Concrete deterioration due to sulfide-bearing aggregates: Mineralogical and chemical characteristics of pyrrhotite in concrete aggregates from Trois-Rivières, Canada

In Trois-Rivières, Canada, the use of sulfide-bearing aggregates has led to internal sulfate attack (ISA) that compromised the durability of the concrete foundations of many homes since the early 2000’s. This study investigates the mineralogical characteristics of both fresh and damaged concrete aggregates, focusing on the occurrence of pyrrhotite and its detrimental effects. Mesoscopic and microscopic analysis has shown that pyrrhotite distribution is largely influenced by the rock type, with gabbroic rocks showing the highest concentrations in the quarries studied. The main sulfide phases identified also include pyrite, chalcopyrite, and pentlandite, with pyrrhotite being the most affected by oxidation, exhibiting grayish bands and dark spots across the crystal surface. Magnetic colloid treatment revealed complex intergrowths of monoclinic (4 C) and hexagonal (NC) pyrrhotite, resulting from exsolution during crystallization. Electron probe microanalysis (EPMA) confirmed the occurrence of sulfur-rich pyrrhotite in fresh aggregates, while sulfur-poor pyrrhotite was prevalent in damaged concrete samples. These chemical disparities reflect the significant sulfur loss through oxidation over the service life of the structures. Sulfur was gradually removed from the pyrrhotite crystal structure and subsequently leached out of the aggregate particles. Sulfur isotopic analyses showed distinct δ 34 S values for the two pyrrhotite species, with higher values in 4 C pyrrhotite and lower values in NC pyrrhotite. This contrast is attributed to isotopic fractionation that occurred during the cooling of the rock unit and the formation of two generations of pyrrhotite crystals. Pyrrhotite isotopic signatures can influence its susceptibility to oxidation in the context of ISA in concrete. • Pyrrhotite distribution in Maskimo and B&B is controlled by rock type. • Colloid treatment distinguishes between magnetic and non-magnetic pyrrhotite. • Sulfur is released from the pyrrhotite structure long before visible signs develop. • δ 34 S-depleted NC pyrrhotite may be more prone to oxidation than 4 C pyrrhotite. • Optical and chemical assessments can identify pyrrhotite oxidation stage in concrete.