Correlative optical, electron, and electrochemical microscopy assessment of localised sulphide reactivity in concrete aggregates

Abstract

Iron-sulphide-bearing aggregates can compromise concrete durability when reactive sulphide phases, particularly pyrrhotite (Fe_1–xS), oxidise and contribute to internal sulphate attack, cracking, and premature deterioration. Existing aggregate assessment methods, including bulk sulphur measurements, petrographic examination, oxygen consumption test, and accelerated mortar bar test, provide useful screening information but do not directly resolve phase-specific sulphide reactivity at the microscale. This study applies a correlative optical, electron, and electrochemical microscopy (OEEM) framework to evaluate pyrite–pyrrhotite reactivity in aggregate from the Trois-Rivières, Canada. Optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and supervised phase classification were first used to identify pyrrhotite, pyrite (FeS₂), pentlandite ((Fe-Ni)_xS_x), and silicate regions within polished aggregate sample cross-sections. AC-SECM was then used for mapping localised oxygen-consumption by monitoring the probe response above selected sulphide-bearing regions in aerated aqueous solution with and without chloride. In the absence of chloride, the pyrrhotite area exhibited lower probe-tip current and higher impedance than pyrite area, indicating stronger local oxygen depletion on pyrrhotite due to its greater electrochemical reactivity. After chloride exposure, the electrochemical contrast between pyrite and pyrrhotite was reduced, suggesting surface modification of these sulphide phases through dissolution and partial formation of hydrated iron oxyhydroxide products. These results demonstrate that AC-SECM, when correlated with SEM and EDS-based mineral identification, can provide spatially resolved insight into sulphide reactivity that is not accessible through bulk testing alone. The proposed OEEM framework offers a complementary microscale screening pathway that supports future standardisation of phase-specific reactivity assessment for sulphide-bearing concrete aggregates.