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> They mix the CO2 with water and then pump that slurry deep underground where it eventually becomes solid rock.

How's that now?

Water plus carbon dioxide gives you a carbonic acid solution which when mixing with metal oxide-bearing rocks like basalt (rich in calcium and magnesium, etc.) forms stable carbonate minerals like calcite and magnesite.

This is high school chemistry stuff, right? Or was my poor Southern state public HS education actually exceptional?

The chemistry part is not surprising [1], but I have very litle idea of the composition of rocks, I expected them to be salts like silicates, carbonates, sulfates that don't react with carbonic acid. [2]

[1] I had a specialization in Chemistry in my high school, so I don't know it it's normal to know that.

[2] IIRC someone posted a project to make huge beaches of malaquite. (It would be weird/nice, becuse it's green.). The idea is that it reacts with the CO2, and the waves would do much of the work to break them in small pieces. If I guess correctly, the reaction is Cu2CO3(OH)2 + CO2 -> 2 CuCO3 + H2O

Possibly forming concrete underground? Doesn't concrete absorb CO2 while curing?

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