SULFATE ATTACK MECHANISM AND PREVENTION IN CONCRETE Prepared for Dr. Mesfer Al-Zahrani Done by Al-Sulaiman, Abdulrahman 216217 Al-Gazlan, Basseer 224080.

SULFATE ATTACK MECHANISM AND PREVENTION IN CONCRETE

Prepared forDr. Mesfer Al-Zahrani

Done byAl-Sulaiman, Abdulrahman

216217Al-Gazlan, Basseer

224080

IntroductionIntroduction

Sulfate attack is a chemical breakdown mechanism Sulfate attack is a chemical breakdown mechanism where sulfate ions attack components of the cement where sulfate ions attack components of the cement pastepaste..

The compounds responsible for sulfate attack are water-The compounds responsible for sulfate attack are water-soluble sulfate-containing salts, such as alkali-earth soluble sulfate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) (calcium, magnesium) and alkali (sodium, potassium) sulfates that are capable of chemically reacting with sulfates that are capable of chemically reacting with components of concrete.components of concrete.

Sulfate attack might show itself in different Sulfate attack might show itself in different forms Depending on forms Depending on ::

The chemical form of the sulfate The chemical form of the sulfate

The atmospheric environment which the The atmospheric environment which the concrete is exposed toconcrete is exposed to

What happens when sulfates get into What happens when sulfates get into concrete?concrete?

It combines with the C-S-H, or concrete paste, and It combines with the C-S-H, or concrete paste, and begins destroying the paste that holds the concrete begins destroying the paste that holds the concrete together. As sulfate dries, new compounds are formed, together. As sulfate dries, new compounds are formed, often called often called ettringiteettringite..

These new crystals occupy empty space, and as they These new crystals occupy empty space, and as they continue to form, they cause the paste to crack, further continue to form, they cause the paste to crack, further

damaging the concrete.damaging the concrete.

Sulfate sources:Sulfate sources:

1. 1. Internal Sources:Internal Sources:

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

portland cement might be over-sulfated.portland cement might be over-sulfated.

presence of natural gypsum in the aggregate.presence of natural gypsum in the aggregate.

Admixtures also can contain small amounts of Admixtures also can contain small amounts of sulfates.sulfates.

2. External Sources:2. External Sources:

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

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

Nature of reaction: (chemical, Physical)Nature of reaction: (chemical, Physical)

SULFATE ATTACKSULFATE ATTACK processes decrease processes decrease the durability of concrete by changing the the durability of concrete by changing the chemical nature of the cement paste, and chemical nature of the cement paste, and of the mechanical properties of the of the mechanical properties of the concrete. concrete.

Chemical process:Chemical process:

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

C3A.Cs.H18 + 2CH +2s+12HC3A.Cs.H18 + 2CH +2s+12H = = C3A.3Cs.HC3A.3Cs.H3232

C3A.CH.H18 + 2CH +3s + 11H C3A.CH.H18 + 2CH +3s + 11H == C3A.3Cs.HC3A.3Cs.H3232

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

NaNa22SOSO44+Ca(OH)+Ca(OH)22 +2H +2H22O = O = CaSOCaSO44.2H.2H22OO +2NaOH +2NaOH

MgSOMgSO44 + Ca(OH) + Ca(OH)22 + 2H + 2H22O = O = CaSO4.2HCaSO4.2H22OO + Mg(OH) + Mg(OH)22

Tow forms ofTow forms of Chemical reaction Chemical reaction depending ondepending on

Concentration and source of sulfateConcentration and source of sulfate ions ions . .

Composition of cementComposition of cement paste in concrete. paste in concrete.

Physical process:Physical process:

The complex physico-chemical processes of "sulfate The complex physico-chemical processes of "sulfate attack" are interdependent as is the resulting damage.attack" are interdependent as is the resulting damage.

physical sulfate attack, often evidenced by bloom (the physical sulfate attack, often evidenced by bloom (the presence of sodium sulfates Na2SO4 and/or presence of sodium sulfates Na2SO4 and/or Na2SO4.10H2O) at exposed concrete surfaces.Na2SO4.10H2O) at exposed concrete surfaces.

It is not only a cosmetic problem, but it is the visible It is not only a cosmetic problem, but it is the visible displaying of possible chemical and microstructural displaying of possible chemical and microstructural problems within the concrete matrix. problems within the concrete matrix.

Both chemical and physical Both chemical and physical phenomena observed as sulfate phenomena observed as sulfate

attack, and their separation is attack, and their separation is inappropriate.inappropriate.

DiagnosisDiagnosis

Spalling due sulfate attack. Spalling due sulfate attack.

Spalling due sulfate attack.Spalling due sulfate attack.

Spalling due sulfate attack.Spalling due sulfate attack.

Microscopical examinationMicroscopical examination

Prevention measuresPrevention measures

Main factors affecting sulfate attack:Main factors affecting sulfate attack:

1.Cement type and content:1.Cement type and content:

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

Cement type and content:Cement type and content:

2. Fly ash addition2. Fly ash addition

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

3. Sulfate type and concentration:3. Sulfate type and concentration:

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

4. 4. Chloride ionsChloride 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 sulfate attackControl of sulfate attack

1. The 1. The quality of concrete, specifically a low quality of concrete, specifically a low permeabilitypermeability, is the best protection against , is the best protection against sulfate attack.sulfate attack.

Adequate concrete thicknessAdequate concrete thickness

High cement contentHigh cement content

Low w/c ratioLow w/c ratio

Proper compaction and curingProper compaction and curing

Effect of Effect of w/c ratiow/c ratio

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

Exposure Concentration of water-soluble sulfates Exposure Concentration of water-soluble sulfates In soil per cent In In soil per cent In

water ppmwater ppm

Mild <0.1 <150Mild <0.1 <150

Moderate 0.1 to 0.2 150 to 1500Moderate 0.1 to 0.2 150 to 1500

Sever 0.2 to 2.0 1500 to 10000Sever 0.2 to 2.0 1500 to 10000

Very sever >2.0 >10000Very sever >2.0 >10000

Questions?Questions?