How Efflorescence Works

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.

A Simple Moisture Test

Contractors can use sophisticated equipment to determine the rate at which masonry is giving off moisture, which can make it prone to efflorescence. You can perform a simple test yourself by taping an 18-inch-square (46-centimeter-square) section of plastic sheeting to a concrete slab. Leave it there 16 hours. If you see condensation on the side of the plastic nearest the slab, or a damp darkening of the concrete underneath, you'll know you have a moisture problem. Efflorescence may appear -- or return after it's been removed -- if the source of water is not eliminated [source: Bannister].