A concrete structure exposed to a rich sulfate environment or contained sulfate salts in its mix constituents (water, aggregates, or cement) can degrade rapidly and causes reduction of strength. Sulfate with the presence of water and cementitious paste will result in a process called Sulfate Attack.
WHAT IS SULFATE ATTACK
Sulfate attack on concrete is a chemical breakdown mechanism where sulfate ions attack components of the cement paste. The compounds responsible for sulfate attack on concrete 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.
When sulfate enters into concrete it combines with hydrated calcium aluminate and/or the calcium hydroxide component of hardened cement paste and begins destroying the paste that holds the concrete together. As sulfate dries, new expansive 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. The volume increase and the needle-like shape of ettringite crystals are responsible for creating internal stresses that cause cracking. This is known as a chemical process. Sulfate attack can also be the result of a physical process, when water soluble salts travel through the concrete and get deposited at the surface, causing it to scale and eventually spalls.
Sulfate ions attacking the concrete can come from internal or external sources. For it to occur internally, sulfate ions must have been incorporated into the concrete when mixed. Examples include the use of sulfate-rich aggregate, excess of added gypsum in the cement, or the use of admixtures containing small amounts of sulfate. Proper screening and material testing procedures should generally avoid internal sulfate attacks. 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. Seawater is considered to have a “moderate” level of exposure of water soluble sulfate ions.
DAMAGES ASSOCIATED WITH SULFATE ATTACK
Sulfate attack generally causes a pattern of micro cracks to the concrete. As a result, surface scaling and disintegration set in, followed by mass deterioration. Sulfate attack can be visually identified by: spotting salts buildup on concrete surfaces, surface scaling, spalling with traces of salts, de-bonding between the aggregates and paste, and the change of texture of concrete becoming like tooth paste. This happens at a developed stage of the Sulfate Attack.
This phenomenon is mainly observed in buried or close-to-the-surface concrete members, such as piers, columns, slabs, foundation, and basements, where sulfate contamination is found in the surrounding soil or water.
TESTING FOR SULFATE CONTENT
There are laboratory tests to determine the water-soluble sulfate ion content in the cement. Furthermore, the water and soil surrounding a concrete body can be tested to examine the level of sulfate contamination. The concentration of sulfate is established using gravimetric determination. By adding Barium Chloride, the sulfate is converted to Barium Sulfate precipitates which are highly insoluble. Therefore, we can collect it, dry it, and weigh it again to figure out the initial concentration of sulfate in the sample.
PREVENTION OF SULFATE ATTACK
There are several methods to prevent or reduce sulfate attack. Here are some of them:
Adequate concrete thickness
Use a cement type that is resistant to sulfate attack
Use low-permeability concrete
Use low water/cement ratio (<0.45)
Apply good compaction and curing
Remove all salts from the mix constituents
Apply a proofing membrane for concrete surfaces in contact with sulfate contaminated soil/water
Apply good compaction and curing
Peter H. Emmons. “Concrete Repair and Maintenance Illustrated”, 1993 §The Constructor. “Sulphate Attack on Concrete – Process and Control of Sulphate Attack”. https://theconstructor.org/concrete/sulphate-attack-on-concrete-prevention/2162/[viewed on December 12, 2018].
Peter H. Emmons, Concrete Repair and Maintenance Illustrated, 1993
https://publish.illinois.edu/concretemicroscopylibrary/sulfate-attack/ (Credit: Paul Stutzman, NIST)
[viewed on December 12, 2018].