Sulphate Attack

Sulphate attack has long been recognised as responsible for concrete deterioration in a wide variety of structures.

Sulphate attack in concrete has been known to occur when sulphate solutions, derived either from a constituent in the concrete such as aggregate or from external sources such as groundwater, react with the calcium aluminate hydrates present in the hardened cement to form the hydrated calcium sulfoaluminate known as ettringite which can occupy over twice the volume. This reaction can inflict serious damage on concrete, leading to weakening and perhaps ultimately failure of the affected structure.

Recent investigation of concrete structures has identified another form of sulphate attack which has taken place in concretes which were specifically designed to provide sulphate resistance. In this type of deterioration sulphate solutions react with calcium silicate hydrate phases in the presence of calcium carbonate ions within the hardened cement paste to form the mineral thaumasite. This mineral is a more complex salt than ettringite and forms at low temperatures (below 15 C) and is associated with the presence of finely divided limestone sometimes used as filler, though limestone aggregate may also promote the reaction. Thaumasite formation renders the cement paste soft with concomitant loss of strength and disintegration of the concrete.

Sandberg have a vast experience in concrete investigation and our UKAS accredited laboratories can provide a wide range of services to assess concrete condition.

A comprehensive package of site investigation and laboratory techniques to appraise this newly appreciated mechanism is offered. Laboratory examination includes petrographic and other analytical techniques, including scanning electron microscopy (SEM) with electron probe microanalysis and X-ray diffraction (XRD) facilities also available where necessary.