Sulphate attack is the reaction between concrete constituents and sulphate ions in the pore solution of concrete that results in the production of product with a fairly large molar volume.
The sulphate attack can have an internal origin which means the attack occurs without any contribution from the surrounding environment and external origin in which sulphate in soil or groundwater penetrates into concrete and develops sulphate attack.
Both types lead to an expansion in volume due to the delayed production of ettringite which is an expansive product. This generates stresses that exceed the tensile strength of the concrete, resulting in cracking and disintegration of the concrete.
1. Internal Sulphate Attack
It develops internally without any contribution from the environment. It occurs when a source of sulphate is incorporated into the concrete during the mixing process. For instance, the use of sulfate-rich aggregate and an excess of added gypsum in the cement or contamination.
Delayed ettringite formation is a special case of internal sulfate attack. It occurs in concrete which has been cured at elevated temperatures such as steam curing or in the case of massive structures.
The word ‘delayed’ indicates that ettringite could not form during the cement hydration, because of an overly elevated temperature. It then appears several weeks, months or years after the casting. Damage to the concrete occurs when the ettringite crystals exert an expansive force within the concrete as they grow.
Source of Internal Sulphate Attack
Sulfate ions in the concrete ingredients like cement, water, aggregate and admixture are the reason for internal sulfate attack.
Effect of Internal Sulphate Attack on Concrete
- It creates a crack network on the structure surface. Cracking not only impairs the ability of a structure to carry its design load but may also affect its durability and damage its appearance.
- It leads to the generalized deterioration of the concrete.
2. External Sulphate Attack
It is a more common type of sulphate attack which develops due to the ingression of sulphate present in soil or groundwater into concrete. Then the sulphate reacts with concrete constituents and subsequently the expansion product is formed.
The concrete would suffer from various detrimental influences which lead to the loss of strength. Generally, deterioration starts at the contact zone between the concrete and the sulfate-containing environment. The influences of external sulphate attack are discussed below.
Influence of External Sulphate Attack on Concrete
- Extensive cracking
- Loss of bond between the cement paste and aggregate
- Alteration of paste composition, with mono-sulfate phase converting to ettringite and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste
Sources of Sulphate Attack
- Solutions of sodium sulphate
- Potassium sulphate solution
- Magnesium sulphate; cause the most severe attack for the same concentration compared with other solutions.
- Oxidation of sulfide minerals in clay adjacent to the concrete - this can produce sulfuric acid which reacts with the concrete.
- Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide which dissolves in water and then oxidizes to form sulfuric acid.