Tech-Notes on Efflorsecence
What is it?
Efflorescence is a change on the surface to a powdery substance upon exposure to air, as a crystalline substance through loss of water. It causes the surface to become encrusted or covered with crystals of salt or the like through evaporation or chemical change. One sure way to identify efflorescence that is present on a finished surface is to taste it. Because it is made up of soluble alkalies, it tastes salty.
Where does it come from?
Efflorescence comes from any free salts in the raw materials. Sand, cement and brick most commonly contain soluble salts, but fly ash and water can also be a source of contamination. There is no good test for efflorescence that can be easily performed in order to determine the source of contamination. As far as water is concerned, in general, as long as the water is potable there should be no problems.
When does efflorescence occur?
Efflorescence occurs when free salts are present in one or more of the raw materials and when these salts are dissolved and carried to the surface by water as drying occurs. This happens most frequently when there are low temperatures and high humidity. In the North this happens most commonly in the early spring when there are intermittent rains, but the temperatures are still only in the 40’s and 50’s. In the South, there are places that never experience these conditions and therefore rarely if ever have any problems with efflorescence.
How can you reduce the possibility of efflorescence occurring?
There are some measures that can be taken in an effort to avoid efflorescence. If your cement contains free salts, you could change to a low alkali cement which, by definition, is .6% or less. Obviously, the lower the alkali content, the less likelihood of efflorescence. If the sand is contaminated, you could use a washed sand which would eliminate salts. If fly ash is causing the efflorescence, the best thing to do is totally eliminate it from your mix. In general, we do not recommend fly ash being used in any architectural concrete application because of the difficulty in receiving fly ash that is consistently one color. If however, one can obtain uniformly colored fly ash, it can in fact help reduce efflorescence by eliminating air-spaces and therefore creating fewer routes for water travel and consequently fewer salt deposits.
The addition of water repellent admixture chemicals can also help in preventing the occurrence of efflorescence. This group of chemicals comes under many names and formulations. The most commonly used products in this area have historically been stearates of one sort or another. The most dramatic effect produced by these products is often the densification of the product being treated, thereby inhibiting the migration of water into or out of the product after it is produced. Any reduction of the passage of water into and out of a unit will reduce the possibility of migration of soluble salts to the surface where they may show up as efflorescence. Due to the increased chances of incompatibility of color and multiple admixtures it is generally our opinion that a minimum of admixture chemicals be employed when color is used. It has been our experience, however, that some water repellent admixtures can help prevent efflorescence and if a problem does develop on a particular project their use can be quite beneficial.
What part does color play?
Color itself cannot cause efflorescence. When working with colored products or colored mortar, however, any efflorescence problems which do exist tend to be accentuated. All pigments used to color concrete and mortar are virtually 100% inert and water insoluble. Therefore, there is nothing in any of the pigments that could possibly dissolve in the concrete or mortar and be deposited on the surface. This means that color may aggravate any efflorescence problem by making it more visible, but it does not actually cause any problem on its own.
What to do when efflorescence occurs?
Even if one does everything correctly to reduce the possibility of efflorescence occurring, there are cases where parts of a building may effloresce. So, you need to know what to do about it. The best thing to do is absolutely nothing. In time almost all efflorescence will disappear. If you can wait for one year before doing anything to the building, 95% of the time all of the salts will work themselves to the surface and the problem will solve itself.
When waiting one year is not an acceptable option, then it is necessary to wash the building. The best thing to use is a detergent based cleaner and follow the manufacturers directions exactly. Usually prewetting the building is necessary as well as thoroughly rinsing the building after applying the cleaner. Never use high pressure water in the cleaning process because it can leave stains of its own. Even if all the directions are followed and the cleaner is used properly, there is still a possibility that the cleaner could change the color. Therefore, prevention is always the best possible answer.
You, as a manufacturer of architectural block, precast, or colored mortar, must bear in mind that addition of pigment will increase the likelihood of seeing efflorescence if it exists. Therefore, it is extremely important to take every precaution possible to prevent the appearance of efflorescence in any colored work.