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Comment by silisili | Hacker News Reader

Interesting. I went looking for studies after it was found that Roman concrete had volcanic ash in it.

Apparently, adding wood ash to concrete actually makes it much weaker. So I'm wondering which forces are at play here.

Volcanic ash is glass (the stone melts during an eruption due to the heat, then explodes into millions of splinters as clouds which lock down air travel because it melts again in the engines and destroys them) and can be found thousand of kilometers away afterean eruption (e.g. Yosemite eruption), creating a deadly environment for every animal where it's falling down.

Thank you for coming to my ~~Ted Talk~~ infodump.

It's very different from wood ash.

Thus, the question remains why "coffee grounds", which in this case should be the same as wood ash, add strength.

Pyrolysis isn't exactly burning stuff, it's carbonizing it by heating without access to air. So no, it's not the same as wood ash because it's not ash.

Wood ash isn't carbon. It's mostly remnant minerals like sodium, potassium, and calcium after the carbon has burned away (hence "potash" = "pot ash" as potassium's namesake).

The proper comparison here would be charcoal.

(Volcanic ash, on the other hand, contains common rock-forming elements: silicon, magnesium, and iron, with the proportions depending on the type of volcano.)

Worth noting that wood ash becomes corrosive when you add water which is probably bad for the concrete.

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