21-04-2014, 11:48 AM
SULFATE ATTACK MECHANISM AND PREVENTION IN CONCRETE
Introduction
Sulfate attack is a chemical breakdown mechanism where sulfate ions attack components of the cement paste.
The compounds responsible for sulfate attack are water-soluble sulfate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulfates that are capable of chemically reacting with components of concrete.
What happens when sulfates get into concrete?
It combines with the C-S-H, or concrete paste, and begins destroying the paste that holds the concrete together. As sulfate dries, new compounds are formed, often called ettringite.
These new crystals occupy empty space, and as they continue to form, they cause the paste to crack, further damaging the concrete.
External Sources:
External sources of sulfate are more common and usually are a result of high-sulfate soils and ground waters, or can be the result of atmospheric or industrial water pollution.
Soil may contain excessive amounts of gypsum or other sulfate.
Ground water be transported to the concrete foundations, retaining walls, and other underground structures.
Industrial waste waters.
Physical process:
The complex physico-chemical processes of "sulfate attack" are interdependent as is the resulting damage.
physical sulfate attack, often evidenced by bloom (the presence of sodium sulfates Na2SO4 and/or Na2SO4.10H2O) at exposed concrete surfaces.
It is not only a cosmetic problem, but it is the visible displaying of possible chemical and microstructural problems within the concrete matrix.