Magnetic Properties of Pyrrhotite-containing Concrete from Northeastern Connecticut, U.S.A

A quarry in northeastern Connecticut provided pyrrhotite-containing aggregate that was used in concrete foundations. As pyrrhotite oxidizes with time it serves as a source of sulfate ions which can lead to internal sulfate attack and premature concrete deterioration. This study combines room-temperature measurements of magnetic susceptibility and magnetic remanence with susceptibility and remanence measurements performed at low (15K - 300K) and high (300K - 973K) temperatures to determine the differences in magnetic properties between deteriorating (pyrrhotite-bearing) and non-deteriorating (pyrrhotite-free) concrete samples. A better understanding of the magnetic properties may aid in the interpretation of existing data and the use of rapid, and inexpensive magnetic tools for pyrrhotite detection and quantification.

Pyrrhotite-bearing samples are characterized by relatively low magnetic susceptibility (χ < 300 × 10^-8 m³/kg) and high anhysteretic (ARM) and isothermal (IRM) magnetic remanence. Both ARM and IRM correlate strongly with sulphur and pyrrhotite content, while magnetite content is the primary control on magnetic susceptibility. ARM/IRM ratios are low (< 0.02) and unsuited to distinguish the magnetic mineralogy. Magnetic susceptibility, measured between 300K and 975K, show that most pyrrhotite exists in the ferrimagnetic monoclinic phase, which is corroborated by low-temperature analyses of magnetic remanence, which reveal a well-developed Besnus transition.

The systematic changes in magnetic remanence with pyrrhotite content suggests a relatively homogenous source and confirms that up to this date all affected homes were supplied with aggregate from one quarry. The observed low ARM/IRM ratios indicate coarse-grained (at least PSD, likely MD) pyrrhotite, which makes reliable detection of pyrrhotite through petrographic means feasible.