> You need to burn a lot of fuel to produce cement, which itself emits carbon during the calcination process. The CO2 emitted by the calcination process is re

> You need to burn a lot of fuel to produce cement, which itself emits carbon during the calcination process. The CO2 emitted by the calcination process is reabsorbed when the Portland cement sets. You won't get back the CO2 expended in the heating process, but the calcination itself is more or less CO2-neutral. It's roughly this: Making cement: CaCO3 (limestone) + (some other stuff, varying with the type of cement) + lots of heat -> CaO + CO2. Setting cement: CaO + H2O -> Ca(OH)2 ("portlandite"). Ca(OH)2 + CO2 (from atmosphere) -> CaCO3 + H20 (chemically much the same as the original limestone, but now set into a useful form). I'm too lazy to balance all the equations, and there are a lot of other things mixed into modern cements, but that's the basic idea. It seems unfair to charge cement with the CO2 driven off during manufacture, while ignoring the fact that an equal amount of CO2 is absorbed during setting.