Depends on if it can short its charge to ground I guess, and how fast. An uncharged concrete capacitor is of course pretty harmless even if it cracks, so perhaps you could pour some strain gauges into it so you get advance warning of impending problems.
I know nothing of the topic, but this was the type of thing I reflexively wondered about. Hopefully knowledgeable HNers can fill us in on the likely risks or downsides, which are not typically the focus of university press releases.
Carbon black is produced by burning petroleum productd, so that oughta bring on the poo-poo squad directly.
Would it be possible to produce carbon black from atmospheric CO₂?