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Sulfate (sulphate) Content Testing

Introduction


Sulphates can be present in concrete either from an external source, usually soil in the case of buried concrete or, rarely, from contamination of the aggregates. Sulphates can attack concrete causing an expansive disruptive effect resulting in gradual deterioration or eating away of the cement matrix. The reaction occurs between the sulphate salts and the tricalcium aluminate (C3A) phase of the cement. The reaction product, if allowed to grow unhindered, is in the form of needle like crystals of ettringite (calcium sulphoaluminate). The crystals have a considerably larger volume than the reactants and, if growing in pores in restricted space, exert a bursting pressure on the concrete, causing cracking and disruption.

If an external source is involved, the problem can be tackled by the use of a high quality impervious concrete using either ordinary Portland cement or for higher sulphate levels, a sulphate resisting cement. Admixtures of Portland cement and pfa or granulated blast furnace slag may also be used. Advice in this respect can be found in BRE Digest 250 obtainable from HMSO.

The sulphate content of the concrete is determined by chemical analysis of the crushed materials.

The procedure is detailed in BS1881:Part 124.

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