SULPHATE ATTACK

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.

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.

• Sulphate attack manifest in the form of expansion & cracking of concrete.

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

• 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).

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.

• 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.

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

 Scanning electron microscope image of sulphate attack in concrete.

• 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.

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.

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.

• 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

• 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.

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.

Reference

• Concrete Technology by M S Shetty.

• Concrete, Microstructure, Properties and Materials by Metha, P K and Monteiro.

• Understanding Cement by Nicholas B Winter.