X-RAY DIFFRACTION ANALYSES OF SULFATE MINERALS RESPONSIBLE FOR CRUMBLING CONCRETE IN NORTHEASTERN CONNECTICUT

Pyrrhotite (Fe_1-xS) is a structurally complicated and reactive sulfate mineral that is believed to responsible for the premature deterioration of concrete. Often referred to as internal sulfate attack, the transformation of pyrrhotite to expansive minerals (e.g. ettringite and thaumasite) can lead to structure failure. In northeastern CT, quarry aggregate containing significant pyrrhotite (>1%) was used to build hundreds (if not thousands) of new construction projects from the early 1980s to as recently as 2016. Current methods to identify pyrrhotite have not been effective in providing quantitative results. Consequently, there is not an established standard for the minimum concentration of pyrrhotite in concrete structures. Petrographic analyses via polished section observations are time consuming and not reliable for samples that may contain very low (<1%) concentrations that could still be susceptible to degradation. X-ray diffraction (XRD) analysis is a method that can be useful to identify pyrrhotite, ettringite and thaumasite, but since concentrations are often low, XRD should be combined with other complementary independent analyses including direct sulfur and magnetic susceptibility measurements. With high enough resolution, it may be possible to use XRD scans to subtract out non-pyrrhotite sulfate minerals and attempt to precisely quantify pyrrhotite concentrations in afflicted concrete.