Whoops, turns out concrete is actually a carbon sink

From the “settled science” department and the University or California Irvine, comes this inconvenient fact. For years we’ve been told by academics that cement is another nasty global warming contributor, because of CO2 released during production.

“Cement manufacturing is responsible for 5 to 8 percent of global CO2 emissions,” notes Del Gado, a theoretical physicist who is part of Georgetown’s Institute for Soft Matter Synthesis and Metrology. “Although there have been calls for creating so-called ‘green cement,’ the sustainability and science communities have yet to find a way to reduce CO2 emissions while retaining the efficiency, durability and cost efficiency of cement. Our study could help change that.”

Now, not so much.

Concrete jungle functions as carbon sink, UCI and other researchers find

Cement-based materials eventually reabsorb much of the CO2 released during creation

An international team of researchers including UCI Earth system scientist Steven Davis has found that over time, cement reabsorbs a significant portion of the CO2 emitted when the material was manufactured. CREDIT Steven Davis / UCI
An international team of researchers including UCI Earth system scientist Steven Davis has found that over time, cement reabsorbs a significant portion of the CO2 emitted when the material was manufactured. CREDIT
Steven Davis / UCI

Irvine, Calif. – Cement manufacturing is among the most carbon-intensive industrial processes, but an international team of researchers has found that over time, the widely used building material reabsorbs much of the CO2 emitted when it was made.

“It sounds counterintuitive, but it’s true,” said Steven Davis, associate professor of Earth system science at the University of California, Irvine. “The cement poured around the world since 1930 has taken up a substantial portion of the CO2 released when it was initially produced.”

For a study published today in Nature Geoscience, Davis and colleagues from China, Europe and other U.S. institutions tallied the emissions from cement manufacturing and compared them to the amount of CO2 reabsorbed by the material over its complete life cycle, which includes normal use, disposal and recycling. They found that “cement is a large, overlooked and growing net sink” around the world – “sink” meaning a feature such as a forest or ocean that takes carbon dioxide out of the atmosphere and permanently tucks it away so that it can no longer contribute to climate change.

Cement manufacturing is considered doubly carbon-intensive because emissions come from two sources. CO2 molecules are released into the air when limestone (calcium carbonate) is converted to lime (calcium oxide), the key ingredient in cement. And to generate the heat necessary to break up limestone, factories also burn large quantities of natural gas, coal and other fossil fuels.

Davis and his fellow researchers looked at the problem from a different angle. They investigated how much of the gas is removed from the environment over time by buildings, roads and other kinds of infrastructure. Through a process called carbonation, CO2 is drawn into the pores of cement-based materials, such as concrete and mortar. This starts at the surface and moves progressively inward, pulling in more and more carbon dioxide as years pass.

More than 76 billion tons of cement was produced around the world between 1930 and 2013, according to the study; 4 billion tons were manufactured in 2013 alone, mostly in China. It’s estimated that, as a result, a total of 38.2 gigatons of CO2 was released over that period. The scientists concluded, however, that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were gradually reabsorbed during that time frame.

“Cement has gotten a lot of attention for its sizable contribution to global climate change, but this research reinforces that the leading culprit continues to be fossil fuel burning,” Davis said.


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November 21, 2016 10:01 am

These are the goofballs that want to run the global economy. God help us.

Reply to  chaamjamal
November 21, 2016 11:41 am
November 21, 2016 10:06 am

Sheesh, I’ve known that for at least 20 years. And I’ve never bothered to actually study this subject.

Reply to  MarkW
November 21, 2016 10:10 am

Everyone except the Carbon Moonies charged with finding new things to worry about knew that.

Monna Manhas
Reply to  gymnosperm
November 21, 2016 12:42 pm

Well, I didn’t know that. But then I never really thought about it, either. Learned something today.

Reply to  MarkW
November 21, 2016 4:56 pm

About the only useful observation that came out of the Biosphere debacle was that concrete absorbs CO2 in around 1993 https://en.wikipedia.org/wiki/Biosphere_2#Challenges

November 21, 2016 10:08 am

We are going to have to pave the world over n order to save it.

