Sulfate attack

Sulfates are frequently present in soils, but the rate of sulfate attack on concrete is dependent upon the soluble sulfate content of the groundwater. Thus the presence of sodium or magnesium sulfate in solution is more critical than that of calcium sulfate, which is relatively insoluble. Soluble sulfates react with the tricalcium aluminate (C3A) component of the hardened cement paste, producing calcium sul-foaluminate (ettringite). This material occupies a greater volume than the original tricalcium aluminate, therefore expansion causes cracking, loss of strength and increased vulnerability to further sulfate attack. The continuing attack by sulfates depends upon the movement of sulfate-bearing groundwater and in some cases delayed ettringite formation may not be apparent for 20 years. Delayed ettringite formation is sometimes observed in precast concrete which has been steam cured, or when the temperature within the in-situ mass concrete has risen excessively during the curing process. With magnesium sulfates, deterioration may be more serious as the calcium silicates within the cured concrete are also attacked. The use of sulfate-resisting Portland cement or combinations of Portland cement and fly ash (pulverised-fuel ash [PFA]) or granulated blastfurnace slag (GGBS) reduces the risk of sulfate attack in well-compacted concrete. In the presence of high soluble sulfate concentrations, concrete requires surface protection.

The BRE Special Digest 1: 2005 describes provision for combating sulfate deterioration, including the more rapid form of attack in which the mineral thau-masite is formed. Thaumasite sulfate attack has seriously affected concrete foundations and substructures including some bridges on the UK M5 motorway. This type of sulfate attack is most active at temperatures below 15°C.

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