Pyrrhotite quantification in sulfide-bearing concrete aggregates: Insights into optical versus automated petrography

The presence of sulfur-containing aggregates in concrete can lead to significant durability issues, compromising the long-term performance of structures. However, due to their intrinsic properties, not all sulfur-bearing mineral phases react readily under concrete environmental conditions. Among these, pyrrhotite stands out as one of the most reactive sulfide minerals, posing a risk even in trace amounts. Therefore, the use of rapid and accurate methods for quantifying pyrrhotite in concrete aggregates is essential for ensuring structural integrity and resilience. Currently, no available technique can accurately quantify pyrrhotite according to industry needs due to limitations in resolution, accuracy, or cost. This study compares pyrrhotite quantification results from traditional optical petrography, a widely used industry method, with those from modern automated mineralogy techniques, including SEM-QEMSCAN® and ZEISS Mineralogic®. Reflected light microscopy analysis of aggregates containing varying amounts of sulfide minerals revealed that factors such as crystal size, spatial distribution, and textural fabric significantly influence quantification results. Additionally, depending on the methodology used, optical petrography can serve as a fast, cost-effective, and semi-quantitative approach for mineral phase assessments. However, the accuracy of results remains highly dependent on the expertise and experience of the operator. Automated mineralogy is a valuable tool for accurately selecting and assessing concrete aggregates, offering high resolution and efficiency by generating thousands of data points per sample. The overall sulfide values obtained from both techniques are largely consistent. However, discrepancies in pyrrhotite content are attributed to operational factors, particularly differences in the mineral classification libraries used by each technique.