Reply to  rxc
November 21, 2016 1:05 pm

I wonder if that’s one reason why the US is a carbon sink according to that CO2 observing satellite.

Larry D
Reply to  MarkW
November 21, 2016 11:08 pm

It doesn’t hurt, but forest growth and soil absorption are likely more significant. If only because there’s a lot more soil and forested land.

November 21, 2016 10:11 am

Now only if CO2 made a difference wrt warming…wait a minute – now only if the planet was warming…All CO2 talk is irrelevant (unless you’re a plant that is)

Alan Ranger
Reply to  jimmy_jimmy
November 21, 2016 4:41 pm

Precisely. And OTOH Patrick Moore points out that activities like cement making actually contribute to the depleted bank of CO2 that plants are starved of and whose increase is (beneficially) greening up the planet.

November 21, 2016 10:12 am

Per IPCC AR5 Figure 6.1 the biospheric carbon balance contains 46,713 Gt of C.
38.2 Gt of CO2 = 10.4 Gt of C or 218 ppm of the total biospheric carbon.
Doesn’t look so big now, does it?
A 57%/43% sequester/retained split means a difference of 2 ppmv in the atmosphere.
And this is all spread out over 83 years.

Reply to  Nicholas Schroeder
November 21, 2016 11:26 am

I’m not sure I follow this. There seems to be some confusion between C and CO2.

Reply to  Nicholas Schroeder
November 21, 2016 3:00 pm

Yeah, me too, Nicholas Schroeder. Can you explain this, or is it explained on the AR5 figure that I haven’t looked up yet (but I will).

38.2 Gt of CO2 = 10.4 Gt of C or 218 ppm of the total biospheric carbon.

Reply to  MRW
November 21, 2016 5:34 pm

NOAA has a formula for converting carbon into co2. Without looking it up its 3.67 or 3.62. I think that it to further confuse the issue. Further they have to account for a substantial amount of co2 that is vanishing over and above the stated 50% that is acknowledged to be sinking. . There is no indication of why or how so much co2 is not making its way into the atmosphere. NOAA has also recently changed one of its metrics, disallowing one of their estimates to lower co2 production.
I see it as a desperate attempt to shore up their failing analysis. It’s like the tropical hotspot or the heat hiding somewhere.

Phil R
Reply to  MRW
November 22, 2016 8:25 am

It’s based on the atomic weights of carbon and oxygen and the molecular weight of CO2. the atomic weights of C and O are (with rounding for simplicity) 12 and 16, respectively. Therefore, the molecular weight of CO2 is 44 (rounded).
So carbon makes up approximately 27% (27.3) of CO2 and oxygen makes up 72.7% of CO2. To convert weight (mass) of CO2 to either C or O, multiply the mass CO2 by the respective %. For example, 38.2 Gt CO2 * 27.3% (for C) = 10.4 GT C.

Reply to  MRW
November 22, 2016 11:11 pm

Thank you, rishrac and Phil R. Very much appreciated.

michael hart
November 21, 2016 10:21 am

Metal carbonates (CO3-2) would actually be a net sink because they can fractionally absorb some extra CO2 to form bicarbonates (HCO3-), this being part and parcel of the oceanic pH buffering system. But it’s not important on the grand scale of things, because of the practical difficulties of digging up all the worlds limestone and spreading around to absorb a little bit more atmospheric CO2.
Why this guy describes his findings as “counterintuitive” baffles me. Perhaps he never actually studied chemistry before.

Alan Robertson
Reply to  michael hart
November 21, 2016 10:39 am

“Why this guy describes his findings as “counterintuitive” baffles me. Perhaps he never actually studied chemistry before.”
I think the statement speaks to the audience at large. ChemEs, etc. are a tiny fraction of the population.
Consider the near universal human frailty whereby any true and valid information which contradicts what an individual believes about a subject becomes difficult- to- impossible for the individual to accept.

Eustace Cranch
November 21, 2016 10:28 am

This is a bit of a double-edged sword. As much as I dislike massively subsidized, ineffective, and downright destructive wind power installations, we can’t use the “carbon footprint” of concrete as an argument against them. I’ve seen that argument made here before.

