The mineral at the heart of the problem is pyrrhotite, a non-stoichiometric iron sulfide with the general formula Fe1−xS, where x can range from zero to 0.125. When even less than one weight percent of this mineral is present in concrete aggregates, its oxidation can trigger a chain of reactions known as internal sulfate attack, or ISA. As pyrrhotite reacts with oxygen and moisture, it releases sulfate ions and acidic species into the surrounding cement paste. These ions then react with hydration products in the cement to form gypsum and ettringite, minerals whose growth generates expansive pressures that crack the concrete matrix. The Canadian standard CSA A23.1:19, Appendix P formally recognizes this combined mechanism of iron-sulfide oxidation and subsequent sulfate reaction as the driver of ISA deterioration.

The damage is not hypothetical. Aggregate from the Maskimo quarry in St. Boniface, Quebec, has been linked to severe ISA affecting more than 1,700 housing foundations in Trois-Rivières, Quebec, many of which suffered serious damage after only five to ten years in service. Earlier work by Rodrigues and colleagues identified pyrrhotite as the principal reactive mineral phase in the damaged Trois-Rivières concrete. In the Maskimo rock, pyrrhotite and pyrite occur in an approximate ratio of one to two, with pentlandite and chalcopyrite frequently found as accessory inclusions within them. The quarry itself consists of a noritic microgabbro dominated by plagioclase feldspar, orthopyroxene, magnesiohornblende, and biotite, with total sulfur contents ranging from 0.85 to 1.8 weight percent, all bound in sulfides.

To capture the deterioration in action, a team led by Nikolas Å. Oberhardt of the Norwegian University of Science and Technology, together with colleagues including Klaartje De Weerdt, examined a sequence of samples spanning the entire life cycle of the reaction. They studied fresh Maskimo aggregate, mortar bars subjected to accelerated mortar bar testing for 12, 27, and 52 weeks, and a severely damaged concrete drill core taken from a Trois-Rivières foundation wall. The accelerated test, described in the Canadian standard, involves repeatedly immersing mortar prisms in sodium hypochlorite bleach, twice weekly for three hours, with interim storage first at 80 degrees Celsius and 75 percent relative humidity to accelerate sulfide oxidation, and later at 4 degrees Celsius and full humidity to promote thaumasite formation. The mortar bars expanded by 0.22 percent after 12 weeks, 0.27 percent after 27 weeks, and 0.47 percent after 52 weeks.

The analytical centerpiece of the study was scanning electron microscopy-based automated mineralogy, which uses quantitative energy-dispersive spectroscopy to classify every pixel of a polished thin section into a mineral phase. The researchers deployed several analytical modes: coarse field mapping at 10 and 20 micrometer step sizes for bulk mineralogy, line scanning at 1 micrometer for quantitative bulk composition, and high-resolution field mapping at 2 micrometers to resolve the fine-grained oxidation products and cement hydration phases. Complementing this, thermomagnetic analysis measured the magnetic susceptibility of powdered samples from room temperature to 650 degrees Celsius, allowing the team to detect and quantify two pyrrhotite superstructures, the hexagonal antiferromagnetic NC-type and the monoclinic ferrimagnetic 4C-type, using the characteristic lambda transition near 220 to 245 degrees Celsius and the Curie point at 315 degrees Celsius.

The results reveal a strikingly ordered oxidation sequence. In the pristine aggregate, pyrrhotite grains showed only incipient surface oxidation along mineral boundaries, with minor magnetite, goethite, and slightly oxidized pyrrhotite as the characteristic secondary phases. After 12 weeks of accelerated exposure, oxidation had penetrated the mineral interiors through etch pitting, and an oxidation rim of iron oxides, mainly hematite and magnetite, had grown around the grain peripheries. Ferrihydrite, an iron oxyhydroxide, appeared at around 1.4 weight percent, typically adjacent to the iron oxides rather than directly on the pyrrhotite. By 27 weeks, the grain interiors were severely altered and largely replaced by iron oxides and oxyhydroxides, leaving pyrrhotite only as relic-like structures. After 52 weeks, the crystals were almost entirely consumed, with ferrihydrite rising to 1.79 weight percent and ettringite increasing from 0.13 to 0.37 weight percent.

