrete. The second type of water-repellent admixtures is based on fatty acid materials like stearic acid, or tall-oil fatty acids like oleic acid. These fatty acid compounds make concrete pores hydrophobic and can be used in a wide range of dosages. This type of admixture may require high dosages to achieve significant water repellency in concrete.
The third type of water repellent/efflorescence control admixture is based on silicon chemistry and uses silane or siloxane as a base. Silicon compounds are more efficient than fatty-acid materials. These silicon-based admixtures can be used at lower dosage rates, are more chemically stable and are chemically bonded to the concrete surface.
Chart A shows graphically how silanes/siloxanes outperform calcium stearates at lower dosages (and lower cost per treated cubic yard) relative to water absorption by weight of concrete. “More importantly for many architectural precast applications, silane/siloxane improves the color vibrancy of properly cured pigmented concrete,” continues Kruse.
“Silanes and siloxanes have also been used for years as post-applied sealers,” explains Jamie M. Gentoso, P.E., regional sales manager for Sika Corp. “A disadvantage of these products as sealers is that they require reapplication after some period of time. There is a large advantage, however, to using these products integrally in that as the precast surface wears, no reapplication is required and the concrete is still protected from
Gentoso warns that “admixtures are not a substitute for good concreting practices. Materials should be chosen wisely and mixes should be designed to include supplementary cementitious materials to improve strength, density and durability, and bind calcium hydroxides. Further, water should
not be utilized in finishing operations. Concrete should be properly cured and stored in a manner such that it is not exposed to extraneous water during early ages. When all these measures are followed, admixtures have the greatest ability to perform and help keep concrete free of efflorescence
throughout its service life.”
Quality control manager shares tips
Wade Gaking is the quality control manager for Colorado Precast Concrete Inc. in Loveland, Colo. He has 15 years of experience with both architectural precast and underground precast structures (manholes, cisterns, grease interceptors). “We’ve found that the water-to-cement ratio is the biggest factor in preventing efflorescence,” says Gaking. “Second to a low water content would be controlling the quality and purity of the raw materials – sands and aggregates – that go into the mix. Any organic impurities in the aggregates can lead to efflorescence.”
ASTM’s C40 test for sand, “Standard Test Method for Organic Impurities in Fine Aggregates for Concrete,” is useful in preventing any salt impregnation of the concrete. This test can provide a visual indicator of the level of silts in your fine aggregates that could carry salts. Sieve analysis can provide additional indicators of the health of the fine or course aggregates used in concrete by the amount of material left in the pan (this is the material remaining in the very bottom container of the sieve screens). If there is substantial material left in the pan, the possibility of this material containing salts and other impurities is very likely.
“Producers are sometimes limited by their local sources of aggregates, and we found we had to change our aggregate supplier in order to obtain cleaner materials with less efflorescence-inducing salts and contaminants,” Gaking continues. “The main thing is to reduce the permeability of the concrete if a producer wants to avoid efflorescence. Another factor in reducing the affliction of efflorescence is to gain as much cure time as possible before exposing the product to snow or water saturation.”
This practice is especially useful in products that contain color. Iron oxides integrated into concrete can effloresce quickly and in greater quantity if exposed to snow or heavy wet conditions before the hydration process has matured. The introduction of low-alkali cements into mix designs has the benefit of reducing the salt levels that exist in standard type cements but does not completely eliminate salts. “The best ways to reduce the problem, in my opinion, is to maintain a low water-to-cement ratio, examine the quality of raw materials and practice solid curing processes before exposing products to extreme weather conditions.”
High expectations for excellent finish aesthetics in precast architectural products, from patio pavers and retaining walls to sophisticated building facades, are a reflection of technological advances, informed quality control and creativity in the industry. Along with high user expectations comes an understandably low tolerance for unexpected flaws, even superficial ones such as that of efflorescence. Efflorescence is not a mystery, but because potential sources of water at a building site are often unpredictable, the precaster must do everything possible to understand and minimize production-related factors that can contribute to the unwelcome emergence of efflorescence.
Sidebar: Known Facts on Efflorescence
1. Three requirements for efflorescence: presence of soluble (unhydrated) salts in the concrete; a source of water; and a path for water through cracks or porous concrete that pulls (capillary or hydrostatic action) water containing salts to the surface where water evaporation occurs and the salts precipitate out as residue.
