The role of biotite in the deterioration of pyrrhotite within concrete aggregates: Trois-Rivières case study

In Trois-Rivières, Canada, numerous commercial and residential structures suffered severe concrete degradation due to internal sulfate attack (ISA). The deterioration was attributed to the use of sulfide-bearing aggregates obtained from nearby quarries. The objective of this study was to investigate the mechanisms involving pyrrhotite oxidation and the role of biotite in this process. Detailed analysis, including reflected light optical characterization, mineral chemistry, and EPMA chemical mapping, revealed significant oxidation of pyrrhotite within damaged concrete, resulting in the formation of iron oxyhydroxides as secondary phases. Furthermore, a textural association between biotite mica and pyrrhotite was observed in fresh and damaged aggregate samples. While the chemical composition of biotite remains unchanged, it accelerates the deleterious reactions when in direct contact with pyrrhotite. As oxidation proceeds, naturally occurring biotite crystals near pyrrhotite act as pathways, facilitating the ingress of oxygen and water while simultaneously channeling the release of iron and sulfur compounds.