When Michelangelo started painting his famous frescoes on the ceiling of the Sistine Chapel in 1508, an unsightly white film appeared, marring parts of his masterpiece. "I am in a great quandary," the artist wrote of the situation [source: Ford]. The problem that stumped him was efflorescence, a powdery deposit that appears on masonry walls and concrete slabs, and believe it or not, that same discoloration has caused similar frustrations for many an unhappy homeowner in more recent times.
The word efflorescence comes from a term meaning "flowering," but the issue is no bed of roses for either artist or homeowner. It can affect brick walls, stucco, concrete slabs, patio stones -- almost any form of masonry. Stained concrete is a popular form of flooring these days, and efflorescence is all the more noticeable on the dark concrete slabs used for this purpose.
The culprit here is the soluble salt often found in concrete, brick or mortar. Water drawn to the surface through porous openings brings the salt with it. As the water dries, the salt is left behind as a solid.
As mentioned, those salt deposits are usually white or gray, but they also can be yellow or green, depending on the type of salt involved. They may appear chalky or as a light scum. Sometimes, they form as crystals or "whiskers" that create a kind of fuzz in a small area. Other times, they cover a wide surface [source: Delaware Quarries].
Efflorescence is usually not a structural problem, but a matter of appearance. In rare cases, the accumulation of salts can weaken the masonry or cause spalling, which is a chipping or blistering of the surface. Persistent efflorescence can also point to a moisture problem, which might result in mold [source: Bannister].
Unsightly efflorescence can be a vexing issue, as it often appears without warning and can be difficult to cure. It may even leave homeowners and contractors in the same quandary as Michelangelo, who had to chip away damaged sections of his painting and start over. But don't get too worried just yet. On the next page, you'll read about the three conditions needed to produce efflorescence, which may point you toward some ways of solving the problem.
Causes of Efflorescence
Three factors must be present wherever efflorescence appears:
- A soluble salt. A variety of salts and salt-forming chemicals can be present in masonry. They include an array of sulfates and silicates and even ordinary table salt, sodium chloride. An alkali, such as calcium hydroxide (lime), can form a salt when exposed to air.
- Water. Moisture is the vehicle that carries the salt to the surface of the masonry.
- Channels. Concrete and bricks have microscopic pores. These tiny channels allow water to move by wicking action. If the moisture reaches the surface and evaporates, it leaves behind the dissolved salts as efflorescence [source: Portland Cement Association].
Natural salts and salt-forming chemicals may be found in the sand or gravel used to make concrete, mortar or cast patio blocks, although most sand and gravel is washed free of salts. Salts can occur in Portland cement, another ingredient of concrete. They could be dissolved in the water used to make concrete or mortar, if it's drawn from a source contaminated with salt [source: Delaware Quarries].
Where does the unwanted moisture come from? A concrete or mortar mix that's too wet is a common source. Water is required for mixing concrete, but adding too much to the mix leaves behind moisture that will eventually work its way to the surface. Water might also seep into masonry from the ground, and water splashing onto slabs or walls from roof runoff or sprinklers can soak in as well.
Masons refer the deposits that appear soon after a slab is poured or a wall built as "new building bloom," or primary efflorescence [source: International Masonry Institute]. It's usually the result of the water used in the building process, and it can appear anywhere from a few weeks to several months after the job is finished. The good news is that normal weathering usually removes this type of efflorescence, and it does not reappear.
Secondary efflorescence, which is caused by water entering the slab or wall, can be more persistent. It may show up at any time and will continue until the moisture problem is solved.
Efflorescence also can be seasonal. Cool, humid weather -- the conditions under which Michelangelo began his famous frescoes -- slows the evaporation of moisture from masonry. That means salts have time to reach the surface and are left behind as deposits of efflorescence. In dry summer weather, on the other hand, moisture evaporates before salts reach the surface, so they remain locked inside the stone.
If efflorescence has appeared on your masonry, don't despair. Read on to discover some of the ways to remove it.
How to Remove Efflorescence
Time is often the best cure for efflorescence. On a slab, such as a basement or garage floor, or on patio blocks, for example, wear and foot traffic will eventually remove the discoloration. New building bloom on a brick or stucco wall will usually not reappear if washed away by rain or brushed off by the homeowner. Especially on naturally colored concrete, where the white deposits hardly show, just waiting for it to wear off might be your best bet. If you're itching to take some sort of action, though, there are a few options:
- Simple washing can sometimes remove efflorescence. Scrub with a stiff brush and mild detergent or plain water. Efflorescence is most soluble when it first appears, so sooner is better than later if you want to try this approach. Just wetting efflorescence can make the film seem to go away (it actually becomes transparent), but you'll need to apply some elbow grease to do a thorough job. Always be sure to rinse thoroughly. If you leave dissolved salts on the surface, they'll return as new efflorescence.
- Power washing also can be effective in removing surface deposits. Keep the pressure as low as you can to do the job. A spray that's too intense may actually open pores in concrete or brick and encourage further efflorescence.
- Sand blasting is effective, but should be used with care. The abrasion may damage surfaces, making bricks and mortar more porous. If you choose to try sand blasting, seal the surface you're working on after you're done.
