Forum Message



Name: Jay  Lukkarila
Heading: 1.1.2.2RE:Reactive Aggregates
Message:
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?

Thanks,

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Warren Coalson


Warren,
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.
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Alan Kirby


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


Warren/Brain,

There are many concrete experts within this forum that have far more experience than I,
(Alan is one of them) but the one thing that I can say about a pyrite aggregate within concrete is staining. Pyrite is FeS2 (iron sulfite). It is the iron that oxidizes (rusts) and stains the concrete surface with time. Once the concrete industry finds out about this - they will limit the use of your aggregate. People don't want their concrete block, buildings, retaining walls, and driveways constantly bleeding a series of rust stains. The asphalt guys are not as particular with their aggregates, as they encapsulate the aggregate with the asphalt oil. You could even sell rubber tires, slag, or Chat to the asphalt guys. Once the asphalt guys find out you can't sell the fine aggregates to the concrete industry, they will offer you what the sand is worth – practically nothing. No one said the asphalt guys weren't smart.

Lime is usually added to an asphalt mix to reduce the problem of stripping, which is the bond between the asphaltic oil and the aggregate. Hydrated lime is usually added to a concrete mortar mix by a mason to slow down the curing time, increase the workability, and waterproof the mortar. Hydrated lime normally has nothing to do with the type of aggregates.

I am not sure what the objections of which agency that you are referring to, but there usually is enough gray area within the specifications of most DOT state agency's that give them the right to reject a fine aggregate with a suspect mineral such as pyrite.

I would be very hesitant to utilize a fine aggregate with pyrite within concrete. Pyrite is usually not "measured" within an aggregate source but its presence alone sets off alarm bells.

Does anyone out there remove pyrite though their process to eliminate it?


Jay Lukkarila
Superior Processing
Aggregates Consultant
ICRUSHROCK@aol.com

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Jay Lukkarila
 
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