[thedoc (OP)]

Alvin Gabriel Cepeda asked the Naked Scientists:
   Is it possible to capture CO and/or CO2, convert to liquid form, and use that liquid form to be mixed with composite materials that can perhaps be used to form cement or concrete? In the same way the cement aggregate is made?
What do you think?

[Atomic-S]

Ordinary concrete is based on the reaction of mineral substances with water. CO and CO₂ are very different substances than water, and undoubtedly could not be used with common cement.  The one thing that might change this picture somewhat is that lime Ca(OH)₂ can absorb carbon dioxide to form a cementious material, which I think actually takes place in some plasters. The CO₂ does not have to be in liquid form, but is absorbed out of the atmosphere. The process, of course, takes a while.  If one is thinking of such a process as a way to clean the atmosphere of excess CO₂, there is the little problem that the manufacturing of the lime takes place by the reverse chemical reaction, and therefore puts as much CO₂ into the atmosphere as will later be recovered.  So if one wishes to do this stuff ecologically, one is going to have to find a different chemistry. I don't know what the outlook for that might be. 

[chiralSPO]

To do this, we need something that reacts with CO₂ to make calcium carbonate (or magnesium carbonate). Unfortunately, the leaves us with calcium (or magnesium) oxide or hydroxide, neither of which exists as such naturally (because it would already have absorbed CO₂ from the atmosphere). Instead, we roast calcium carbonate to produce calcium oxide, which can react with water to make the hydroxide--but the roasting process releases exactly as much CO₂ as it could absorb.

One could also imagine starting with a different calcium-containing mineral like gypsum (calcium sulfate) or fluorite (calcium fluoride). But converting these to the oxides or hydroxides would produce sulfuric or hydrofluoric acids, which are fairly nasty. Of course, both of these acids have industrial uses, but the scale at which we would want to absorb CO₂ is several orders of magnitude greater than the demand for hydrofluoric acid...