Abstract
The oxidation of pyrrhotite and other sulfide minerals in concrete aggregates, leading to internal sulfate attack, is a major durability issue that has, in an increasing number of cases, resulted in the premature deterioration of concrete structures. Although the general principles of sulfide oxidation are established, the mechanisms of early-stage pyrrhotite oxidation in concrete remain insufficiently understood. This study presents a comprehensive mineralogical investigation of pyrrhotite oxidation using pristine aggregate from the Maskimo quarry (Canada), a series of laboratory mortar bar samples and a concrete field sample. The laboratory mortar bar specimens were subjected to accelerated expansion testing for 12, 27, and 52 weeks. The field sample was obtained from Trois-Rivières, QC, Canada, representing the final stage of ISA-affected concrete. Scanning electron microscopy (SEM)- energy-dispersive spectroscopy (EDS)-based automated mineralogy (AM), thermomagnetic analysis, and LECO sulfur quantification were applied to investigate the oxidation sequence and secondary mineral associations. Pyrrhotite alteration is controlled by the contrasting mobility of iron and sulfur during oxidation. Iron remains largely immobile, and AM revealed a progressive transformation of pyrrhotite from surface oxidation to near-complete replacement by Fe-oxides in the AMBT samples. In contrast, sulfur reacted to sulfate and migrated from the aggregate into the cement paste, resulting in the formation of gypsum, ettringite, and a Fe-O-S-Ca phase. The AM results suggest that secondary Fe-oxides and hydroxides are the primary contributors to volume increase and minor expansion during the initial phase of internal sulfate attack in the laboratory mortar bar samples. The identification and quantification of secondary phases using automated mineralogy are valuable tools for assessing aggregate reactivity, evaluating oxidation progression, and predicting ISA-related deterioration in pyrrhotite-bearing concrete structures.