The occurrence of thaumasite as a product of seawater attack

This paper describes the mechanism of the rapid deterioration caused by seawater attack on the bedding mortar of recently constructed harbour wall steps in South Wales. Within 2 years, the mortar had suffered severe cracking and spalling. The degraded mortars were examined under optical and electron microscopes and also using X-ray diffraction analysis. The reaction products formed included thaumasite, ettringite, brucite and hydrated magnesium silicate. A further end product of deterioration was the precipitation of large, non-interlocking crystals of calcite (termed popcorn calcite) in the areas of the mortar previously occupied by cement paste. Investigation of degradation of associated structural concrete from this site also showed the early stages of the thaumasite form of sulfate attack.