PYRRHOTITE IN CRUMBLING CONCRETE FOUNDATIONS: THE LATEST SOCIETAL USE FOR MODERN GEOLOGIC MAPPING IN CONNECTICUT

Concrete foundations in untold numbers of homes in eastern and central Connecticut are crumbling, due in large part to weathering of pyrrhotite and other sulfide minerals present in coarse aggregate of the concrete mix. An investigation sponsored by the CT Office of the Attorney General (Wille and Zhong, 2016) validated the chemical weathering sequence where pyrrhotite oxidation leads to the formation of expansive minerals, ferrihydrite and sulfate. Further reactions, involving gypsum in the presence of tricalcium aluminate in cement and carbon from calcite or environmental CO₂, form ettringite and thaumasite. These secondary expansive minerals cause horizontal cracking in concrete walls, which propagates outward and upward, and lift homes off their foundations. Accelerated degradation by infiltration of moisture along cracks and within enlarged pore spaces, increases potential for catastrophic failure.

Rock aggregate in the failing concrete foundations was largely mined from a single quarry, working a stratified metamorphic unit in eastern CT mapped as Ordovician Brimfield Schist. The gray, rusty brown to orange yellow weathering rock, is a medium to coarse grained, interlayered schist and gneiss, composed of oligioclase, quartz, K-feldspar, biotite and commonly garnet, sillimanite, graphite, and pyrrhotite. Iron sulfides of the unit are associated with acid drainage, which degrades surface and groundwater quality.

The Connecticut Geological Survey has provided assistance to the Governor’s Office, US Army Corp of Engineers, and the CT Congressional delegation, in the assessment of the potential scope of the issue. This includes providing maps of pyrrhotite and other sulfide bearing rocks as well as information on bedrock chemistry. Modern geologic map projects, integrating mineralogy, petrology, rock fractures, structural data, and chemistry, are contributing to the growth of a statewide geochemical database being compiled by the Connecticut Survey. This initiative will document the distribution of major and trace elements in bedrock, including metals, sulfides, and others of environmental concern. These geoscience data serve many societal applications, including environmental quality investigations, resource assessments, economic interests, and consumer protection.