Efflorescence

This white powder or stain that appears on the surface of masonry, stucco or concrete is a common problem for paint contractors. But profits and customer satisfaction will soar for those contractors who understand how to spot potential problems and prescribe appropriate cures before painting.

The Facts
Efflorescence occurs when moisture migrates from within either new or old masonry. As water moves through the brick, mortar, cement or concrete, it dissolves the salts in the masonry and mortar. The salts are then deposited on the surface as the water evaporates, leaving dirty white crystals.

There are two classifications of efflorescence—primary and secondary—distinguished by the point in time at which they occur in relation to the curing process.

Primary efflorescence occurs during the curing process. Excess water used in the mixing of the product bleeds to the surface and evaporates leaving behind deposits of calcium hydroxide crystals. These deposits react with the carbon dioxide in the atmosphere to form non-water soluble calcium carbonate, which hardens in the surface pores.

Secondary efflorescence occurs in cured concrete and composites that are in contact with moisture or are subjected to cycles of re-wetting and drying, resulting in the same chemical reaction.

In most situations, efflorescence causes little damage. Its greatest aggravation is its tendency to appear within a short period after a building is constructed. The so-called New Building Bloom can appear within a few weeks of construction. “Depending on the voids and how the moisture can move through the masonry, if the wall is constructed with places for the moisture to get out rapidly generally there is no problem,” says Lunsford, but adds, “when the moisture can’t get out rapidly the salts feed upon themselves and grow resulting in higher alkaline levels. As the alkaline level of the substrate increases this can modify the polymer of the coating and result in paint failure.”

“Paint films subjected to increased pH exposures above manufacturers recommendations begin to saponify. The process changes the molecular structure and the film will lose adhesion, blister or lose elasticity, becoming brittle,” says Andrew Kinnen, architectural product manager for Cleveland, OH-based Sherwin-Williams Company.

A Matter Of Timing
The rule of thumb for all masonry paint jobs is to check the pH of the surface before beginning the job. Masonry often is highly alkaline especially if it’s new, which will lead to problems if alkalinity is not addressed early on. The closer the pH is to neutral (7), the better.

“New masonry should be allowed to cure for a month before it’s coated,” says Lunsford. This allows the chemical reactions between the raw materials to stabilize and the pH level to drop. Even in new construction, concrete blocks may have had an opportunity to cure. They may have been sitting in a storage yard for months before being trucked to the construction site.

Some basic forms of stucco have been around for centuries. Though generally easy to paint, a new surface needs curing time. “One of the biggest problems faced with stucco is alkalinity. New stucco has a hot surface. The alkalinity drops if the stucco is allowed to cure from 15 to 30 days, but general contractors and building owners often don’t want to wait that long,” Tim Flood, General Manager for Sacramento, CA-based Vista Paint Company.

“If a general contractor doesn’t want to wait 30 days before painting the stucco, paint contractors should have the general contractor sign a waver. We have had court rulings that say if you ignore the manufacturer’s recommendations and paint sooner than 28 days, the paint contractor is partially at fault for what happens,” says Lunsford.

The Paint Manufacturers, Portland Cement Institute and the Gypsum Plaster Association all recommend that cementitious products, including stucco, cure for 28 days before painting. These organizations also recommend that the newly applied stucco be fogged (periodically spraying with a fine water mist) and kept damp for 48 to 72 hours.

“By following these guidelines, the water you put on the surface fills the capillaries of the stucco, allowing the water-soluble salts to migrate to the surface,” said Lunsford. The carbon dioxide in the air converts the calcium hydroxide and calcium sulfate—both of which can have pH levels approaching 14—into calcium carbonate, which is pH neutral.

In humid climates, nature takes care of part of the hydration process. If the stucco is not fogged in a dry climate, surfaces will remain hot for several months. In hot climates the stucco dries very quickly; adding moisture while curing will allow a surface to be damp, the salts to move to the surface and then calcify and become closer to neutral. “Even after 28 days, you can still have surfaces with a pH of 11 or 12, and the only thing that’s going to bring it down is time and moisture,” says Flood.

If the 28-day curing period has expired and the pH still is on the high side, it is recommended that the plastering contractor or owner spray the building with water two or three times a day for a day or two. A garden hose with a nozzle and a fine spray mist is all that’s needed. “That helps the stucco finish going through the hydration process,” says Lunsford.

Paying Heed To pH
The best tool for field testing the pH values is a pH pencil. This requires wetting the surface and marking the surface with the pencil. After 10 minutes the color of the pH pencil mark will change and by comparing the color with a chart the pH value of the substrate is determined. Typically the higher green and blue colors indicate higher pH values. Also commonly used is a liquid product named pHydron. This product is available in a small kit and performs the same as the pH pencil.

Phenyl-saline, the same product used to test swimming pool water, is also used to test pH. “The problem with phenyl-saline is that it reacts up to about a pH 11. It tells you the substrate is hot but not how hot,” says Lunsford.

Paint contractors should also be careful about how they measure the pH level, too. It’s important to only measure the surface the paint will adhere to; if the pH pencil scratches through the calcium carbonate layer, the pH will always be higher.

“It’s important for a painter to know that the wall about to be coated has a neutral pH, or as close to neutral as he can get it,” says Lunsford. A paint contractor should also know when the stucco was applied and what system was used (single, double or triple coats) and if the wall was fogged and cured properly. Knowing these things will help those involved make much better decisions about how to coat the surface.

“Efflorescence is not a paint problem,” says Lunsford. “We have spoken to several paint manufacturers about their products and none can guarantee that by using their paint or primer/sealer on a masonry surface it will not effloresce, and none will provide a warranty. The painter’s job is to make sure the paint sticks to the surface, not waterproof the structure,” adds Lunsford.

The Cure For Moisture
The first thing a paint contractor should do is to look for the cause of the efflorescence. “There must be moisture intrusion into the substrate to cause the problem and if the job is a parapeted wall structure, chances are guaranteed there will be cracks on the exposed surface” says Lunsford.

Unless the moisture is stopped, washing off the efflorescence with a dilute muriatic acid solution and repainting will not solve the problem—after the next rain the efflorescence will reappear. “Another thing that people don’t realize is than in dry climates dew settles on the tops of these walls and roofs from November to March every year, which will migrate into the substrate. Typically the worst places are on the upper three feet of the wall, worse on the south and east facing walls because the moisture moves to the warm side of the substrate,” says Lunsford.

Stopping the moisture from entering the wall is the first job. This can be done by applying a 1/8-inch high grade elastomeric, reinforced with an alkaline-resistant mesh, backed with a second layer of 1/8-inch elastomeric over the cracks. All of the moving cracks, either thermal or structural, should be repaired by making control joints—all good elastomeric manuals have instructions for this type of crack repair. After repairing the points of entry for moisture it is advisable to roll the sealer coat on as rolling will help fill voids.

When coatings are required on surfaces where efflorescence has been treated, there remains the possibility that entrapped or hidden moisture may cause the efflorescence to reappear and cause paint failures. “It is advisable after eliminating the cause of efflorescence to wait 15 to 30 days before re-coating,” says Lunsford.

Several good sealer/primers are available for masonry, while high quality acrylic paints are available through local paint stores.