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Experts challenge thaumasite criteria

THREE KEY criteria said by the Department of the Environment, Transport and the Regions to be needed to trigger thaumasite sulphate attack in concrete have been disputed by experts this week.

The DETR had claimed that thaumasite only occurs in subsoil structures with an external source of sulphates and at low temperatures. The other key elements, which are not in dispute, are wet conditions and a source of calcium carbonate.

But two independent thaumasite experts have come forward with evidence to show that thaumasite can form in concrete above ground, using an internal source of sulphate and at relatively high temperatures.

Structural and materials consultancy STATS director, Dr Ian Sims, revealed on Friday that he had discovered a case of thaumasite in concrete above ground more than two years ago. The case is believed to have affected dozens of houses constructed using concrete blocks which were made with a fine-grained limestone aggregate. An internal source of sulphate was produced in the blocks by the biological oxidation of a pyrite impurity in the aggregate.

Golder Associates materials scientist Stephan Jefferis, who has spent 30 years investigating unusual chemical conditions, also claimed that low temperatures are not necessarily needed for thaumasite to form.

'Although thaumasite may form readily in cold conditions, say 4C, it can also form at higher temperatures but probably not above 25C,' he said.

Jefferis was one of the specialists to identify thaumasite four years ago in 70 year old grout surrounding a London Underground tunnel near Old Street station, despite the fact that temperatures were considerably above 5C. He described thaumasite crystals as fine 'straw-like' structures which can crack the concrete-cement matrix apart.

A DETR spokesman said any new evidence of conditions needed to produce thaumasite would be considered by the Thaumasite Expert Group, which is due to report in October.

Matthew Jones

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