Reply to  Eustace Cranch
November 21, 2016 1:07 pm

There are two aspects to the CO2 footprint of concrete. The first is the CO2 released from the materials being processed. The second is all the energy that goes into processing all those materials.

Reply to  Eustace Cranch
November 21, 2016 1:59 pm

Concrete for windmills is mostly buried, isn’t it? It’s primarily used as a below ground mass to secure and counter-balance the massive, vibrating towers.
If so, it would be largely shielded from atmospheric CO2 by its surrounding earth.
That would presumably greatly reduce its total CO2 reabsorption, wouldn’t it?

Reply to  Ben
November 21, 2016 4:23 pm

I suspect that soluble carbonates like sodium carbonate and sodium bicarbonates, that abound in moist or wet soils along with carbonic acid deliver plenty of CO2 to the buried concrete footings.

November 21, 2016 10:36 am

Whoa—you mean maybe they were wrong about concrete? I had always been told it is contributing to global warming.

November 21, 2016 10:39 am

“More than 76 billion tons of cement was produced around the world between 1930 and 2013, according to the study; 4 billion tons were manufactured in 2013 alone, mostly in China. It’s estimated that, as a result, a total of 38.2 gigatons of CO2 was released over that period. The scientists concluded, however, that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were gradually reabsorbed during that time frame.”
Please note:
38.2gt emitted
4.5gt reabsorbed
Not a net sink, just 12% less bad than was assumed.

Reply to  Abuzuzu
November 21, 2016 10:57 am

I noticed that as well and wondered if maybe it was a typo: 4.5 instead of 14.5. So I tried 14.5/38.2 which is 38%. In the right ball park but not really close enough. Somebody needs to take another look at those numbers.

Reply to  sagalout
November 21, 2016 12:10 pm

“or 43 percent of emissions from LIMESTONE conversion – were gradually reabsorbed during that time frame.”

Reply to  sagalout
November 21, 2016 1:22 pm

@ Marcus
Yes, you’re right. I was reading it wrong. But it was the juxtaposition of the numbers and the fact that emissions from limestone conversion weren’t separated out that confused me.

Reply to  sagalout
November 21, 2016 3:54 pm

@sagalout…I agree, it was confusing at first glance…

Reply to  sagalout
November 22, 2016 3:23 pm

They pulled a switch to make it look like a smaller amount absorbed by the 43% being calculated on the 10.4Gt of C and not on the CO2.

Clyde Spencer
Reply to  Abuzuzu
November 21, 2016 7:04 pm

Something to consider is that it only takes a matter of hours to release the CO2 when calcining limestone, but it takes years to decades for the carbonation to take place. They said that they took the entire life cycle into account, but many of the projects, like Hoover Dam and the Aswan Dam have yet to be demolished when the useful life is over. It will take considerable energy (and probably release of CO2) to crush and transport the old concrete to whatever its final resting place will be. So, I doubt if the full life-cycle has been fully accounted for. Fundamentally, the full accounting has been delayed and their optimistic figures only represent the situation at an intermediate point in time.

Tom Halla
November 21, 2016 10:40 am

Aww! Only 43% off? Some of the variables in the IPCC GCM computer models are several hundred percent undetermined, like the effect of doubling CO2 levels. So what is the big deal about a petty 43%?/sarc

Tom O
November 21, 2016 10:41 am

Best I can figure, the only contribution to “global warming” that cement makes is the fact that it retains heat into the night, thus raising the overnight lows. As far as I can tell, that is also the ONLY warming that we are experiencing, meaning that day time highs haven’t gone up, just night time lows giving us a higher “average daily temperature.” Maybe I can’t figure though.

Richard Saumarez
November 21, 2016 10:42 am

Does this mean that Climate Scientists should live in concrete towers rather than ivory towers?

Terry Gednalske
Reply to  Richard Saumarez
November 21, 2016 12:28 pm

Think of all the elephants that woul be saved.

November 21, 2016 10:47 am

So we would be at “normal” or below normal temps today if LBJ had paved over Vietnam?

