Cut CO2? Change concrete!

[Dr Mordrid]

http://www.livescience.com/environment/070129_clean_concrete.html

The making of cement accounts for up to 10 percent of the world's total emissions of carbon dioxide, a key gas involved in global warming. Now scientists and engineers are developing a cleaner way to manufacture cement.
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Most of the carbon dioxide emissions in cement manufacturing result from heating the kiln to a temperature high enough to transfer energy into the powder. Civil engineer Franz-Josef Ulm at MIT and his colleagues are now developing materials that are hopefully as strong and cheap as cement while requiring lower temperatures during production.

In the end, the researchers suggest this cool solution can slash carbon dioxide emissions during cement manufacture by up to 10 percent. This would accomplish a fifth of the goal of the Kyoto Protocol,
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The researchers suggest they could swap out C-S-H (calcium-silicate-hydrate) with a material that stacks up just as well but requires less heat to produce, thus cutting down on carbon dioxide emissions. For instance, Ulm said candidates include compounds resembling C-S-H that replace calcium with magnesium.

"Magnesium is an earth metal, like calcium, but it is a waste material that people must pay to dispose of," he explained.

Ulm and his colleagues report their findings in the January issue of the Journal of the Mechanics and Physics of Solids.

[Brian Ellis]

Yes and no.

There are two sources of CO2 in cement manufacture. One is from the fuel burnt in calcining the mix and reducing the temperature would result in a small drop of emissions. However, most of the CO2 comes from the limestone (CaCO3) which reacts with the silicon dioxide in snd/clay/marl/fly ash etc. to produce a mixture of two calcium silicates. This CO2 is typically twice as much per unit weight of cement as is produced by the fuel combustion. The most prevalent one is (CaO)3.SiO2, so you can see that for each molecule of this substance, three molecules of carbon dioxide are released. Now, if you use dolomite (magnesium carbonate, MgCO3) instead of limestone, calcining at c. 1350°C instead of 1400-1500°C, you will get exactly the same amount of CO2 emissions from the materials themselves.

Of course, you can also have either calcium oxide (quicklime) or magnesium oxide as your starting point, rather than the carbonate, but the calcining temp is the same. You are not saving anything, though, as these oxides never occur free in nature and have to be made by "burning the lime". This is done by heating the limestone or dolomite to about 900°C to release the CO2, so we are back to square one, except that the material is heated twice, causing more CO2 emissions from fuel sources.

There have been magnesium cements available for as long as I can remember. If I remember correctly, they are manufactured from dolomite which has been burnt and calcined with relatively little silicon dioxide but some hydrochloric acid is added to the mix. (Sorry, if my memory fails, but it is nearly 60 years since I studied cements). Also, as I remember it, this cement is extremely hard and strong but it absorbs water like nobody's business so is unsuitable for structural applications. I believe its main use is - or was in my misspent youth - as a cheap binder for mock-marble floors.

Anyway, I do not believe that you can resolve 1/5th of the Kyoto goal by this method. They have only 5 years to do this, as the Kyoto Protocol is finished in 2012.