One of the problems with calculating CO2 emissions from concrete is the process involves cooking off the CO2 out of a carbonate mineral, but when you actually use it to construct something, the chemical reaction to make it involves pulling that same CO2 back out of the atmosphere into the concrete as it cures. A big chunk of it in the first few days of curing, and in theory all the rest in a long tail of curing which takes decades (in practice not actually all of it).

So how do you account for this – temporary – CO2 emission when doing your calculations?

Often it depends on the story you're trying to tell... lying with statistics and all.