HIGH RESOLUTION MEASUREMENTS OF PYRRHOTITE AND OTHER MINERALS IN COARSE CONCRETE AGGREGATE

Pyrrhotite (Fe_1-xS, 0<x<0.125) is known to take part in deleterious chemical reactions with the hydrated ordinary Portland cement paste that binds concrete together. The series of chemical reactions that start with pyrrhotite reacting with water and oxygen inside concrete have damaged thousands of homes and other buildings in North America. While the basic reaction process is known, there are gaps in the knowledge of how pyrrhotite damages concrete. These gaps are particularly challenging because studies have shown that a small amount (0.23% by volume) of pyrrhotite in concrete can cause damage. Some recent results have also suggested that other metallic minerals in the aggregate may contribute to the damage process.

One of the activities in the project “Impact of Aggregates Containing Pyrrhotite and Other Sulfides on Canadian Concrete Structures” has therefore involved investigating the use of alternative methods for measuring pyrrhotite and other mineral content in coarse concrete aggregate. This presentation provides results from two such methods. High resolution, quantitative X-ray diffractometry was carried out at the Canadian Light Source synchrotron facility. Commercial mineralogical samples were used to develop a calibration curve for the measurements and confirm their accuracy. Samples of aggregate taken from two locations in the Maskimo quarry, a known source of pyrrhotite bearing concrete aggregate that is located near Trois-Rivières in Quebec, were then analysed to determine their mineralogy.

Pump probe microscopy, which was originally developed to investigate femto-second chemical and biological processes, has recently been employed at the National Research Council Canada as a tool for investigating geological samples. In a transient-reflectivity mode, the method provides surface area information similar to that from optical petrography, with good potential for process automation.

In addition to describing the approach taken to the measurements and presenting results, the presentation will discuss how the data from the measurements is being used in the overall project and the potential for further work with each method.