November 21, 2016 10:47 am

The numbers do not add up.
CO2; MW = 44
CaO; MW = 56
Ratio = 44/56 = 0.79
CaCO3; MW = 100
Ratio CaCO3/CaO = 100/56 = 1.79
So 76 tons cement produced gives 60 tons CO2 from 136 tons CaCO3!
This is the conversion from calcium carbonate to calcium oxide. No account for the fuel is made here, just the chemical conversion.
(the ratios are all the same so use grams, kg, tons, gt, etc.)
They throw out 38.2 gt. as a working number. Then they say 4.5 gt absorbed, well OK.
Then they say 4.5 gt = 43% of 38.2! Well, it mostly is not.
And the number for CO2 from CaCO3 conversion is 60 gt, not 38.2 gt.
This does not make sense.

Alan Watt, Climate Denialist Level 7
November 21, 2016 10:51 am

More than 76 billion tons of cement was produced around the world between 1930 and 2013, according to the study; 4 billion tons were manufactured in 2013 alone, mostly in China.

Are we talking concrete or cement? The figure of 4 billion metric tons for 2013 agrees with what I read elsewhere for concrete, which is about 10-15% cement. The rest is water and aggregate (sand, gravel or recycled concrete). It’s the manufacture of cement which releases C02.
So I think the figures here need to be scaled back to arrive at cement production.

Reply to  Alan Watt, Climate Denialist Level 7
November 21, 2016 11:15 am

Good point. Just a quick look at mixing and matching concrete or cement numbers (using your 10-15%) in the data above, and they still do not make any sense.
Lets try this:
76 gt concrete * 0.15 (your handy number) = 11.4 gt cement, and 9 gt CO2
They say 4.5 gt absorbed so
4.5/9.0 = 50%, more or less close to the claimed 43%.
At this point I feel like I am just playing with numbers and making stuff up. As astrologer could do as much.
But then again, they say cement, and give numbers.

Reply to  TonyL
November 21, 2016 12:24 pm

AW, you’re correct, they have confused cement and concrete. For example Assoc. Prof. Davis reportedly said “The cement poured around the world since 1930….” Concrete gets poured, not cement. So we can’t be sure if they are always talking about concrete, or are sometimes talking about cement. They also mention units of both billion tons and gigatons. These are equivalent.
The rule of thumb is around 1 tonne of cement produces 1 tonne of CO2. This has come down in recent years with increased efficiency of cement plants, and also substitution of carbonate raw materials. However, as noted previously their numbers just don’t add up anyway.
The carbonation of concrete is well known, but for high strength, dense concretes it is a very slow process – maybe a few millimetres penetration into the concrete over several decades. The concrete has to be reduced to rubble and left exposed to the air for appreciable carbonation to occur.

Reply to  TonyL
November 21, 2016 1:14 pm

Yes, I think this basically BS. I have known for a long time that lime white wash absorbs CO2 into its suface and basically reverts to being limestone. However, I had presumed that concrete was too dense for this happen at anything more than a trivial thickness.
Most construction involving poured concrete also uses vibrators to help the air bubbles rise out. This results in a dense compacted finish against the shuttering.
Even when buildings get pulled down they are pulverised, they remain fairly massive lumps of concrete.

Reply to  TonyL
November 21, 2016 3:38 pm

But concrete remains permeable to water, and the water which enters the concrete contains some CO2, so it can get all into the concrete, not just at the surface.
In fact, it is well known that concrete continues to strengthen indefinitely under most conditions because the hydration process never stops. Needles shaped crystals grow ever longer over time, interlocking and making the concrete stronger as they grow.
As CO2 concentration in the atmosphere goes up, it stands to reason that there will be more of it dissolved in the water which infiltrates the concrete and is incorporated into the crystalline matrix.
So even buried concrete likely absorbs CO2 over time.

