Cracking of residential concrete foundations in eastern Connecticut, USA from oxidation of pyrrhotite

Abstract

Extensive cracking in thousands of residential foundations in eastern Connecticut is found to be due to expansions from oxidation of pyrrhotite in crushed gneiss coarse aggregate in concrete. Sulfates released from pyrrhotite oxidation reacted with aluminous phases in paste to cause internal sulfate attacks and further expansions. These two-stage expansion processes took as long as 15 years of service in the presence of oxygen and moisture to develop extensive cracking to crumbling. The unsound pyrrhotite-bearing quartz-feldspar-biotite-garnet gneiss of Ordovician Brimfield Schist formation came from a quarry on a hydrothermal vein of significant pyrrhotite crystallization. Microstructural, chemical, and mineralogical evidences of pyrrhotite oxidation and resultant internal sulfate attack are presented from a residential concrete foundation in Mansfield, Connecticut. Ferrihydrite oxidation product of pyrrhotite, bands of oxidized iron in iron sulfide bodies, and microcrystalline fibrous secondary ettringite deposits intermixed with the cement hydration products in paste as well as deposited in cracks, voids, and porous areas of paste are the products of two-stage expansions that have contributed to the distress. Occurrences of numerous pyrrhotite bearing metamorphic rocks of the Appalachian mountains along eastern US pose concerns of similar distress of many homes with urgent need for standardized testing protocols to control pyrrhotite-related distress. A five-stage testing protocol is proposed to screen potentially deleterious pyrrhotite-bearing aggregates for mitigating this distress in future constructions.