10-1 The Role of Hexagonal and Monoclinic Pyrrhotite Phases in Concrete Stability

Session: Geochemistry and Mineralogy (Posters)

Presenting Author:

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Abstract:

The role of pyrrhotite (Fe_1-xS) in concrete deterioration is well understood, but as pyrrhotite (po) is a mixture of monoclinic (mcl) Fe₇S₈ and several hexagonal (hx) mineral phases (e.g., Fe₉S₁₀ or Fe₁₁S₁₂) it is unclear whether some pyrrhotite phases are more deleterious than others. Thermomagnetic analyses of concrete or aggregate can rapidly and accurately detect even small amounts of the mineral (as low as 0.1% by weight) and, in combination with direct measurements of Sulfur, allow for a semi-quantitative determination of pyrrhotite concentrations.

In a thermomagnetic analysis magnetic susceptibility (χ) is measured continuously between room-temperature and 700°C and the presence of pyrrhotite is usually determined through a decrease in χ at 325°C, the Curie-temperature of monoclinic pyrrhotite. Here we further analyze thermomagnetic curves to estimate the relative abundances of hexagonal and monoclinic pyrrhotite phases. We associate an increase in χ between 200°C and 220°C with the conversion of antiferromagnetic hexagonal po into ferrimagnetic phases and the decrease in χ near 325°C with the thermal demagnetization of monoclinic po.

We compared the magnetically estimated hx/mcl ratios of over 1000 foundation samples from Connecticut and Massachusetts to their observed deterioration status and found that overall pyrrhotite concentrations drive concrete stability. While monoclinic pyrrhotite phases appear to be slightly less stable than hexagonal ones, the ratio between these two phases plays a minor role in the deterioration of concrete foundations.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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