Jason McVetta wrote:
> On Sat, Feb 27, 2016 at 10:17 AM, John Young <jya@pipeline.com
> <mailto:jya@pipeline.com>> wrote:
[snip]
>A few years ago this guy, a chemical engineer, who worked at a local
>startup that was attempting to do some "Green Tech" by leasing a piece
>of a nearby cement plant to experiment with sinking CO2 into the
>concrete as a way to help the environment, needed to find another job
>because the 'suits' were having too much fun driving their leased
>Ferraris to look for funding.

The portion of concrete which binds it together, called "cement", is calcium hydroxide, or Ca (OH)2.   Mixed with gravel and sand and water, this makes a slurry called "concrete".  It quickly (hours) cures (hardens), and it begins to absorb CO2.  That conversion makes old concrete harder and stronger than newly-poured concrete.

Poured concrete continues to absorb CO2 for months, years, and even decades after it is poured.  

However, to make the initial cement, calcium hydroxide,  a chemical called Calcium Carbonate (CaCO3) (limestone is nearly all CaCO3) is heated strongly, and it emits CO2, forming calcium oxide, CaO.  Adding a bit of water forms calcium hydroxide, Ca(OH)2.

So, the CO2 that curing concrete absorbs is matched by the CO2 that the CaCO3 originally released during the making of the calcium hydroxide.  Further, heating that process requires some sort of fuel, which itself usually releases CO2 as well.

             Jim Bell    BS Chemistry, MIT, 1980.