Deterioration Caused By Sulfide-Bearing Aggregates: Performance Test Development

Recently in Quebec Canada, concrete structures suffered very rapid deterioration within 3 to 5 years of construction. The deterioration was caused by an iron sulfide, namely pyrrhotite, in the coarse aggregate that suffered oxidation inside concrete and promoted sulfate attack; indicated by the presence of ferric oxyhydroxides (“rust”), gypsum, ettringite, and thaumasite. The goal of the current work was to reproduce this reaction under accelerated laboratory conditions, in progression of a performance test. Conditions to promote pyrrhotite oxidation and internal sulfate attack were provided; exposure cycles were tested with heating and cooling, and saturation in oxidizing agents or lime solution. Oxidation was induced in concrete samples, however, other mechanisms contributed to deterioration. The bleach was found to promote NaCl and Friedel’s salt formation, furthermore, it seemed to mitigate expansion from sulfate attack. Sulfoaluminate decomposition was also found to cause secondary ettringite formation. More optimization to the test methods was recommended.