The element heat maps told an equally important story about where the atoms went. Iron remained largely immobile, locked in place within the relic zones of the oxidizing grains as iron oxides and oxyhydroxides replaced the original sulfide. Sulfur, by contrast, leached away as sulfate anions into the cement paste, and the sulfur maps showed a progressive increase in sulfur concentration within the cement matrix as exposure time lengthened. In the field sample, the cement paste adjacent to reacted aggregates contained as much as 10 to 12 weight percent sulfur. The team also identified a cryptocrystalline composite phase, labeled FeOSCa, averaging roughly 34 percent oxygen, 10.5 percent sulfur, 24.5 percent calcium, and 31 percent iron, which formed along cracks and near oxidizing pyrrhotite and may warrant further study with transmission electron microscopy or electron probe microanalysis.

Comparing the laboratory samples with the field concrete proved especially revealing. The field sample contained between 1.06 and 2.30 weight percent pyrrhotite, far more than the 0.19 to 0.37 weight percent measured in the mortar bars and the 0.50 weight percent in the fresh aggregate, underscoring the link between the amount of reactive pyrrhotite available and the magnitude of expansion. The field sections showed gypsum and the FeOSCa composite phase together reaching approximately 1.87 weight percent, against less than 0.05 weight percent gypsum in the laboratory samples, along with distinct cracking in the interfacial transition zone between aggregates and paste. The mortar bars, by contrast, showed little to no expansion cracking, suggesting that their measured expansion stemmed primarily from the formation of secondary iron oxides and hydroxides rather than from ettringite-driven paste expansion. Thermomagnetic analysis indicated that roughly two-thirds of the pyrrhotite in all samples was the hexagonal NC-type, a variant previous work suggests is more prone to oxidation than the monoclinic 4C form.

The study also carries a practical warning for laboratories running accelerated tests. Sulfur measurements by high-temperature combustion showed that the aggregate batches used in the Norwegian pilot testing contained only 0.44 to 0.51 weight percent sulfur after cement paste removal, considerably lower than the 0.85 to 1.08 weight percent reported for the reference Maskimo aggregate, which plausibly explains why the Norwegian mortar bars expanded less than Canadian counterparts tested under the same conditions. The authors recommend routinely determining total sulfur content in every test batch before accelerated mortar bar testing to ensure results are comparable and not misinterpreted. They further note that pre-oxidation of sulfide minerals, whether in the geological deposit, the quarry stockpile, or after concrete production, could accelerate sulfate attack, making the degree of pre-oxidation an important quality assurance consideration.

Beyond its immediate findings, the work demonstrates that automated mineralogy can serve as a diagnostic tool for assessing aggregate reactivity and predicting ISA-related deterioration in pyrrhotite-bearing concrete. By correlating a continuous transition from incipient surface alteration to complete pyrrhotite replacement with distinct secondary mineral assemblages, magnetite and goethite in fresh rock, ferrihydrite, gypsum, ettringite, and the FeOSCa phase in mortar and concrete, the researchers have established a mineralogical fingerprint of internal sulfate attack at each stage. Although the study drew on a single, well-characterized aggregate source, the authors expect the observed oxidation progression to be relevant to other ISA cases, noting that reactive aggregates from regions such as Connecticut exhibit comparable mineralogical characteristics. For the homeowners of Trois-Rivières and engineers elsewhere facing similar mysteries of crumbling concrete, the invisible enemy has finally been caught in the act.

Subject of Research: Pyrrhotite oxidation and internal sulfate attack in concrete aggregates and cement paste

Article Title: Pyrrhotite oxidation in aggregate and cement paste of accelerated laboratory and concrete field samples

Article References: Oberhardt, N. Å., Danner, T., Lode, S., Lindgård, J., Aasly, K., & De Weerdt, K. (2026). Pyrrhotite oxidation in aggregate and cement paste of accelerated laboratory and concrete field samples. Case Studies in Construction Materials, 25, Article e06544. https://doi.org/10.1016/j.cscm.2026.e06544

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06544

Keywords: pyrrhotite, internal sulfate attack, concrete deterioration, automated mineralogy, cement paste, Trois-Rivières, Maskimo quarry, thermomagnetic analysis, iron sulfide oxidation, ettringite, ferrihydrite, accelerated mortar bar test