Concrete production actually does have crazy CO2 emissions. Wikipedia says it's up to 5% globally:
https://en.wikipedia.org/wiki/Environmental_impact_of_concre...
https://en.wikipedia.org/wiki/Environmental_impact_of_concre...
It seems to say that the reason for this is that it must be heated to absolutely insane temperatures, which is, obviously, energy-intensive.
5% is huge though. I still have trouble believing it. I wish I could see a pie chart with the major sources of CO2 emissions. After things like cars, planes, factories, POWER GENERATION, etc, I would have thought those were the major ones.
if you Google "pie chart of co2 emissions" it's hard to see where just concrete fits in. Oh well.