[Gibbon1]

One of the inputs in cement manufacture is limestone which is Calcium Carbonate. That is ground and heated at high temperatures with clay minerals to produce portland cement.

So CO2 comes from the reaction between limestone and clay. And comes from burning natural gas or coal to generate the heat needed to drive the reaction.

[tachyonbeam]

So we use solar instead. We might not even need solar panels, we could potentially use one of those solar thermal reactors with an array of mirrors pointed to a central focal point. They can produce heat up to several hundred degrees C AFAIK.