Limestone used in it is baked at up to 1,450 degrees Celsius (2,640 degrees Fahrenheit) in enormous kilns that are fired almost exclusively with fossil fuels. The chemical reactions involved produce even more carbon dioxide as a by-product. Making one kilogram of cement sends one kilogram of CO2 into the atmosphere. Worldwide every year cement and concrete production generates as much as 9 percent of all human CO2 emissions.
— source scientificamerican.com | Feb 1, 2023