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Name: Alan  Kirby
Heading: Aggregates
Our company is looking at a site for use in concrete and asphalt aggregates that has finely diseminated pyrite crystals in it. The pyrite occurance is quite low (as a percentage of the total volume), but none-the-less it is present. We are attempting to have the site classified as an important aggregate resource, but are running into difficulty with agency staff due to the presence of pyrite. Questions we have are related primarily to the suitablity of the rock for use with Portland Cement Concrete. We have some references, but they are all quite vague and fall short of making recommendations for testing.

Can anybody help us with specific studies and/or research data that can provide us with a pathway to determine the suitability of these resources for PCC use?


Warren Coalson

Not all forms of pyrites are reactive. You can check for reactivity by placing the aggregate in a saturated solution of lime. If the aggregate is reactive, a blue-green precipitate of ferrous sulphate appears within a few minutes - see AM Neville's Properties of Concrete.
Alan Kirby

Will a hydrated lime solution work? And will this work with all aggregate types?
Brian Burr

I don't know if a hydrated lime solution will work - you may have to ask a chemist. I believe that the saturated lime solution test will work for all aggregate types. There may be more information in the reference that Neville quotes - The staining of concrete by pyrite, HG Midgley, Magazine of Concrete Research Vol. 10, No. 29.
For completeness of my original response (to Warren), I should have added that the specification used for concrete aggregates in the US is generally ASTM C-33. Appendix XI, which is "nomandatory information", gives details on Methods for Evaluating Potential Reactivity of an Aggreagate", but does not specifically mention pyrites.
Alan Kirby
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