Efflorescence in Concrete
WHAT CAUSES EFFLORESCENCE AND HOW DO YOU REMOVE IT?
Efflorescence comes from soluble salts and other water dispersible materials that rise to the surface of the laid concrete and in some mortars.
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Causes can be from lower temperatures, rain, at the time of laying, dew, or any type of moist conditions, condensation can also have an effect and when water is sprayed onto the surface of freshly laid concrete, generally done to help with trowelling.
Efflorescence can occur as the concrete dries or within a few days of setting. The condition can be caused after exposure to moist or colder conditions or rise from within the concrete gradually and not be noticeable in the first instance, coming from within the concrete or from the subgrade.
Any material containing Portland cement results in efflorescence.
(Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-speciality grout. It was developed from other types of hydraulic lime in England in the mid 19th century, and usually originates from limestone.(wikipedia)
The most common reaction occurs when Lime (calcium hydroxide) formed in the hydration reaction of the cement is conveyed by the water in the concrete to the surface through passageways (tubes – vessels – capillaries) in the concrete. Then Lime (calcium hydroxide) formed in the hydration combines with carbon dioxide from air and produces calcium carbonate and water.
It should be noted that efflorescence can also be created or caused by hydroxides and sulphates of either sodium or potassium, and these become much more soluble in water than calcium. These salts can come from cement, aggregates, water, or admixtures. They form efflorescence more rapidly than calcium hydroxide.
The colour of efflorescence normally shows in white and will be more prevalent in darker colour concrete than in lighter colours like white softer greys interestingly in contrast it only take around 5.6 grams of calcium carbonate per square meter of surface to cause a significant shift in the colour. Thus the complaints from the customer about the concreter not supply the colour ordered. Some forms of efflorescence are very difficult and sometimes impossible to remove, while other forms are easier particularly if they are caught and removed as they form.
The best time to remove calcium hydroxide efflorescence is defiantly before it combines with carbon dioxide.
The efflorescence will dissolve in water, so using a pressure washing or wet broom scrubbing to remove the residue will mix into a solution with water, then is can be rinsed away. Care must be taken to rinse the total surface with clean water so that no residue of the efflorescence is left drying on the surface of the concrete. So that the efflorescence does not reappear the surface needs to be dry, this can be achieved by using blow driers or a wet vacuum removing any water left. Any water remaining in the surface will cause new efflorescence to appear.
Once the efflorescence proceeds to the calcium carbonate stage, it becomes insoluble and as such is more difficult to remove, sometimes almost impossible. The first course of action is to apply a mild acid solution over all the affected areas. Note - High levels of acid containing muriatic acid, or citric acid can be very dangerous requiring the wearing of proper safety clothing. Muriatic acid comes in full strength so it must be diluted before use.
After acid washing, the concreted areas all need to be rinsed thoroughly and forcibly neutralized. Baking soda (sodium bicarbonate) or an equivalent is the best remedy. Remaining acid can harm the garden and plants. Keeping in mind that the reaction products of acid on concrete are all soluble calcium and iron salts, thus can cause more efflorescence.
In times where the efflorescence cannot be removed using the acid washes as mentioned above, other commercial products can be obtained. One of those is ethylene diamine tetra acetic acid (EDTA), which when applied will dissolve calcium salts. It needs to be remembered that EDTA can damage concrete, so it is essential to test it on an inconspicuous sample area first.
It should also be noted that in every case where efflorescence arises is NOT the fault of the concreting contractor.
There are a number of things that can be done to reduce the possibility of efflorescence accruing. Including either Class-F fly ash (Class F and Class C fly ash are products of the combustion of coal in large power plants. Fly ash is collected in electrostatic precipitators or baghouses, then transferred to large silos for shipment. When needed, fly ash is classified by precise particle size requirements, thus assuring a uniform, quality product.) or Met kaolin (Meta kaolin is a DE hydroxylated form of the clay mineral kaolinite. Stone that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of Meta kaolin is smaller than cement particles, but not as fine as silica fume.) can lock up significant amounts of calcium hydroxide in the concrete.
As mentioned, the efflorescence reaction is determined by water, either water from on top or below the concrete. It is only from vapour barriers that are able to prevent the movement of moisture from the subgrade to the top or surface of the concrete slab. The application of sealers and special coatings will help stop the surface water from penetrating the concrete. These should be applied when the concrete surfaces are clean and dry.
Efflorescence naturally occurs on all concrete
This is import to understand efflorescence naturally occurs in all concrete and as such your concrete contractor can be placed into a situation where they are not aware of an issue arising until the concrete is laid and then they are placed into a position where they need to advise a customer that a change has accrued and there could be further costs involved above the original quotation for acid cleaning washing and drying, plus the cost of sealants and the application of those sealants should the customer wish to go further.
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