2. There are two kinds of efflorescence: primary efflorescence – a thin, white, uniform deposit that occurs soon after fabrication or during construction as structures begin to dry out (typically disappears soon thereafter with weathering); and secondary efflorescence – a thicker, more localized, white crystalline deposit that occurs after construction when the concrete is exposed to cycles of wetting and drying. Secondary efflorescence, or lime weeping, is not readily removed by weathering.
3. Salts in concrete: All concrete contains salts like calcium hydroxide, and only a small amount can lead to efflorescence. Other alkaline salts (calcium sulfate, sodium sulfate, potassium sulfate) can appear as efflorescence but are soluble in water and therefore do not cause aesthetic problems in architectural concrete. Even a very small amount of hydrated (slaked) lime or calcium hydroxide Ca(OH)2, a (10 to 25 percent) byproduct of portland cement hydration, can lead to efflorescence on the surface of concrete if a source of water is able to permeate through cracks or pores in the hardened concrete and the water is able to evaporate at the surface. All cements contain lime, or water-soluble calcium oxide, CaO.
4. Sources of water: excess mix water; groundwater; sprinkler systems; rain, snow, ice and high humidity; poor site drainage; poorly constructed joints, mortar or caulk detailing; roof leaks or leaks behind walls; and surface water beneath structures.
5. How efflorescence forms: Calcium oxide, CaO, in concrete is soluble in water and becomes calcium hydroxide, Ca(OH)2, that is easily transported by water from any source (building leaks, improperly applied caulking or joint construction, landscape sprinklers, groundwater). Water can be pulled to the surface through tiny cracks or pores in the concrete by capillary or hydraulic action. When the salt-laden water reaches the surface, the water alone evaporates and calcium hydroxide combines with CO2 (carbon dioxide) in the air and becomes calcium carbonate (or calcite), CaCO3, a whitish crystalline coating (the notorious efflorescence) that stubbornly clings to the concrete and is not readily removed through weathering. The chemical
reaction that creates efflorescence on the surface of concrete is: Ca(OH)2 + CO2 → CaCO3 + H2O.
6. Construction errors: Design or construction errors may allow water ingress and precipitate efflorescence on structural surfaces.
7. Precast strength and durability: Efflorescence is unsightly and can lead to architect, owner and producer dissatisfaction but it is not
detrimental to concrete strength, function or durability.
8. Color: Because most efflorescence is white, it is more noticeable on dark or colored concrete, precast block or tiles; efflorescence can be brown, green or yellow, depending on the salts involved. Efflorescence is less noticeable on light or white-colored concrete.
9. Temperature and humidity conditions conducive to efflorescence:
Efflorescence tends to occur most often in damp, moist or cold weather that creates a ready source of water to the structure. Calcium hydroxide is most soluble in cold water (about 50 F/10 C), and that is why efflorescence in temperate regions occurs more in the spring and fall when weather is cooler and rainfall is greater.
10. Weathering: Some concrete efflorescence will weather away with time, cycles of wetting and drying, and the effect of wind on surface evaporation. Efflorescent growth that has crystals firmly attached to the finished surface may be more stubborn to remove naturally (without mechanical or chemical remediation). Therefore, it is difficult to predict how long efflorescence will last without remediation, as its natural removal depends on local climatic and weather conditions. In most cases, however, even if no attempt is made to remove the efflorescence, it is more than likely that it will disperse after a period of time in line with the chemical reaction: CaCO3 + CO2 + H2O = Ca(HCO3)2. In other words, soluble calcium hydrogen carbonate (Ca(HCO3)2) is gradually formed from the insoluble calcium carbonate (CaCO3) under the continued influence of carbon dioxide and water, and the soluble calcium hydrogen carbonate is eventually washed away with rainwater. This natural process of carbonation can take several years.
11. Predictability: Efflorescence is unpredictable; its formation is erratic and depends on local conditions. No architect, engineer, producer or builder can predict potential sources of water or saltladen groundwater that may eventually reach the concrete in a completed structure.
12. Terminology: Worldwide, efflorescence is known by many names: crystalline deposits; whiskers; laitance; grout residue; whiting; lime bloom; new building bloom; lime weeping; salt crystallization; lime spots; lime runs; lime deposits; salty residue; and my personal favorite: white powdery scum.
