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MIT scientists propose power storage using cement blocks
(www.datacenterdynamics.com)
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If it's just a byproduct of other industries, like existing coal power plants, it might be seen as carbon neutral. And lithium batteries also use it.
Carbon black is produced bij burning a hydrocarbon with a limited amount of air. It's not a byproduct, but uses organic materials. This can be of renewable sources like vegetable oil, but it is made a lot from the heavy fractions in fossil oil.