Until thaumasite sulphate attack (TSA) was discovered in the foundations of an M5 overbridge in 1998 very few cases had been recorded worldwide. Concrete deterioration in areas with high sulphate ion levels in groundwater was universally attributed to what became known as ettringite sulphate attack.
This involves a long-term reaction between the sulphate ions and the calcium aluminate hydrates (C3A) in the hardened cement paste to form ettringite – expansive calcium sulphoaluminates that disrupt the cement matrix. However, a typical CEM 1 cement contains only about 10% C3A, which contributes little to concrete strength.
Specifying a cement with a low C3A content is the normal reaction to a site survey that discovers high groundwater sulphate levels.
Research now shows that even low C3A cement concretes can be vulnerable in very wet ground conditions – if a source of calcium carbonate is present, if temperatures are often below 15oC, and particularly if there is mobile groundwater.
TSA targets calcium silicate hydrates (C3S), the main strength-giving constituent of the cement matrix. As the C3S is slowly converted to the softer thaumasite mineral the concrete surface gradually erodes away, eventually exposing the reinforcement.
This is potentially more serious than ettringite sulphate attack, particularly as around 40% of UK structural concrete contains limestone aggregates, high in calcium carbonate. At least 60 cases of TSA have now been identified, mainly on the iron pyrites-rich Lower Lias and Kimmeridge Clays in the West Country, although TSA has also been found in the North East where concrete is in contact with pyrites-containing red shale colliery waste backfill.
Iron pyrites (FeS2) can oxidise, releasing sulphate ions into the groundwater. Disturbed Lower Lias clays have been shown to have accelerated rates of oxidation after being used to backfill excavations.
In 2001, BRE’s Special Digest 1, Concrete in aggressive ground advised that carbonate-containing aggregates can still be used in most cases, in combination with appropriate cement types. Research by the University of Sheffield suggests that the most effective cement contains 65% by weight of ground granulated blastfurnace slag – although a bitumen coating was equally effective.