The MIT Technology Review publishes a post with the title Cement isn’t as terrible for the climate as we thought. The article notes that making cement pumps out a lot of CO2 through the chemical process of cooking limestone at high temperatures, and then Michael Reilly writes:
But a new study in Nature Geoscience says that once a building is built (and even after it’s been torn down) the mortar, concrete, or rubble soaks back up a fair amount of carbon dioxide through chemical reactions with air and water. In all, the study suggests that this carbon sponge effect may account for as much as 43 percent of what was emitted in the first place.
But that is only half the CO2 picture.
The problem with cement is not just the chemical reaction, it is also the fossil fuels used to heat limestone (calcium carbonate) at 1000 degrees C to get it to turn into lime (calcium oxide), the main ingredient of cement. Fully half of the CO2 produced making cement comes from that. The actual study Substantial global carbon uptake by cement carbonation, says this very clearly, my emphasis:
In total, we estimate that a cumulative amount of 4.5 GtC has been sequestered in carbonating cement materials from 1930 to 2013, offsetting 43% of the CO2 emissions from production of cement over the same period, not including emissions associated with fossil use during cement production.
So in total, the CO2 absorbed by the cement component of concrete is at best 22 percent of the CO2 created making cement, not 43 percent.
This is not news, it's chemistry.
Everybody in the concrete biz knew this already. The carbonation of concrete is part of the curing process, where CO2 reacts with the calcium to make calcite. It actually increases the compressive and tensile strength of the concrete ( and corrodes the reinforcing). It continues for years; that's why concrete buildings get stronger over time. According to Understanding Cement,
Within a few hours, or a day or two at most, the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process penetrates deeper into the concrete at a rate proportional to the square root of time. After a year or so it may typically have reached a depth of perhaps 1 mm for dense concrete of low permeability made with a low water/cement ratio, or up to 5 mm or more for more porous and permeable concrete made using a high water/cement ratio.
Tear down that wall!
In fact, in my complaints about about the demolition of brutalist buildings I have noted that they are taking them down before they have even completely hardened. Fortunately, there is a great way to speed up this process of carbonation that takes decades or centuries: knock it down. The study is the first to take demolition into account; breaking up the concrete exposes more of it to atmospheric CO2, increasing the absorption more than previously thought. So now all those haters of brutalism have a good justification: all that rubble is a CO2 sponge, although the older the building, the more carbonated the concrete. This appears to be happening:
Furthermore, the rate of sequestration by carbonating cement is increasing rapidly (by an average of 5.8% per year during the period 1990–2013) as the stock of cement buildings and infrastructure increases, ages and gets demolished and disposed.
Yes, but we can assume all of those demolished buildings are being replaced by new concrete buildings, (this is a Chinese study; why else demolish existing buildings and infrastructure?) so there is actually going to be a serious net gain of CO2.
Concrete, the new carbon sink.
The authors of the study acknowledge in their conclusions, that the CO2 produced from fossil fuels used to cook the limestone is a problem. But hey, it’s not so bad. Because they now call the concrete itself a carbon sink and they can just capture the rest of the CO2.
Future emissions inventories and carbon budgets may be improved by including this cement sink. Moreover, efforts to mitigate CO2emissions should prioritize the reduction of fossil-fuel emissions over cement process emissions, given that produced cement entails creation of concomitant carbon sink. Indeed, if carbon capture and storage technology were applied to cement process emissions, the produced cements might represent a source of negative CO2 emissions.
In their dreams.
There is no doubt that this study (and the 43 percent number) will ping around the Build with Strength crowd, and will be used to attack the trend toward wood construction. But however you slice it, wherever possible we should be building with materials that sequester carbon right now instead of waiting for fifty or even a hundred and fifty years (and demolition to boot) for a quarter of the CO2 to be sucked back in. This is no justification for concrete, and the stuff is pretty much as bad for the climate as we always thought.