Efflorescence
Efflorescence is the “frost” that sometimes appears on concrete products.
Anything made with cement is susceptible to “frost” or efflorescence. This includes: concrete blocks (and the mortar used to hold them in place), segmental retaining wall units, concrete roofing tiles and pavers, poured concrete for roads, sidewalks, patios, and driveways.
The good news is: we know how and why efflorescence occurs. The bad news is: efflorescence is almost impossible to prevent. The silver lining is that efflorescence eventually goes away–although it may take a year or two.
Efflorescence comes from within the masonry itself.
All mortar and masonry products contain cement, and cement inherently contains lime. To make a concrete product, in addition to cement, a manufacturer adds specific amounts of other raw materials including sand, rock, water, chemical admixtures, and pigments. These materials are mixed together then poured or molded into a specific shape. After all this happens, the material sits or cures for a period of time.
Although the concrete product is solid and usable after curing, it contains microscopic pores or capillaries that go from the surface of the product into the interior. As soon as the product starts to harden (which it will do for quite a while), the process of efflorescence may start.
How does Efflorescence occur?
Water (rain, moisture from dew, water from a sprinkler system) reacts with the free lime (Calcium Oxide) in the masonry product; this forms Calcium Hydroxide. As the masonry product dries out, the water migrates to the surface through the pores in the masonry product–carrying with it the Calcium Hydroxide. When the Calcium Hydroxide comes in contact with Carbon Dioxide in the air, a hard white substance (Calcium Carbonate) forms, which settles on the surface of the masonry. After the moisture evaporates, a “frost” of Calcium Carbonate is left behind. This "frost" is efflorescence. It will continue to form so long as the masonry product is curing and the Calcium Hydroxide is free to move to the surface. The efflorescence phenomenon will stop when no more Calcium Hydroxide forms.
Technically, efflorescence is Calcium Carbonate Precipitates (or Calcites):
CaO + H2O ---> Ca(OH)2,
Ca(OH)2 + CO2 -> CaCO3, H2O
Masonry-Born Efflorescence
Another aspect of efflorescence is the build-up of salts in masonry and concrete substrates. Regular moisture movement within the substrate dissolves these salts. As moisture travels toward the surface, it carries the dissolved salts with it. When the moisture reaches the surface, it dries, causing the salts to crystallize. The salts and calcites then accumulate on the surface of the substrate.
Just be patient. Efflorescence will go away.
The hard, white "frost" of efflorescence will eventually dissolve and not return, but it takes time. If you live in an area of frequent rain followed by sunny days, the entire process of efflorescence (formation to elimination) will happen quickly. On the other hand, if you live in a dryer, more arid climate, this process will take longer.
Technically speaking, the "frost" degrades to water soluble Calcium Bicarbonate:
CaCO3 + CO2 + H2O --> Ca(HCO3)2
As for crystallized salts, rainwater will eventually dissolve them and wash them off of the substrate surface.
What if I can't wait?
There are two alternatives to waiting for efflorescence to run its course. The first is simple; the second should be done with caution.
Method 1: Clean the masonry surface with Cathedral Stone Products’ Efflorescence Remover.
Method 2: Use diluted Hydrochloric (Muriatic) Acid
You must know, however, that this acid may alter the appearance of your masonry. This change becomes especially noticeable on products that are colored with iron oxide pigments.
Follow these steps:
• Put on protective goggles and gloves for mixing and application. A long sleeve shirt should also be worn.
• Wet surface thoroughly with water.
• Mix water with hydrochloric (muriatic) acid. To dilute the acid, add 1 part acid to 10-20 parts water, making certain to add the acid to the water carefully.
• Use a stiff brush to apply the diluted acid.
• Allow the solution to stay on the surface for about 15 seconds.
• Wash it off thoroughly with water.