- Chemical cleaning might be needed for some cases of efflorescence. However, always soak the surface with water before using chemicals in order to keep the cleaner from penetrating into the stone and further opening pores that encourage efflorescence. If you choose this strategy, you can follow the directions of one of the proprietary cleaners on the market, or you can use diluted muriatic acid, citric acid or vinegar. Make sure you wear gloves and goggles when handling acids or cleaners. After cleaning, neutralize the acid with a baking soda solution and finish by washing the surface thoroughly with water [source: Nasvik]. Acid cleaning may discolor stained concrete, so test it on a small section first.
A general rule for cleaning efflorescence is to try gentle methods first before moving on to harsher techniques [source: Koski]. But really, the best approach to dealing with efflorescence is to keep it from forming in the first place. Find out how to do that on the next page.
How to Prevent Efflorescence
An ounce of prevention can eliminate many of your efflorescence headaches, but you'll have to rely mainly on your contractor. Good building practices are essential. Remember, you're trying to avoid the three essentials of efflorescence: keeping salts out of the masonry, keeping water out of the slab and avoiding porosity in your stone work.
Here are some of the key things you should insist on from your masonry contractor:
- Use high quality concrete that contains minimal water. In the business, this is known as "low slump" concrete. Compact and finish the concrete well to minimize its porosity.
- Make sure the sand and gravel used in concrete has been washed and that the water in the mix is pure and salt-free. Adding fly-ash to concrete also reduces the water and cement that are needed and binds some salts, minimizing efflorescence [source: Bannister].
- Use low-alkali mortar for stone or brick work so that alkali salts don't leach into the masonry. Mortar should be firm and free of cracks. Make sure the manufacturer of fired clay bricks has added chemicals during manufacture to make salts in them insoluble and limit efflorescence.
- Cure concrete or stucco properly. Keeping a concrete slab wet and covered with plastic while curing makes it denser and leaves fewer channels through which salts can wick to the surface [source: Nasvik].
- Consider a sealer or paint that will minimize efflorescence. Since some sealers can trap the deposits and make them harder to remove, this decision should be left to an expert.
- Install a vapor barrier under the slab in order to keep moisture from seeping up from below. Seal footings with plastic so water doesn't seep from the ground into the foundation. Proper flashing, wall covers, roof overhangs and window caulking all protect masonry, too.
Once the masonry is in place, it's up to you to discourage efflorescence. Watch out for lawn sprinklers, excessive washing and splashing -- any wetness encourages the problem. Improve drainage by routing runoff away from your house, patio or wall.
The bad news is that efflorescence may not be entirely preventable. But the good news is that however annoying it may be, it's usually not a huge problem and may even go away on its own. Don't be impatient. When the restoration of Michelangelo's Sistine Chapel ceiling began in 1975, the first step was to waterproof the vault to prevent any further efflorescence. And when the 19-year project was completed, the art work didn't look half bad [source: Capel].
Author's Note: How Efflorescence Works
I'm sitting in a trendy coffee bar on New York's Lower East Side, sipping a trendy espresso and listening to Tom Waits growling on the sound system when I happen to notice the exposed brick wall. Exposed brick is so common today as to be beyond trendy. But my eye is immediately drawn to a whitish-gray film on some of the bricks. Efflorescence in action!
A serious problem for the café owner? Time to whip out the muriatic acid or bring in the sand blaster? Hardly. That powdery deposit makes the bricks look old. It's just what the interior decorator ordered. Character. Ambiance.
It goes to show, what's grungy to some is fashionable to others. Kind of like Tom Waits's voice.
- Bannister, Doug. "Contractor's Guide to Efflorescence," ConcreteNetwork.com. (April 20, 2012) http://www.concretenetwork.com/doug_bannister/efflorescence.htm
- Capel, Chris. "Conservation Skills." Routledge. 2000, page 102.
- Delaware Quarries. "Efflorescence Causes, Removal, and Prevention."(April 20, 2012) http://www.delawarequarries.com/cleaners/efflorescence.html
- Environmental News Service. "Bacteria 'Trained' to Clean Ancient Artworks," June 15, 2011.(April 20, 2012) http://www.ens-newswire.com/ens/jun2011/2011-06-15-02.html
- Ford, Aleid. "Sistine Chapel - spot the differences!" Italy Magazine, June 23, 2010.(April 20, 2012) http://www.italymag.co.uk/italy-featured/roma/sistine-chapel-spot-differences
- International Masonry Institute. "Efflorescence & New Building Bloom."(April 20, 2012) http://www.imiweb.org/imi_toolkit/pdf/Efflorescence%20and%20New%20Building%20Bloom.pdf
- Koski, John A. "Removing efflorescence," Masonry Construction Magazine, May 1, 1992.(April 20, 2012) http://188.8.131.52/data/pci/technical/specmix/Masonry%20Magazine%20-Removing%20Efflorescence(Print%20Article)(Efflorescence).pdf
- Nasvik, Joe. "What Causes Efflorescence and How do You Remove it?" Concrete Construction, December 2005.(April 20, 2012) http://www.concreteconstruction.net/concrete/what-causes-efflorescence-and-how-do-you-remove-it.aspx
- Portland Cement Association. "Preventing Efflorescence," cement.org. (April 25, 2012) http://www.cement.org/masonry/cc_fn_preventing_efflorescence.asp