While concrete does absorb CO2 there is a large misconception that it can absorb as much as is output during production. In reality any concrete worth its cost can only absorb a very small fraction of the production output.
A largely overlooked issue with the idea of concrete absorbing CO2 is exposed surface area. Imagine a hydro dam, one side is saturated with water, the other side is air. The dam is also usually quite thick, several meters to hundreds of meters. That really limits the available absorption surface especially considering the ratio of surface area to volume. Concrete foundations, epoxy coated parkades, even painted surfaces start to drastically reduce the possibility of CO2 being absorbed.
Think of concrete like a membrane or a sponge. The thicker the membrane you want to pass a fluid through, the higher the pressure you would require. So using our pressure as atmospheric, getting a high depth of CO2 absorption requires a well connected pore structure, and a long time frame.
Add to this membrane metaphor the issue of size. We design concrete structures to prevent fluid transfer, especially hydro dams! H2O is a smaller molecule than CO2, and the static pressure on the ‘wet’ side of a hydro dam increases 1 atmosphere every 10m.
As concrete sets and gains strength, its porosity decreases. This process happens rapidly in the first few days, and only ever stops when the cement runs out of free water to absorb. CO2 absorption depth is a pretty common test when evaluating a structure for rehabilitation purposes (carbonation dept testing) as CO2 will mess with the concrete PH and corrode reinforcing steel which (depending on many factors) can be as shallow as 5cm from the concrete surface. I’ve seen structures with up to 5cm of depth, but that is outside of the norm. 1-2cm of CO2 absorption depth for a structure from the 50’s is pretty common in my area.
Modern concrete, through many different means, has a highly disconnected pore structure (compared to the concrete of the 50’s). New concrete, is designed to reduce any type of fluid transfer especially after the first few days of curing. This will reduce the depth of CO2 absorption further.
So bringing this all back together, consider the absorption depth of 2cm, a structure with minimal exposed surface area, and some sort of coating or cladding… and you very quickly realize that you are not going to be off setting CO2 production outputs by any significant margin, and its not in your best interests either.