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Sulfates and concrete.

Sulfates react with concrete resulting in an expansive formation of ettringite or gypsum in hardened concrete causing cracking and exfoliation. If there is a continuous supply of sulfate, through movement of groundwater, the cracks will allow contact with more concrete surface and the reaction can lead to a softening and further disintegration of the concrete.

A rare form of sulfate attack is through the formation of thaumasite as a result of the reaction between calcium silicates in cement, calcium carbonate in limestone aggregates and sulfates usually from an external source.

This reaction causes the concrete to soften and progressively disintegrate from the contact surface. For this reaction to occur the following conditions must apply: temperature below 15oC; consistently high Rh; supplies of calcium, sulfate and carbonate; initial presence of reactive alumina (0.4 - 1.0%).

Sulfates can also be found in de-icing salts and can attack highway structures.

Acknowledgement: The Concrete Society

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TR54 Diagnosis of deterioration in concrete structures- identification of defects, evaluation an