MAGNETIC PROPERTIES OF PYRRHOTITE-BEARING CONCRETE

At this point Trinity College has analyzed over 1000 samples of concrete, concrete aggregate and pyrrhotite (Fe_1-xS)-bearing rock formations to characterize their magnetic properties. Measurements of magnetic susceptibility χ(T) between room-temperature and 700°C have been used reliable to detect and quantify the presence of monoclinic pyrrhotite (Fe₇S₈) which is common in concrete from Connecticut. We have since complemented our thermomagnetic analyses with X-ray analyses of magnetic extracts to better understand the details of thermomagnetic χ(T) curves. In addition, we have conducted analyses of rock-magnetic properties measured at room temperature to better discriminate between pyrrhotite-bearing and pyrrhotite-free specimens.

Simple concentration-dependent parameters, such as magnetic susceptibility (χ), saturation magnetization (M_S), anhysteretic remanent magnetization (ARM), or isothermal remanent magnetization (IRM) are poor predictors of pyrrhotite concentration as these parameters are often dominated by the presence of magnetite. Ratios between remanent magnetization and susceptibility (ARM/χ or IRM/χ) are significantly higher (ARM/χ > 100 A/m, IRM/χ > 15 kA/m) for pyrrhotite-bearing samples than pyrrhotite-free samples. Furthermore, pyrrhotite-bearing samples have hysteresis properties that are similar to samples containing single-domain magnetic particles.

X-ray analyses show that increases in magnetic susceptibility near 220°C are due to the conversion of antiferromagnetic hexagonal pyrrhotite into a ferrimagnetic phase as evidenced by the presence of characteristic pyrrhotite peaks between 43-45 degrees 2Θ (Cu – Kα). These analyses further our understanding of the thermomagnetic properties of various pyrrhotite phases and pave the way for the development of more rapid semi-quantitative analyses of pyrrhotite in natural and man-made samples.