TROJAN HORSE SULFIDE, PYRRHOTITE IN AGGREGATE CAUSING THE FAILURE OF CONCRETE FROM WITHIN

Failure of concrete foundations from the weathering of the iron-sulfide mineral, pyrrhotite, is an extreme reminder that geologic materials in disequilibrium with conditions at the earth’s surface can drive chemical reactions with structural consequences. Quarried and crushed stone aggregate has little time to equilibrate with our oxidizing atmosphere before incorporation in concrete. In northeastern Connecticut, stone from a local quarry containing pyrrhotite was used locally as aggregate in the construction of house foundations. The iron in pyrrhotite, present as both Fe⁺³ and more-reduced Fe⁺², has a greater affinity for oxygen than sulfur. The resulting cascade of reactions results in the breakdown of the mineral, of the soundness of the aggregate, and of the binding cement while substantial increases in product volumes lever open the structure accelerating deterioration. This has brought into focus the importance of evaluating stone to be used in aggregate and the applicability of methods and reference materials available for analysis of sulfur and sulfide/sulfate ratios. Open issues include how methods of construction might accelerate or retard the progression of this failure mode. Details of the breakdown process as well as advances in the investigation into the susceptibility of aggregate, methods of analysis, and potential effects of construction techniques and strategies for remediation, are likely to be informative to projects across a range of scales where crushed-stone aggregate is used in concrete.