The Constructor

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:

What happens when sulphates get into concrete?

When sulphates enters into 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.

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.

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:

2. Physical Process of Sulphate Attack

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

Diagnosis of Sulphate Attack on Concrete

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:

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.

Fig: Effect of water-cement ratio on sulphate attack

2. The use of sulphate resisting cements provide additional safety against sulphate attack
Exposure Concentration of water-soluble sulphates in soil percent Concentration of water-soluble sulphates in water ppm

Mild

<0.1

<150

Moderate

0.1 to 0.2

150 to 1500

Severe

0.2 to 2

1500 to 10000

Very severe

>2

>10000

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