[GSBN] embodied energy and sequestration

[Derek Roff]

I have no disagreement that concrete carbonates.  I would love to get a 
better handle on the percentage/time curve.  John says, "Porous lower 
strength concrete can carbonate several inches in a decade".  I would 
hope that the columns and beams in buildings would not be at the 
porous, lower end of the spectrum.  With thick (large cross-section), 
denser concrete, John's statement would indicate that the amount of 
carbonation at 10 years would be a moderate fraction of the total 
concrete.  It also seems that hand-in-hand with the carbonation comes a 
decrease in strength and safety, if the reinforcing steel is corroding. 


But my main question of the moment seeks to understand the carbon 
footprint of the concrete that does carbonize.  What is the balance 
(ratio) between the CO2 that is released by a unit mass of 
proto-portland cement, in the heat-driven chemical reaction during 
calcining, and the CO2 that is later re-absorbed by that same mass of 
portland cement, in the concrete after it is in place in a building?  I 
have read that this is a 1:1 ratio.  That is, all the CO2 absorbed is 
just the equivalent mass of CO2 that was chemically released earlier. 
Limestone in, limestone-ish stuff out, after final carbonation.  So 
that no net sequestering can occur.  That part of the reaction is at 
best a zero sum game (when carbonization is complete).  Is this 
correct?

Running all of the fuel-burning machinery during all of the processing 
is obviously releasing fossil fuel CO2 into the air.  But if the 
chemical reaction is zero-sum, as described above, then the claim of 
sequestration has no foundation.  On the other hand, if concrete is 
absorbing CO2 in excess of what is released earlier, I'd like to 
understand that better, and get some kind of handles on the quantities 
involved.

Can anyone shed any light on this part of the equation?  John?  Bruce?

Derek

--On Friday, May 7, 2010 9:40 AM -0400 John Straube 
<jfstraube at gmail.com> wrote:

> Concrete absolutely carbonates and consumes CO2.
> This is one of its problems: carbonation reduces the high initial pH
> of 13 and when it drops below 10 or so (some say 9, others 11!),
> steel within it becomes much more susceptible to corrosion.
> The industry has been working for years to produce very dense
> concrete, and even produce CO2 blocking coatings, to reduce this
> problem.
> Porous lower strength concrete can carbonate several inches in a
> decade, whereas high strength, low w/c ratio, high fly ash and silica
> fume concrete carbonates much much slower (at least 10 times more
> slowly)
>
>
> The CO2 released by the coal or nat gas during the firing of lime
> cant ever be reabsorbed only the CO2 released by the chemical
> decomposition is. Portland is likely the same.



Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu