What Is A Sulfate Attack In Concrete?
What Is A Sulfate Attack In Concrete?
Sulfate attack on concrete occurs when sulphate ions interact with the cement paste, causing chemical deterioration. This can happen when concrete is exposed to water-soluble Sulfate salts like calcium, magnesium, sodium, or potassium Sulfate.
The strength and development of cement is primarily influenced by two silicate phases: tricalcium silicate and dicalcium silicate.
When cement is hydrated, the main resulting compounds are calcium silicate hydrates and calcium hydroxide.
The calcium silicate hydrates act as the glue in the hardened cement paste and provide cohesion. Cement also contains two aluminate phases, which when hydrated, form mineral compounds that can lead to concrete degradation and structural failure when exposed to sulfates present in certain soils. This is known as sulfate attack and is most commonly seen in ground floor slabs.
Sources Of Sulphates In Concrete
Sulphate attack can occur from two sources: internal and external. Internal sources originate from materials used in the concrete-making process, such as cement, fly ash, aggregate, and admixtures. External sources are more common and come from high-sulphate soils and ground waters, or from atmospheric or industrial pollution.
These external sources can cause excessive amounts of gypsum or other sulphates to be present in the soil and water which can be transported to underground structures like concrete foundations and retaining walls. Additionally, industrial waste waters can also be a source of sulphates.
Reactions Of Sulphate Attack On Concrete
The durability of concrete can be decreased by chemical and physical reactions known as sulphate attack. This process alters the chemical makeup of the cement paste and affects the mechanical properties of the concrete.
The chemical process of sulphate attack involves the combination of sulphate ions, hydrated calcium aluminate and/or calcium hydroxide, and water, resulting in the formation of ettringite or gypsum.
The physical process of sulphate attack is a complex interdependent process that can lead to visible damage, such as the appearance of sodium sulphates on exposed concrete surfaces.
This damage is not only cosmetic, but it also indicates potential chemical and microstructural problems within the concrete matrix. Both chemical and physical aspects of sulphate attack are interconnected and cannot be considered separately.
Factors Affecting Sulphate Attack
The main factors that influence the severity of sulphate attack include the type of cement and its mineralogical composition, the addition of fly ash, the concentration and type of sulphates present, and the presence of chloride ions.
Other factors that can affect sulphate attack include the level and fluctuations of the water table, groundwater flow, soil porosity, the form of construction, and the quality of the concrete.
What Happens If Sulfate Attack The Concrete?
When sulfates attack concrete, it can cause decalcification, an incredibly destructive process in which hydrated calcium silicates decompose and the elastic properties of the concrete are severely weakened.
This reaction also increases the internal volume of the concrete as ettringite and gypsum crystals form, resulting in expansive internal stresses that can eventually cause cracking if they exceed the tensile strength of the structure.
If this happens, water may be able to penetrate the cracks up to the surface level, where it can then expose reinforcement steel to corrosion due to its contact with oxygen from outside.