Reply to  Alan Watt, Climate Denialist Level 7
November 21, 2016 11:19 am

Concrete and cement are two different things …?
Jeeze, next thing you’ll be telling me is temperature and heat are two different things as well.
And please please please don’t tell me my models have to be accurate as well as precise, ’cause then somebody would ask what the difference is (and I don’t (care to) know).

bill johnston
Reply to  DonM
November 21, 2016 3:17 pm

And isn’t that the same as using carbon and carbon dioxide interchangably

Reply to  DonM
November 21, 2016 3:41 pm

Since CO2 is not harmful but beneficial, and it likely has very tiny influence on the temperature of the Earth, it is all just an exercise in tedious analysis of a complicated equation that ultimately is inconsequential anyway.
IWO, big deal…so what?

Jim Barber
Reply to  DonM
November 21, 2016 10:48 pm

Just think of it this way. Cement is to concrete what flour is to bread. A component.

Myron Mesecke
November 21, 2016 10:51 am

This made me think of an old Gallagher joke.
“They say that cities are destroying the environment. They’re wrong. It’s the farmers. Farmers plant crops that leach the nutrients from the soil. They rotate the crops, they rotate the nutrients they leach from the soil. Where in the cities they lay down the parking lots and asphalt. Seal in the nutrients.”

Reply to  Myron Mesecke
November 21, 2016 3:45 pm

Reminds me of George Carlin ” The planet is going to be fine”!
The Earth will eventually get around to going what is was going to do anyway, and shake us off like a case of the fleas.
The only stuff lost to the planet is anything we launch into space which does not come back.
Everything else is just reshuffling atoms and molecules that the Earth has been shuffling around for billions of years.

November 21, 2016 10:59 am

So, 38.2G tons of CO2 was released making cement, then 4.5G tons (11.8%) of that was more or less slowly incorporated into 76G tons of concrete. What did I misunderstand?
You’d think that concrete in standing structures, with their surface area, would grab up even more than buried limestone. Who has a CRC HandBook and a P-chem text with the figures for the industrial process?
Over 2.2*10^11 pounds of calcium carbonate are produced, 4.4*10^10 pounds of sodium carbonate…. no… water softeners… magnesium carbonates…dolomite… naaaah, I’ve got nothing useful here. Help.

Reply to  Mib8
November 21, 2016 3:47 pm

Nothing useful here?
You might consider a career in government financed money wasting I mean climate science.

November 21, 2016 11:04 am

This is bad news. Plants need all the CO2 they can get. And CO2 is a weak warmer. We need more warmth.

dean hicks
November 21, 2016 11:06 am

What about all the pure traditional lime mortar and plaster that has been used around the world prior to the invention of portland cement also? This is still reabsorbing CO2 and will do so for hundreds if not thousands of years. The book Plastering Plain and Decorative by William Millar published in 1897 mentions this stonemasons proverb “When a hundred years are past and gane then gude mortar will turn into stane”, Not that it matters as its great plant food!

November 21, 2016 11:12 am

The list of solutions for solving nonexistent problems is virtually endless. The money to fund such nonsensical research is not. Just think of the progress forfeited in lieu of studies such as this. Not that it’s worthless, only that it’s not worthy of priority .

November 21, 2016 11:18 am

Down is the new up. Does this mean we can stop promoting hay bale construction techniques for buildings?

Reply to  Resourceguy
November 21, 2016 11:23 am

Straw Bales….
(and sorry to continue with this ….)

Reply to  DonM
November 21, 2016 12:10 pm

You’re excused, but only this time.

Reply to  Resourceguy
November 21, 2016 8:25 pm

What it does is turns “The Donald’s” wall into a “green” project.

November 21, 2016 11:19 am

You see! Those large cement foundations required for wind farms aren’t so bad after all! And when the wind generator eventually dies, we can leave the skeleton there to keep absorbing evil CO2 because it would require a non-carbon sink machine to fix it. (end sarcasm)

November 21, 2016 11:46 am

What will now become of all those collegiate researchers who have devoted years towards development of a green cement?

Reply to  arthur4563
November 21, 2016 11:58 am

Now, I like this term (‘Green cement’). I can abuse the ignorant by saying 100% of rigid paving structures on project ‘X’ will indeed be green (concrete), and that while green concrete is on the site the specifications for working on and around are modified to protect such.
In my book Green concrete is defined as per:

November 21, 2016 12:03 pm

CO2 in the atmosphere only matters to plants.
Thermalization and the complete dominance of water vapor in reverse-thermalization explain why CO2 has no significant effect on climate. Terrestrial EMR absorbed by CO2 is effectively rerouted to space via water vapor.
CO2 is not merely harmless, it is profoundly helpful. It is helpful in that it is plant food and, perhaps more importantly, it reduces plant’s need for water.

