In the 2000s, the perception of concrete began to change. In addition to focusing on high strength, the production of concrete with long-term durability has started to gain importance. As long as concrete is in service, it must maintain the required strength and be durable enough to withstand both chemical and physical effects. Concrete with long-lasting durability can only be produced if all of the inputs are carefully controlled. One of the durability problems that can occur in the medium- and long-term is sulfate attack. In this article, we will discuss how sulfate attack on concrete occurs and what must be done to prevent it.
What is Sulfate Attack?
Sulfate attack is a form of expansion and cracking of concrete due to penetration of SO4 ions. When concrete cracks, permeability increases, and water penetrates more easily, thus accelerating the deterioration.
Picture 1. Sulfate Attack
Sometimes the expansion of concrete may cause serious structural problems, such as the displacement of building walls due to horizontal thrust by an expanding slab.
Sulfate attack can also take the form of a progressive decrease in the strength and loss of mass due to loss of cohesiveness of the cement paste.
Picture 2. Mortar Subjected to Sulfate Attack
How does Sulfate Attack on Concrete Occur?
There are 3 fundamental reasons for the reaction known as sulfate attack or sulfate effect that results in harmful expansion in harden concrete:
1. C3A phase in cement,
2. Groundwater or seawater-borne sulfate
3. Portlandite (Calcium Hydroxide), a product of cement hydration
One of these three factors must be prevented in order to stop the expansion caused by sulfate attack on concrete.
Picture 3. Sulfate Attack Formation
The ground water in clay soils and seawater contains sulfate ions that are harmful to concrete. Calcium hydroxide (CH) and alumina-bearing phases in hardened cement paste are susceptible to sulfate attack. If the C3A content of the cement is more than 5%, monosulfate hydrate (C3A⋅CS⋅H18) will be formed in hardened cement paste. If the C3A content of the cement is more than 8%, the hydration products will also contain C3A⋅CH⋅H18.
When SO4 penetrates in, both these phases hydrates react with CH and SO4, converted to the high-sulfate form ettringite.
Ettringite that forms in the stable structure of hardened concrete results in harmful expansion and the formation of cracks.
Depending on the cation type associated with the sulfate solution (i.e., Na+, K+, or Mg+2), both CH and C-S-H may be converted to gypsum by sulfate attack.
What’s more, sulfate combines with free MgO resulting in the deterioration of the calcium-silica-hydrate gels (C-S-H) that provide binding to cement paste. The result is a negative effect on concrete strength.
What are the Sources of Sulfate?
- Sulfate containing soils (Lands with white deposits on the surface, lands where plants do not grow except bushes)
- Cement (Must be SO3≤3)
- Seawater, groundwater
Picture 4. Sulfate Sources
What are the Negative Effects of Sulfate Attack on Concrete?
- It has adversely effect on the permeability of concrete, resulting in a structure with gap spaces.
- Deep cracks form in the outer surface of the concrete causing the reinforcement to corrode by damaging the concrete cover.
- Concrete strength decreases.
- It causes deterioration of the aesthetic appearance.
What Must be Done to Prevent Sulfate Attack on Concrete?
- The amount of SO3 in the ground water in the soil to be applied should be determined.
- Cement with a low percentage of C3A should be used:
- Concrete construction that will be exposed to groundwater or seawater will exposed to sulfate attack. For this reason, special cements should be selected in these construction projects.
- According to TS EN 197-1, CEM I SR, CEM III B and CEM III C cements are resistant to sulfate.
Picture 5. Çimsa Resistant SR – Sulfate Resistant Portland Cement – CEM I SR5 42,5 R
3. Mineral additives can be used.
- CH is a cement hydration product and plays a role in the sulfate attack mechanism.
- Mineral additives (fly ash, blast furnace slag, etc.) ensures CH consumption and so prevent sulfate attack on concrete. Mixing water should be analyzed regularly for internal sulfate attack.
4. Mixed water analysis should be done regularly for internal sulfate attack.
5. Concrete permeability should be reduced.
- If the infiltration of sulfate-containing waters into the concrete is prevented or reduced, the concrete can be protected from sulfate attack.
- Reducing the water/cement ratio of concrete as much as possible by using chemical admixtures will reduce concrete permeability.
6. When necessary, the concrete should be sealed.
Sulfate attack is one factor that has a negative affect on concrete durability. Preventing sulfate attack ensures that concrete will be strong and safe with long-lasting durability. Controlling the phases in the cement, using mineral additives, analyzing mixing water, preventing the entry of ground water and seawater and reducing concrete permeability are all measures that prevent sulfate attack. When these measures are taken, the service life of concrete increases.