If I was forced to make a bet, I would guess fly ash. https://patents.google.com/patent/US10669205B2/en?oq=US10669205B2 https://patents.google.com/patent/US106

If I was forced to make a bet, I would guess fly ash. https://patents.google.com/patent/US10669205B2/en?oq=US10669205B2 https://patents.google.com/patent/US10669205B2/en?oq=US10669... I think this is the patent, but they don't mention a "binder" per se. There are several claims specifically relating to the exclusion of "tar" which seems interesting. I think someone better versed in reading patents than I am might be able to make more sense of that. Tar just doesn't seem like their style. Maybe there are already claims on that or they are well known. But "organic matter", "tar", "cement" or "fly ash" still seem like possibilities. The patent also mentions "additive" (e.g. pigment / recycled toner) as improving the strength of the resulting bricks and reducing moisture uptake. It can be added in surprising amounts (5-20%). That doesn't seem very scalable though. Or, of course, they could have come up with something else in the meantime. Distinctly possible.