Over the years, countless papers and a great deal of research have been dedicated to analyzing the persistent problem of efflorescence in manufactured
GARRY CULTON, CCCM, CDT
Over the years, countless papers and a great deal of research have been dedicated to analyzing the persistent problem of efflorescence in manufactured concrete products. Following is a summary of recent data addressing the feasibility of a silver bullet to completely stop efflorescence.
ACI 116R defines efflorescence as a deposit of salts, usually white, formed on a surface, the substance having emerged in solution from within either concrete or masonry and subsequently been precipitated by reaction, such as carbonation, or evaporation. While efflorescence does not compromise concrete’s integrity, its effect on the aesthetic quality of products constitutes a costly problem for the industry.
Primary and secondary efflorescence are distinguished mainly by time of occurrence: Primary efflorescence, caused by excess water during concrete fabrication, typically appears during the first 48-72 hours. Secondary efflorescence occurs when water from an external source (i.e., rain, condensation) seeps into concrete and carries salts out to the surface. Both types are formed by the same process.
Three conditions must be present for efflorescence to occur: (1) presence of soluble salts, (2) availability of water to carry the salts in solution, and (3) a pathway for the solution’s migration to the surface (and water evaporation). Of the most common efflorescence salts Û calcium carbonate, sodium sulfate, and potassium sulfate Û the most prevalent and deleterious, by far, is calcium carbonate. During the cement hydration process, calcium hydroxide, Ca(OH)2, which is slightly soluble in water, is formed. The Ca(OH)2 dissolves and is carried to the concrete surface, where it reacts with carbon dioxide, CO2, in the air to form calcium carbonate, CaCO3, plus water (as shown below).
Ca(OH)2(dissolved) + CO2(dissolved) ? CaCO3(solid) + H2O(liquid)
The water evaporates, leaving insoluble CaCO3 on the surface. Since the residue cannot be simply rinsed off with plain water, its removal requires the application of weak acid and/or abrasion. Efflorescence arising similarly from sodium or potassium salts is water soluble and thus more easily removed.
Since efflorescence is a surface phenomenon typically white in color, a photometer Û using the CIE Lab color system (specifically, the L* value for lightness) Û offers a convenient method to evaluate its extent. Accordingly, a greater L* value would indicate a higher degree of efflorescence (lightness) when comparing two samples. Analytical methods, such as infrared spectroscopy and X-ray diffraction, may be used for analysis as well.
Primary factors affecting efflorescence are cement content, mix water, water/cement ratio, admixtures, curing conditions, and permeability. Greater cement content tends to increase the potential for efflorescence. That principle was demonstrated in samples run with increasing levels of cement and a constant water/cement ratio of 0.35 (Figure 1). All other variables were controlled as well.
Mix water may contain various levels of calcium, magnesium, potassium, or sodium contributing to efflorescence potential. Especially conducive to efflorescence is water softened by ion exchange, during which each calcium and magnesium ion is replaced with two (more water-soluble) sodium ions.
Water/cement ratio is another significant factor, as increasing the w/c ratio leads to a more porous concrete matrix (Figure 2), which increases the potential for efflorescence by adding excessive water and creating easier pathways. Plasticizing admixtures have been shown to help optimize cement content and water/cement ratios in manufactured concrete products.
Permeability is also tied to efflorescence: the less permeable the concrete matrix, the lower the efflorescence potential. Water-repellent (pore-blocking) admixtures have been shown to decrease permeability by repelling water and reducing wicking potential (absorption) of concrete units.
Proper curing of concrete products is essential for cement hydration and strength development. Typically, steam is used to provide high humidity and temperature for accelerated curing cycles. In some cases, carbon dioxide is forced into the concrete matrix Û theoretically, to form efflorescence below the surface and block pathways for further efflorescence on the outermost layer. This process, however, can be difficult to control due to such factors as varying product densities, absorption rates, moisture contents, air circulation, and humidity levels.
SILVER BULLET CURE?
Unfortunately, no silver bullet has been discovered to completely eliminate efflorescence. Yet, taking the following steps can significantly reduce the likelihood of its occurrence.
Optimize mix designs using quality materials
a. Cement Û Too little leads to low density/high permeability; too much increases potential for efflorescence and expense.
b. Water Û Keep water/cement ratios low; use plasticizing admixtures to improve resistance.
c. Aggregates Û Use well blended aggregates; optimization programs are available to help maximize cement efficiencies as well.
d. Admixtures Û Use plasticizers for enhancing production, increasing density, and cement efficiency, plus water-repellent/efflorescence-controlling admixtures for lowering absorptions, enhancing color vibrancy, and improving overall water-repellency characteristics.
e. Fly ash Û Pozzolans or supplementary cementitious materials typically help reduce efflorescence potential; yet, color, consistency and early strength gain are also factors to be evaluated.
Increase product density by using plasticizing admixtures or longer compaction times.
Properly cure concrete products by applying consistent temperature, moisture and air circulation.
Protect products from external water sources for as long as possible
Follow good construction practices, e.g., covering partial masonry walls, using low w/c mortar; and, don’t use high-pressure when cleaning walls.
Seal external surfaces as needed.
In the absence of a single remedy, following the above guidelines and maintaining a quality assurance program will greatly minimize potential for efflorescence in manufactured concrete products.
American Concrete Institute, ACI 116R
Technical report Û Evaluation of the Causes of Efflorescence in Concrete Masonry Products and Recommended Guidelines for Efflorescence Control, Degussa Admixtures (Barbour, Culton, & Nmai), 2005
Cemex MCP Technical Service Manager Garry Culton is a 24-year veteran in the concrete industry. He holds a BS degree in Industrial Management and an AS in Industrial Engineering with a Quality major. Specializing in the area of manufactured concrete products for the past several years, Culton is a Certified Consultant of Concrete Masonry (National Concrete Masonry Association) and Construction Documents Technologist (Construction Specifications Institute). He currently serves on several technical committees for NCMA and Interlocking Concrete Pavement Institute. Û [email protected]