Nigel S
November 21, 2016 12:08 pm

Carbonation makes the concrete less alkaline and reduces the corrosion protection of the steel reinforcement. The depth of concrete over the reinforcement is increased to allow for this. So it’s worse than we thought, increased CO2 is making concrete acidic and destroying buildings! The Romans must be cross that they missed out on Carbon Credits for the Pantheon.
(I’m sure someone will discover that the Pantheon is, in fact, part of some Carbon Credit boondoggle.)

Lance Wallace
November 21, 2016 12:30 pm

From the following Science mag link, a missing phrase appears (4.5 gigatons carbon OR MORE THAN 16 GIGATONS OF CO2…).
“The researchers estimate that between 1930 and 2013, cement has soaked up 4.5 gigatons of carbon or more than 16 gigatons of CO2, 43% of the total carbon emitted when limestone was converted to lime in cement kilns, they report online today in Nature Geoscience. That’s more than 20% of the carbon soaked up by forests in recent decades, they write.”
Since the limestone to lime contribution was about half the total, the rest being fossil fuels, the 43% number would be cut in half to about 20% of the total CO2 getting clawed back by the reabsorbtion.
I tried the link to the original paper in the Science mag but got a 404 notification.

November 21, 2016 12:48 pm


t’s estimated that, as a result, a total of 38.2 gigatons of CO2 was released over that period. The scientists concluded, however, that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were gradually reabsorbed during that time frame.

4.5 Gtn out of 38.2 gigatons released hardly a game changer, is it?

Reply to  Greg
November 21, 2016 12:58 pm

“Cement has gotten a lot of attention for its sizable contribution to global climate change, …”
No, it may have made a sizeable contribution to human emissions, period.
Of about 1 degree C of warming which may have occurred since 1850, something like half is allegedly attrubutable to human influence, much of that is land use and other factors, not CO2. So lets say 0.3 , of which concrete is 5-8%.
That makes 0.015 to 0.024 or 15 to 24 MICROKELVIN of “global warming”.
Sizeable, not really.

November 21, 2016 1:04 pm

The sinks in my house are porcelain, except for one that’s stainless steel.
Where do you go to buy one of these carbon sinks?

Reply to  MarkW
November 21, 2016 1:16 pm

from a carbon trader , of course !

November 21, 2016 1:05 pm

“The researchers estimate that between 1930 and 2013, cement has soaked up 4.5 gigatons of carbon or more than 16 gigatons of CO2, 43% of the total carbon emitted when limestone was converted to lime in cement kilns”
Yes, that sounds about right. As so often, the headline here is misleading. Concrete isn’t a net sink; 43% of the original emissions are eventually reabsorbed.
As many have mentioned, none of this is new. The idea of cement is that you heat limestone to drive of the acid part, CO₂, leaving the base CaO. This is then mixed with silica (acid) with the intent that it will over time form calcium silicate. But some recombines with CO₂ to reform carbonate.

Clyde Spencer
Reply to  Nick Stokes
November 21, 2016 7:57 pm

Your chemistry lesson leaves something to be desired. If you pulverize limestone and wet it to determine the pH, it will be alkaline. If one adds acid, CO2 will be driven off. The CO2 only appears to be acidic when it reacts with water to form carbonic acid, and the CO2 plays the part of the anion. The acidic property is the result of hydrogen ions, supplied by the water, in excess of what is found in pure water. While silica-rich rocks are often called “acidic” by geologists, they are so insoluble that the concept of pH (i.e. “acidity’) is inapplicable. Hydrogen ions have a positive charge. in the calcium silicate, the calcium has a positive charge and the silica has a negative charge.

