Sulphate Attack

• The resulting crystallised products from the reaction cause an increase in the volume of the hardened concrete & contribute to internal disruption or spalling of the concrete .
• Concrete attacked by sulphates has a characteristic whitish appearance with progressive cracking & eventually the concrete may become soft & friable .
• Rate of sulphate attack depends on :
• The % of tricalcium aluminate in the cement
• Higher % higher rate
• Lower % lower rate
• The permeability of concrete , higher rate
• Higher permeability , higher rate
• Lower permeability , lower rate .
• THe solubility (ability to dissolve ) of the sulphates .
• Higher solubilityu , higher rate
• Lower solubility , lower rate
• The concretration of the sulphate solution .
• Higher strength , higher rate .
• Lower strength , lower rate .
• The rate at which the sulphate removed by the reaction with cement can be replenished concrete which is exposed to the pressure of sulphate-bearing water on one side experiences a higher rate of attack .
• The vulnerability of concrete to sulphate attack can be reduced by :

1. the use of cement low in tricalcium aluminate such as sulphate resisting cement .
2. Replacing some of the cement with pozzolanas ( such as pulverised fuel ash or PFA ) which tend to removed free calcium hydroxide & render the attack inactive .
3. The vulnerability of concrete to sulphate attack can be reduced by
4. For concrete structure in sulphate-bearing soils , protective coatings such as bitumens , tars & epoxy resins could be applied on exterior surfaces , although some of these coatings may be eroded away by groundwater flow .
5. Using conctete of higher grade to improve permeability .