Iron sulfide-related internal sulfate attack: microstructure and mechanism

The internal sulfate attack of concrete, which is caused by iron sulfide, was investigated, and the attack mechanism is also proposed in this study. The specimens were collected from a bridge pier suffering from pop-outs and showing visible rust staining. The microstructure and chemical composition of the field and laboratory specimens were characterised by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. Iron sulfides were observed in the oxidised aggregates of the pop-out particles. Iron sulfates, the intermediate product, were found to form a condensed shell surrounding the aggregate. Various secondary products were identified and calcium silicate hydrates were degraded. According to the abundance, spatial location, morphology and associated cracking of the gypsum, it is suggested that the expansive stress is mainly induced by the gypsum. Cracking occurs under the combined effects of mechanical stress and lack of bond strength in the mortar. The deterioration of concrete is caused by iron sulfide oxidation following the internal sulfate attack.