Sulphate attack is the reaction taking place between concrete and sulphur.

1. SULPHATE
ATTACK

2. 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.

3. 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.

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

5. Reactions
Reactions on hardened cement paste
• Sodium sulphate attacking Ca(OH)₂
Ca(OH) +Na SO .10 H O CaSO .2 H O +2NaOH +8H O₂ ₂ ₄ ₂ ₄ ₂ ₂
• Reaction with Calcium aluminate hydrate
2(3CaO . Al O . 12 H O) + 3(Na₂ ₃ ₂ ₂SO₄. 10H₂O)
3CaO. Al₂O₃. 3CaSO₄. 31H₂O + 2Al(OH)₃ +
( ettringite) 6NaOH +
17H₂O

6. • Calcium sulphate reacts with CAH to form calcium
sulphoaluminate( ettringite).
• Magnesium sulphate reacts with Ca(OH) , CAH and₂
completely decomposes CSH making it a friable mass
(easy to crumble).

7. 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.

8. • 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.

9. Other sources of sulphate which can cause sulphate attack
include:
• 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

10. Scanning electron microscope image of sulphate attack in concrete.

11. • 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.

12. 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.

13. 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.

14. • 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

15. • 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.

16. 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.

17. Reference
• Concrete Technology by M S Shetty.
• Concrete, Microstructure, Properties and Materials by
Metha, P K and Monteiro.
• Understanding Cement by Nicholas B Winter.