Sulphate Attack on Concrete – Process and Control of Sulphate Attack

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Sulphate attack on concrete is a chemical breakdown mechanism where sulphate ions attack components of the cement paste. The compounds responsible for sulphate attack on concrete are water-soluble sulphate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulphates that are capable of chemically reacting with components of concrete.

Forms of Sulphate Attack on Concrete

Sulphate attack on concrete  might show itself in different forms depending on:

• The chemical form of the sulphate
• The atmospheric environment which the concrete is exposed to.

What happens when sulphates get into concrete?

When sulphates enters into concrete:

• It combines with the C-S-H, or concrete paste, and begins destroying the paste that holds the concrete together. As sulphate 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.

Sources of Sulphates in Concrete

Following are the sources of sulphates which are responsible for sulphate attack:

1. Internal Sources

This is more rare but, originates from such concrete-making materials as hydraulic cements, fly ash, aggregate, and admixtures.

• Portland cement might be over-sulphated.
• presence of natural gypsum in the aggregate.
• Admixtures also can contain small amounts of sulphates.

2. External Sources

External sources of sulphate are more common and usually are a result of high-sulphate soils and ground waters, or can be the result of atmospheric or industrial water pollution.

• Soil may contain excessive amounts of gypsum or other sulphate.
• Ground water be transported to the concrete foundations, retaining walls, and other underground structures.
• Industrial waste waters.

Reactions of Sulphate Attack on Concrete

Nature of reaction: Chemical and physical reactions Sulphate attack process decrease the durability of concrete by changing the chemical nature of the cement paste, and of the mechanical properties of the concrete.

1. Chemical Process of Sulphate Attack

The sulphate ion + hydrated calcium aluminate and/or the calcium hydroxide components of hardened cement paste + water = ettringite (calcium sulphoaluminate hydrate)

C3A.Cs.H18 + 2CH +2s+12H = C3A.3Cs.H32

C3A.CH.H18 + 2CH +3s + 11H = C3A.3Cs.H32

The sulphate ion + hydrated calcium aluminate and/or the calcium hydroxide components of hardened cement paste + water = gypsum (calcium sulphate hydrate)

Na2SO4+Ca(OH)2 +2H2O = CaSO4.2H2O +2NaOH

MgSO4 + Ca(OH)2 + 2H2O = CaSO4.2H2O + Mg(OH)2

Two forms of chemical reaction occurs depending on:

• Concentration and source of sulphate ions .Diagnosis
• Composition of cement paste in concrete.

2. Physical Process of Sulphate Attack

• The complex physicochemical process of "sulphate attack" are interdependent as is the resulting damage.
• physical sulphate attack, often evidenced by bloom (the presence of sodium sulphates 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.

Both chemical and physical phenomena observed as sulphate attack, and their separation is inappropriate.

Diagnosis of Sulphate Attack on Concrete

• Spalling of concrete due to sulphate attack.

Microscopical Examination of Sulphate Attack

Prevention of Sulphate Attack on Concrete

To prevent the sulphate attack on concrete, we must understand the factors which affect the sulphate attack.

Main factors affecting sulphate attack are:

1. Type of Cement and its Content

The most important mineralogical phases of cement that affect the intensity of sulphate attack are: C3A, C3S/C2S ratio and C4AF.

2. Fly ash addition

The addition of a pozzolanic admixture such as fly ash reduces the C3A content of cement.

3. Types of Sulphate and  its Concentration

The sulphate attack tends to increase with an increase in the concentration of the sulphate solution up to a certain level.

4. Chloride ions

Other factors:

• The level of the water table and its seasonal variation
• The flow of groundwater and soil porosity
• The form of construction
• The quality of concrete

Control of Sulphate Attack on Concrete

Following measures help to control sulphate attack”": 1. The quality of concrete, specifically a low permeability, is the best protection against sulphate attack.

• Adequate concrete thickness
• High cement content
• Low w/c ratio
• Proper compaction and curing

Fig: Effect of water-cement ratio on sulphate attack

2. The use of sulphate resisting cements provide additional safety against sulphate attack