"Rust comets"

[Tom Mazanec]

In the parking lot of my supermarket there are hundreds, if not thousands, of rust spots with "tails" trailing downslope. They look like little red comets every couple inches. They seem scattered at random, not in a regular pattern.
What are they?

[grant hutchison]

Rusting iron pyrites fragments in the aggregate used to produce the pavement. It's a common contaminant, and it will eventually produce rust streaks even through sealed or painted sufaces.
Some sample photos here.

Grant Hutchison

[Van Rijn]

I’ve never seen that. Is it just at a single parking lot? Do cars rust easily where you live? if it is just at the single location, I’d guess there are unique conditions, maybe bits of iron in the surface. If cars rust easily and it is common, I’d guess it comes from cars.

Where I live, it almost never snows, and almost never lasts on the ground if it does (it is decades between snow sightings) so no salt is used. I’ve often had relatives comment on surprise at seeing many good looking older cars and rarely any rust.

[Van Rijn]

Rusting iron pyrites fragments in the aggregate used to produce the pavement. It's a common contaminant, and it will eventually produce rust streaks even through sealed or painted sufaces.
Some sample photos here.

Now I’m wondering if I’ve just never noticed it, or maybe iron pyrites as a contaminant is much more common in some places.

[grant hutchison]

Now I’m wondering if I’ve just never noticed it, or maybe iron pyrites as a contaminant is much more common in some places.

I guess it depends on where you get your aggregate from. Common enough in these parts that I looked up the cause years ago.

Grant Hutchison

[Swift]

Now I’m wondering if I’ve just never noticed it, or maybe iron pyrites as a contaminant is much more common in some places.

My guess it is the later. Concrete makers, at least in the US (though I suspect this is true elsewhere) tend to use local sources for aggregate, as shipping can become the biggest cost of an otherwise cheap material. Given local geology, iron content will vary considerably.

I suspect the "comet" effect just indicates which way the water is flowing.

[grant hutchison]

My guess it is the later. Concrete makers, at least in the US (though I suspect this is true elsewhere) tend to use local sources for aggregate, as shipping can become the biggest cost of an otherwise cheap material. Given local geology, iron content will vary considerably.

Yes. The document in my link recommends that, if you have some particular reason to avoid rust streaks (the topic under discussion there is unsightly and distracting streaks on tennis courts), then you should source your aggregate carefully.

Grant Hutchison

[Van Rijn]

Thinking about it, I suspect it is a bit of both not noticing and location. I’ve lived in my current location for quite some time, but I’ve lived in four states, six distinct geographical locations and visited a good number of other places. Heck, years ago we had an RV, and drove through most of the western states back to Iowa. I doubt I looked closely at concrete though, especially if just visiting.

I will be looking around here to see if I notice it anywhere. I’m sure I would have noticed at my house though - I have quite a lot of concrete here.

[Swift]

Apparently it can be a much bigger problem than appearance. When the water reacts with the pyrite or pyrrhotite impurities, it forms hydrated iron oxide (rust), it also forms sulfuric acid or similar acids, which then react with the calcium carbonate in the concrete to form calcium sulphate, which are weaker also expanding, causing cracks in the concrete. I suspect the cracking allows more water to enter, and just accelerates the problem.

https://cfawalls.org/concretefacts/2...e-in-concrete/
https://lesgoudronsduquebec.com/pyrite-concrete

[John Mendenhall]

Apparently it can be a much bigger problem than appearance. When the water reacts with the pyrite or pyrrhotite impurities, it forms hydrated iron oxide (rust), it also forms sulfuric acid or similar acids, which then react with the calcium carbonate in the concrete to form calcium sulphate, which are weaker also expanding, causing cracks in the concrete. I suspect the cracking allows more water to enter, and just accelerates the problem.

https://cfawalls.org/concretefacts/2...e-in-concrete/
https://lesgoudronsduquebec.com/pyrite-concrete

Fascinating. I had a friend whose motto was, "Don't make your mistakes in concrete."

[grant hutchison]

I don't know about other countries, but there are permissible limits on the sulphate content of materials used to producing concrete in the UK, because of the structural problems high sulphate can cause, as detailed by Swift. The Concrete Society (yes, there is such a thing) says that natural aggregates in the UK rarely exceed the limit.
Of all the small blocks that make up our driveway and path, only one has produced a rust stain in the last thirty years. I find it disproportionately irritating.

Grant Hutchison

[profloater]

The iron in reinforcing rods (rebar) often protrudes and this of course can rust.if badly done it can also cause surface cracks by expansion, and these let water in. If salt water is involved , the cyclic drying and expansion of salt crystals can destroy the structure, an even bigger problem than rust stains.

[Tom Mazanec]

I live in northeast Ohio.

[grant hutchison]

I live in northeast Ohio.

Congratulations / commiserations [delete as applicable]

Grant Hutchison

[publiusr]

I have heard it said that some of the toughest concrete comes from Iran—which seems to have had bits of stainless steel from long ago as well.

Cinder comes have been lost to mixes in past years.

Were it easy to harvest, Moon dust might even be better.

[Tom Mazanec]

They just put new asphalt on the parking lot...you can’t see the comets anymore.