Sidebar: How to Avoid Efflorescence
1. Low w/cm and low-alkali cements: Many precast producers consider the best way to avoid efflorescence is to use as little water as possible in the concrete mix design and use a lowalkali cement (≤ 0.6 percent alkali). Superplasticizing admixtures and water reducers help produce workable mixes with low water-cementitious materials (w/cm) ratios. A rich cement mix used to produce low w/cm, however, is not advised; a rich mix potentially offers an abundance of unhydrated salts (excess calcium hydroxide) to feed the formation of efflorescence if conditions are right.
2. Sufficient curing and thorough hydration: Conditions for efflorescence are increased when concrete is poorly consolidated and not sufficiently hydrated (cured at recommended temperature, humidity and duration) and therefore more porous and susceptible to water ingress. Dense, well-consolidated concrete, regardless of compressive strength, is less vulnerable to efflorescence. Curing is particularly critical for dry-cast products with relatively higher cement contents (16 percent to 22 percent cement with w/cm ≈ 0.40 to 0.45) due to the need for high early strength.
3. Uniform consolidation/compaction: Uniform vibration and consolidation of concrete constituents will produce a uniformly dense concrete. Pockets of unconsolidated aggregate with excess lime can become conduits for water and surface efflorescence.
4. Formwork: The type of formwork, watertightness and amount of release agent used can be a factor in non-uniform concrete surfaces and subsequent porosity.
5. Pozzolans: Use pozzolans that react with calcium hydroxide and lower concrete permeability: fly ash, microsilica, or ground granulated blast furnace slag; these pozzolans bind up free lime in the concrete, helping to prevent efflorescence. Pozzolans react with excess calcium hydroxide and cement alkalis, helping to prevent primary efflorescence. Once the concrete cures, the pozzolanic reaction causes the concrete to become denser and less permeable. This limits egress of moisture, and reduces the lime and alkalis available to migrate to the surface causing primary and secondary efflorescence.
6. Steam curing: Curing with heat and moisture produces lower concrete permeability, resulting in no excess lime to be leached out as efflorescence.
7. Salt-free aggregates and mix water: Use clean aggregates and mix water that is free of dissolved salts (calcium, magnesium, potassium, sodium). Avoid limestone aggregates and salts from any source (sea water, sands and coarse aggregates contaminated with salts, ground water laden with salts, and aggregate moisture containing salts).
8. Admixtures: Use plasticizing admixtures to increase concrete density. Use water-repellant or reducing admixtures to decrease water absorption rates.
9. Storage: Protect finished concrete products from sources of water, including ground water and snow in storage yards. Mockups for precast panel finishes can experience efflorescence in the precast yard and delay approvals for color and texture. Avoid stacking concrete elements close to one another after fabrication, as condensation water can lead to secondary efflorescence.
10. Design and construction detailing: Use appropriate flashing on coping, column caps and sills. Ensure that joint and mortar compounds are of recommended quality for the application and are applied properly. Use proper wall moisture barriers where required. Pay close attention to wall joint materials and execution.
11. Sealers: Make your precast surface as waterproof as possible. Seal concrete surfaces with a penetrating, breathable sealant. Efflorescence can reoccur under ordinary acrylic sealants (crytoflorescence, or formation of crystals under sealants, can lead to spalling). Avoid silicone sealants that can trap water in the masonry or concrete.
Sidebar: How to Remove Efflorescence
1. Mechanical removal: Abrasion with stiff non-metallic brush, power washing or light blasting. Abrasion works best on efflorescence that is of recent occurrence. Sandblasting can adversely affect certain finishes and can make surfaces more porous.
2. Acid treatment: Proprietary chemical or acid cleaning (muriatic acid, diluted hydrochloric acid, HCl). Acid cleaning should always be followed by rinsing the concrete surface thoroughly with water to neutralize acid. Always follow manufacturers’ directions, using protective gear and safe procedures when working with caustic chemicals like muriatic acid. Always test cleaning methods or subsequent sealants on an unobtrusive spot prior to general application.
3. Caution: “While mechanical and acid remediation are effective in removing efflorescence, these methods may weaken the concrete surface and do not prevent efflorescence from recurring,” says Jamie M. Gentoso, P.E., regional sales manager for Sika Corp. “The source of water must be identified and eliminated if possible.” If the water source feeding the efflorescent reaction is not corrected, surface crystallization can reappear. Application of quality sealers is recommended after removal of efflorescence.
Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.