Half of that is from the chemical transformation of limestone to quicklime, and half of it is from the fuel energy currently used to drive the transformation:

Half of that is from the chemical transformation of limestone to quicklime, and half of it is from the fuel energy currently used to drive the transformation: https://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-cement-industry/ https://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-... So if carbon-neutral fuels or clean electricity were substituted in the calcination process, the CO2 burden would be reduced by half without any additional downstream changes.