sulphate attack
TRANSCRIPT
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SULPHATE ATTACK
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Introduction
• Sulphates occur in both soil and ground water.
• Soild sulphate does not attack the concrete severely.
• But water sulphates enter into the porous concrete and react with the HCP products forming a whitish appearance.
• This indicates Sulphate Attack.
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Sulphate Attack
• Increase in the volume of cement paste in concrete or mortar due to the chemical reaction between the products of HCP and solution containing sulphates.
• In hardened concrete, C-A-H can react with sulphate salt from outside, forming calcium sulphoaluminate in the framework of HCP.
• Due to the increase in volume of the solid phase( which can go up to 227%),a gradual disintegration of concrete take place.
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• Sulphate attack manifest in the form of expansion & cracking of concrete.
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Reactions
Reactions on hardened cement paste• Sodium sulphate attacking Ca(OH)₂Ca(OH)₂+Na₂SO₄.10 H₂O CaSO₄.2 H₂O +2NaOH +8H₂O
• Reaction with Calcium aluminate hydrate 2(3CaO . Al₂O₃ . 12 H₂O) + 3(Na₂SO . 10H O) ₄ ₂ 3CaO. Al₂O₃. 3CaSO₄. 31H₂O + 2Al(OH)₃ +
( ettringite) 6NaOH + 17H₂O
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• Calcium sulphate reacts with CAH to form calcium sulphoaluminate( ettringite).
• Magnesium sulphate reacts with Ca(OH)₂, CAH and completely decomposes CSH making it a friable mass (easy to crumble).
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Types
» External » Internal
External Sulphate Attack• Due to the penetration of sulphates from a solution (groundwater) into the concrete from outside.
• Composition and microstructure of concrete changes.
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• These changes may vary in type or severity but commonly include:
• Extensive cracking• Expansion• Loss of bond between the cement paste and aggregate
• This results in overall decrease in strength.
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Other sources of sulphate which can cause sulphate attackinclude:• Seawater• Oxidation of sulphate minerals in clay adjacent to the concrete - this
can produce sulphuric acid which reacts with the concrete• Bacterial action in sewers - anaerobic bacterial produce sulphur
dioxide which dissolves in water and then oxidizes to form sulfuric acid
• In masonry, sulphates present in bricks and can be gradually released over a long period of time, causing sulphate attack of mortar, especially where sulphates are concentrated due to moisture movement
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Scanning electron microscope image of sulphate attack in concrete.
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• Ettringite (arrowed) has replaced some of the calcium silicate hydrate in the cement paste.
• The darker areas of paste have been partly decalcified.
• As a consequence of these alterations, the paste will be weakened.
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Internal Sulphate Attack
• Due to source of sulphate being incorporated into the concrete at the time of mixing, while adding gypsum in the cement etc
• Proper screening and testing procedures should generally avoid internal sulfate attack.
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Delayed ettringite formation
• Delayed ettringite formation (DEF) is a special case of internal sulfate attack.
• DEF occurs if the ettringite which normally forms during hydration is decomposed, then subsequently re-forms in the hardened concrete.
• Sulphate ions released by decomposition of ettringnite are absorbed by CSH. Then sulphate ions are desorbed, reformation of ettringnite take place.
• Damage to the concrete occurs when the ettringite crystals exert an expansive force within the concrete as they grow.
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• DEF causes a characteristic form of damage to the concrete. While the paste expands, the aggregate does not.
Delayed ettringite formation: scanning electron microscope image of limestone aggregate particle
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• The cement paste has expanded and a gap has formed between between the aggregate and the cement paste.
• The aggregate is no longer contributing to concrete strength, since it is effectively detached from the cement paste.
• Often, these gaps become filled with ettringite.
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Conditions necessary for DEF to occur are:
• High temperature (above 65-70 degrees C approx.), usually during curing but not necessarily
• Water: intermittent or permanent saturation after curing• Commonly associated with alkali-silica reaction (ASR)
• Limestone coarse aggregate has been found to reduce expansion.
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Reference
• Concrete Technology by M S Shetty.
• Concrete, Microstructure, Properties and Materials by Metha, P K and Monteiro.
• Understanding Cement by Nicholas B Winter.
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