Earlier this month, researchers, engineers, and laboratory scientists gathered at the Hilton Mystic in Connecticut for the 2nd International Conference on Iron-Sulfide Reactions in Concrete (ICISR 2026). Among the technical papers presented was Petrographic Methods to Identify and Quantify Iron-Sulfide Minerals in Aggregate, co-authored by Michael Baker and Chesney DeTullio of RJ Lee Group, and April Snyder and Blake Restelli of ECS Mid-Atlantic.
The two-day conference brought renewed attention to a question the concrete industry is still working to answer: How do you reliably find something that makes up less than 0.2% of an aggregate but can ruin a foundation if missed?
Connecticut's response to the foundation crisis—codified in Public Act 21-120—limits total sulfur in concrete aggregate to 0.1% by weight. In mineralogical terms, that translates to a pyrrhotite content as low as 0.15% by weight.
That's an extraordinarily small needle in an extraordinarily large haystack. And the standard petrographic toolkit, as useful as it is for characterizing aggregate lithology, was not built to identify it.
ASTM C295, the Standard Guide for Petrographic Examination of Aggregates for Concrete, is the workhorse standard for aggregate characterization. It's well-suited to identifying rock types, textures, and constituents that may affect concrete performance. But the method is primarily oriented toward transmitted-light thin-section analysis — and iron-sulfide minerals are opaque in transmitted light.
For trace pyrrhotite detection, that creates three compounding problems:
ASTM C295 does permit the use of reflected light and SEM when needed—but the standard doesn't prescribe a workflow for trace-level identification and quantification. That gap is precisely what ICISR 2026 was convened to discuss.
Among the technical papers presented at the conference was a collaboration between RJ Lee Group and ECS Mid-Atlantic, Petrographic Methods to Identify and Quantify Iron-Sulfide Minerals in Aggregate. The paper lays out a practical methodology that builds on ASTM C295 rather than replacing it, using two complementary techniques in sequence.
First, reflected-light microscopy at up to 1000×. Polished pucks of representative coarse and fine aggregate are examined for the optical signatures that distinguish pyrite (yellow-white, smooth, weakly anisotropic), pyrrhotite (creamy pinkish-brown, pitted, pleochroic), chalcopyrite (brassy yellow), and pentlandite (creamy white with a pinkish tint, isotropic). For coarse aggregate, every particle larger than 3 mm is numbered and mapped, creating a consistent reference frame for the analysis.
Second, SEM/EDS with backscattered electron imaging and x-ray phase mapping. This stage confirms optical identifications and resolves features that reflected light alone cannot: framboidal pyrite morphology, micron-scale intergrowths, and compositional variations within single grains. EDS x-ray maps then color-code individual sulfide phases by their elemental signatures, supporting quantification by area percent.
The final step closes the loop with the regulatory threshold. The Delesse principle holds that the area fraction of a mineral in a random polished cross-section equals its volume fraction in the bulk. This allows area-percent measurements to be converted to weight percent using mineral density and bulk aggregate density. Paired with independently measured total sulfur values, the method yields a defensible estimate of pyrrhotite content specifically, rather than a generic "sulfides present" finding.
The methodology presented at ICISR 2026 indicates the direction this field is headed. ASTM C295 remains the foundation. But for projects in regions with known iron-sulfide concerns, building reflected-light microscopy and SEM/EDS analysis on top of the standard turns an inconclusive screen into an actionable measurement. That's the difference between "sulfides detected" and "pyrrhotite quantified."
If your project uses aggregate from regions with known iron-sulfide concerns, the right analytical approach depends on the details: aggregate type, regulatory context, and the decisions the results need to support.
Contact our petrography experts to discuss the right approach for your project.