Chapter 3. Pyrite in Construction Aggregates – Using Trace Elements to Assess the Risks
Michael L. J. Maher, PhD, and Maxim Ryskin, PhD
Materials Engineering Group, WSP Golder, Whitby, Ontario, Canada
This chapter is in response to a recent spate of cases of structural damage in buildings caused by the use of aggregates that contain reactive sulfides, typically pyrite or pyrrhotite. Since these problems seem to predominantly hit residential construction, the huge financial and emotional impacts on residents gets the attention of governments. In addition to establishing how homeowners will get redress for the damages, governments also want to see protocols in place to provide confirmation that the damage can be attributed to reactive sulfides and to categorize the severity of the damage. For engineering practitioners, tasked with investigating the problems, they need to be able to apply the protocols so that objective and repeatable results are achieved. Since the development of pyritic damage is slow and progressive, the issue of risk of future more severe damage is a key challenge to establishing the scope and costs of remedial works. In this chapter, we describe two categorization protocols, one developed in Québec and the other in Ireland. While both these protocols have been successful in assisting engineers and geologists in investigating pyrite-induced structural problems, they have some limitations. Thus, most of the chapter is devoted to evaluating an investigative technique based on the geochemistry of the aggregate that may enhance current practices. The proposed analytical technique has promise to assist in the risk categorization of aggregates based on the fact that the reactivity of pyrite is established by its original depositional history. Using analysis of select trace elements, which are markers for the abundance, form and reactivity of pyrite, we hope to be able to establish the depositional history and in turn develop numerical thresholds to assist geologists and engineers in establishing the risk of future pyrite-induced expansion and its severity. The research objective was to explore whether there was a link between the concentrations of the selected redox-sensitive trace elements (Mo, U, V, Re, and Se) in aggregates and the Risk of Pyrite Expansion (RPE). The technique is demonstrated using test data from actual samples recovered from damaged and undamaged houses, as well as from source quarries. From this analysis, Mo and the ratio of Mo to U enrichments in aggregate, seem to provide the best options for distinguishing between RPE categories. Using the same laboratory test data, we also explore the possibility of using trace element data to match unknown aggregate samples with their source quarries. Traceability of aggregates is a key component of an investigation when performance problems arise. In the absence of chain of custody type documentation, a ‘fingerprinting’ methodology would be useful for matching aggregate samples to original source. Using sets of either 50 or 20 major and minor trace elements, good results have been achieved in differentiating aggregate samples from different quarries, even in cases where lithologies were generally similar.
Keywords: aggregates, categorization, pyrite risk, trace element analysis, source matching
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