Thermomagnetic susceptibility as a quick screening measure and quantification of pyrrhotite in concrete aggregates

This study investigates thermomagnetic analysis (TMA) as a rapid screening and quantification tool for detecting pyrrhotite polytypes in concrete aggregates with concentrations as low as 0.1% by weight. Pyrrhotite, particularly in its hexagonal (NC-type) and monoclinic (4C-type) forms, can cause expansive deterioration reactions in concrete when oxidized. Current quality assurance methods, such as differential thermal analysis, optical microscopy, and total sulfur measurement, lack the specificity and resolution to distinguish between pyrrhotite polytypes and concentrations, leading to either underestimation of the risk of not detecting potentially internal sulfate attack reactive aggregates and, on the contrary, to inflate the disposal rates of viable aggregate raw materials. Here, thermomagnetic analysis measures magnetic susceptibility changes across heating cycles to identify and quantify pyrrhotite polytypes based on their magnetic transitions. Pure NC- and 4C-type pyrrhotite reference samples were analyzed and used to develop a TMA quantification correlating magnetic susceptibility peaks with pyrrhotite concentration. Thermomagnetic analyses of crushed aggregates from Norway and North America were tested and compared with results from electron microprobe analysis and automated mineralogy. The proposed quantitative model for the estimation of the pyrrhotite concentration, using single-grain NC- and 4C-type pyrrhotite, describes a novel approach to calculate the pyrrhotite content by weight % in bulk samples. The findings demonstrate that thermomagnetic analysis effectively differentiates between pyrrhotite polytypes reliably and estimates their relative content within acceptable margins compared to automated mineralogy. Thermomagnetic analysis offers a fast, low-cost complement to current testing protocols. Therefore, adopting thermomagnetic analysis as a standard method could help to significantly improve material assessment accuracy, reduce unnecessary aggregate disposal, and mitigate ISA risks in concrete infrastructure. • Thermomagnetic susceptibility analysis is a reliable, sensitive method to detect pyrrhotite. • The Kappabridge can identify pyrrhotite concentrations as low as 0.1% by weight. • TMA offers a rapid and cost-effective protocol for detecting and quantifying NC- and 4C-type pyrrhotite. • Relative pyrrhotite concentrations can be calculated in weight % using the presented model.