[GSBN] embodied energy and sequestration

[Bruce King]

I doubt that ANY lime or portland cement-based concrete (PCC) has ever  
fully carbonated.  To do so would require grinding the lime or PCC to  
a sand consistency and leaving it out in the weather for at least a  
year, spread thin so as to have full access to the air.  So, OK, maybe  
a few thousand tons, total, in history.  IF you could fully carbonate  
such material, then its carbon load consists only of all the fuel used  
to mine it, calcine the limestone, haul rocks or cement or wet  
concrete around from quarry to kiln to building, i.e., its embodied  
energy in the conventional sense.  Which is, I believe, at least half  
the carbon load.

To put that differently:  even if you could fully carbonate your lime  
plaster or concrete foundations or concrete statues of Dick Cheney,  
you would only have decreased the carbon load to the atmosphere by  
half.  That fuel was burned, brothers and sisters, and there ain't no  
unburning it.  The entire sum of lime and concrete human constructions  
-- from Mesopotamia to ancient Rome to New York to Shanghai --  
constitutes a whole bunch of carbon emitted;  a net, large climate  
changer.  Never by any stretch of the imagination carbon sequestration.

Nor has anyone that I've heard of come anywhere close, and not for  
lack of trying, to a large scale replacement for portland cement (or  
lime and all the family of scorched rock binders (SRB's)).  We're  
stuck with Portland cement for the foreseeable future, so the game, in  
the green building view, is to use it sparingly and wisely.  Or,  
wherever possible, as Pete points out, use clay as your binder.

Thanks,

Bruce King
(415) 987-7271
the art and science of building well
bruce-king.com
PO Box 6397
San Rafael, CA 94903 USA




On May 7, 2010, at 8:54 AM, Derek Roff wrote:

> 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
>
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