A thermomagnetic technique to quantify the risk of internal sulfur attack due to pyrrhotite

Abstract

The use of pyrrhotite-containing aggregate in concrete has led to the premature deterioration of building foundations in Connecticut (U.S.A.). Pyrrhotite is a highly reactive iron-sulfide mineral that can initiate internal sulfate attack in concrete structures, because it serves as a source of sulfate ions for secondary minerals. Associated increases in mineral volume can lead to spalling, cracking and a loss of structural integrity. Since pyrrhotite is a strongly ferrimagnetic mineral with a Curie-temperature of 325 °C and several diagnostic phase transitions, we measured the variation of magnetic susceptibility between room-temperature and 700 °C. Such a thermomagnetic measurement serves as a rapid and sensitive semi-quantitative indicator of pyrrhotite in aggregate or concrete samples. In combination with measurements of total sulfur concentrations, pyrrhotite concentrations as low as 0.1% can be detected. The analysis can aid in the quantification of risk of internal sulfur attack due to the presence of pyrrhotite.