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Mixed reaction

THE THAUMASITE form of sulphate attack is potentially much more serious than the much better known ettringite form as it affects calcium silicate hydrates - the main strength-giving hydration products of Portland cement.

But recent investigation by the Building Research Establishment has concluded that the results from traditional tests for sulphate resistance led to any deterioration on site usually being blamed on the ettringite formation.

Ettringite is produced when sulphates react with calcium aluminate hydrates, which are a minority constituent of hardened cement paste and contribute little to concrete strength.

Around 10% of a typical modern Portland cement is tricalcium aluminate, usually referred to as C3A. On its own C3A would produce an immediate flash set when mixed with water, so cement manufacturers add small amounts of calcium sulphate, usually in the form of gypsum, to modify the C3A reaction and extend the setting time to practical levels.

When C3A reacts with water in the presence of gypsum it initially forms insoluble calcium sulphoaluminate, but with time this reverts to various forms of calcium aluminate hydrates. But, in the presence of sulphates from outside, these hydrates are converted to ettringite - expansive calcium sulphoaluminates - which disrupt the structure of the cement matrix and cause the concrete to crumble.

Should a site survey show high levels of sulphates in the ground the usual practice is to specify sulphate-resisting Portland cement, with C3A content limited to 3.5%.

But it has recently emerged that even when SRPC is used the concrete can be vulnerable to sulphate attack - provided that a source of calcium carbonate is present.

Originally it was believed that the calcium carbonate had to be present in a finely divided form, either as the dust from limestone aggregates or as the type of limestone filler which European cement producers have been adding for many years. However, recent research at BRE has shown that even large limestone aggregate particles can trigger thaumasite attack, especially at low temperatures.

Ironically, SRPC usually contains more of the vulnerable calcium silicate hydrates than OPC. In the right conditions of cold and damp these hydrates react with sulphates from the ground water and carbonates from the limestone to form soft translucent crystals of CaSiO3.CaCO3.Ca.SO4.15H2O - better known as thaumasite.

In its report published last year the BRE points out that 40% of all UK structural concrete contains limestone aggregates. And it concludes that the reason the problem had not been discovered before was that tests on sulphate resistance were traditionally carried out with flint aggregates at 20C - and so any deterioration on site was usually blamed on ettringite formation.

Dave Parker

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