Efflorescence and Carbonation:
Working Together to Destroy Your Concrete Slab
I know we have all seen those unsightly white stains and powder materials on the surface of concrete and wondered not only what they were, but how they got there. These formations are most likely caused by efflorescence in combination with carbonation.
Efflorescence occurs when moisture migrates to the surface, bringing with it salts from within the concrete and depositing them on the surface. The primary material that is brought to the surface is calcium hydroxide, but there are many other salts that can also come to the surface.
Once on the surface, calcium hydroxide chemically reacts with carbon dioxide and is converted into calcium carbonate. This chemical process is known as carbonation. Carbonation occurs not only at the surface, but also deep within the concrete.
In northern climates, carbonation occurs during cold weather when concrete is being placed in the same area in which unvented fossil fuel heaters are being used. If carbon dioxide present in the flue gasses comes into contact with the bleed water of freshly placed concrete, it produces carbonic acid. When this acidic bleed water is absorbed back into the concrete, its destructive characteristics are also carried with it. Depending upon how tightly the concrete floor is finished, the bleed water that is absorbed can penetrate deeply into the concrete. Carbonic acid chemically reacts with calcium hydroxide to form calcium carbonate. (Calcium carbonate is a weak white powder that reduces the strength of the surrounding hydrated cement paste.) Concrete containing large quantities of calcium carbonate will become soft and have low abrasion resistance. The calcium carbonate can be washed from the surface of the concrete but, most likely, it will reappear in a few days as moisture continues to migrate from within the concrete to the surfacebringing with it more calcium carbonate.
Testing concrete for carbonation
The removal of efflorescence stains can be as simple as dry bushing or washing with water and stiff brush. Stubborn stains may require scrubbing with a 10% muriatic acid solution diluted with 12 parts water to 1 part acid. Expect some etching of the concrete surface in the processand that efflorescence, unfortunately, will more than likely return with the continued migration of moisture to the concrete surface.
In addition to removing efflorescence stains, a remedy to the carbonation issue should be addressed. When a concrete surface has been carbonated to the point of dusting, an effective solution is to apply a chemical hardener and densifier to the surface. The liquid chemical hardener converts the soft calcium carbonate formed during the carbonation process into durable calcium silicate hydrate. Calcium silicate hydrate is also the crystalline structure formed during the hydration of portland cement. Therefore, by converting calcium carbonate into calcium silicate hydrate, the soft surface of the concrete becomes chemically altered to a very hard dense surface.
"The calcium carbonate can be washed from the surface of the concrete but, most likely, it will reappear in a few days as moisture continues to migrate from within the concrete to the surfacebringing with it more calcium carbonate."
When internal carbonation takes place, the naturally high pH calcium hydroxide is replaced with a lower pH calcium carbonate. This results in an overall lowering of the pH of the concrete. The lower the pH, the greater is the degree of carbonation. Non-carbonated cement paste has a naturally high percentage of calcium hydroxide, which means the pH of fresh cement paste will also be high, about 12.5 to 13. It is well documented that it is this high pH of the cement paste that carries with it the beneficial property of protecting the reinforcing steel from rusting. Without this protection, the structural integrity of concrete members may soon be in question. Fully carbonated cement paste can have a pH as low as 7.
Phenolphthalein, an organic indicator used in the determination of pH, can be used to determine the degree of surface carbonation. The Portland Cement Association (PCA) in its March 1991 edition of Concrete Technology Today reports a method that can be used to determine the pH of cement paste in three ranges:
"Phenolphthalein is an organic indicator used to establish approximate pH values correlated to colors formed in the paste when a 2% solution in ethyl alcohol is applied to a freshly broken portland cement concrete surface. Maximum color change to a deep purplish red occurs at a pH of 9.8 or higher. Below 9.8 the color maybe pink, and at a pH of 8.0 or lower, colorless."
Using the fact that at a pH of 12.5 to 13 carbonation has not occurred and at a pH of 7 the cement paste is fully carbonated, by using the PCA petrographic analysis method an estimation of the degree of carbonation can be made. Once it has been determined that carbonation has occurred, treatment can begin. If it is determined that the pH is very low, the only alternative may be to replace the concrete. An alternative for surface carbonation may be the simple chemical treatment as indicated above or, for deeper instances, diamond grinding first to remove the low strength cement paste and then chemically hardening.
But let's hope that with this short tutorial, the conditions in which carbonation occurs become more rare in the first place.
© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2005/2006.