The CO2 emission is linked to the chemical process of cement, usually 0,9 ton of CO2 for 1 ton of cement. This is mostly because of the furnace (kiln at 1800°) and the CaCO3 (calcium carbonate)...

The world production of cement (concrete mostly) is about 1 cubic meter per person and it is increasing!

To reduce CO2 emissions, several paths:
Linked to the process:

• Electrical furnaces in nuclearized countries...
  
• Solar furnaces...
  

Linked to the use:
* Less cement in concrete... That's the "new concretes", using more fly ashes and additives (chemicals). The ratio of cement in the final product is optimised and reduced, as the use of water (less quantities).

Some future paths:
Calcia's research on catalytic additives has lead to the "millenium concrete" (trade mark) that keeps the concrete free of the usual organic pollution... But there are experiments with other catalytic products with UV (sun) to break down other molecules.
Some of those are tested to de-pollutate sites by acting like a micro-filter with catalytic reactions (still in experimentation and so quite secret!).

Up to now, carbonation was a problem for concretes (pH), but with organic fibers it's not the case... So a special concrete (not very structural but could be used for fillings or cladding) could be used on buildings to either break CO2 in C and O2 (quite a lot of energy there) or to trap the CO2 in some other molecule like calcite or some other CO2 "trap"...!

It's just the beginning of the "customized material" industry :-)

"What can I do, What can I write, Against the fall of Night". A.E. Housman