Deterioration of residential concrete foundations: The role of pyrrhotite-bearing aggregate

Abstract

Premature deterioration of concrete foundations has been reported for a large number of homes in Eastern Connecticut of the United States. The characteristic symptoms are map cracking, whitish deposits at the vicinity of the cracking surface and wide crack opening. It is hypothesized that the volume expansion associated with the oxidation of pyrrhotite-bearing aggregate and the following internal sulfate attack (ISA) due to the delayed secondary mineral formation are the primary causes of premature deterioration. Extensive tests were carried out to validate the hypothesis using a combination of tools such as scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and energy dispersive X-ray (EDX).The results showed that pyrrhotite and its oxidation products, such as ferrihydrite, goethite and sulfur, were present in or at the surface of the aggregates of the deteriorated concrete, suggesting the occurrence of pyrrhotite oxidation. Expansive secondary mineral formations (SMF) such as ettringite and thaumasite were also identified. The abundance of these SMFs, their spatial distribution in the open spaces together with their close association with matrix cracking suggest the cause of premature deterioration.