Reply to  Clyde Spencer
November 21, 2016 8:09 pm

” The acidic property is the result of hydrogen ions, supplied by the water”
My chemistry isn’t weak. Yours is out of date. G.N. Lewis figured this out in 1923. An acid is an electron pair acceptor. CO₂ is an acid, CaO a base (donor), and
CaO + CO₂ → CaCO₃
is a simple acid-base reaction.

Clyde Spencer
Reply to  Clyde Spencer
November 22, 2016 2:57 pm

“Although the Brønsted–Lowry concept of acids and bases as donors and acceptors of protons is still the most generally accepted one, other definitions are often encountered. Certain of these are adapted for special situations only,” i.e. your Lewis Theory.
“Numerous lengthy polemical exchanges have taken place regarding the relative merits of the Brønsted–Lowry and Lewis definitions. The difference is essentially one of nomenclature and has little scientific content. In the remainder of this article the term acid is used to denote a proton donor (following the Brønsted–Lowry terminology), whereas the term Lewis acid is employed exclusively to refer to electron-pair acceptors. This choice is based partly on the logical difficulties mentioned in the last paragraph and partly on the fact (see below Acid–base equilibria) that the quantitative description of acid–base reactions is much simpler when it is confined to proton acids. It also represents the commonest usage of the terms.”
You can take it up with Encyclopedia Britannica: https://www.britannica.com/science/acid-base-reaction/Alternative-definitions

November 21, 2016 1:12 pm

In order for cement to be carbon-neutral, it must be made without burning carbon-based fuels and it must entirely convert back to carbonate. Good luck with it becoming carbonate while still being useful as cement. As the amount of concrete in use in the world grows, so does the amount of CO2 released from converting carbonate to oxide/hydroxide, plus the CO2 produced by burning fuel to produce cement.

Reply to  Donald L. Klipstein
November 22, 2016 12:56 am

The ancient Egyptians knew this – that’s why they mined and moved stone blocks by gangs of slaves instead of internal combustion vehicles. Maybe the pyramids were really carbon sequestration devices?

Lance Wallace
November 21, 2016 1:17 pm

Here is the full abstract. The paper (actually a letter) is by Xi et al.
Calcination of carbonate rocks during the manufacture of cement produced 5% of global CO2 emissions from all industrial process and fossil-fuel combustion in 2013. Considerable attention has been paid to quantifying these industrial process emissions from cement production, but the natural reversal of the process—carbonation—has received little attention in carbon cycle studies. Here, we use new and existing data
on cement materials during cement service life, demolition, and secondary use of concrete waste to estimate regional and global CO2 uptake between 1930 and 2013 using an analytical model describing carbonation chemistry. We find that carbonation of cement materials over their life cycle represents a large and growing net sink of CO2, increasing from 0.10 GtC yr−1 in 1998 to 0.25 GtC yr−1 in 2013. 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. We conclude that carbonation of cement products represents a substantial carbon sink that is not currently considered in emissions inventories.
Full paper here.

November 21, 2016 2:29 pm

“Sheesh, I’ve known that for at least 20 years. And I’ve never bothered to actually study this subject.”
This explains why some are not very good at telling the truth. They read a headline that supports their agenda and do not bother with further study.
There are many environmental impacts besides the trivial climate change issue. While the science in the paper is interesting, it is just a small piece of the puzzle.

November 21, 2016 2:51 pm

Concrete absorbs carbon dioxide and stores it in a process commonly called carbonation. This process gradually hardens up the concrete and is part of its curing process. Carbon dioxide may be absorbed by concrete in many forms such as buildings, bridges and pavements . Concrete does not have to be directly exposed to the atmosphere for this process to occur. Underground concrete piping, tunnels and foundations can absorb CO2 from the air in the soil, and from underground and underwater applications absorb dissolved carbon dioxide (carbonates) present in groundwater, freshwater, and saltwater. Concrete just keeps on giving, as the Romans knew.

November 21, 2016 3:12 pm

But there is no real evidence that CO2 affects climate. There is no such evidence in the paleoclimate record. There is plenty of scientific rational that the climate sensivity of CO2 is really zero. It is all a mater of science.

November 21, 2016 3:45 pm

When C02 and H20 combine they form HC03 Carbonic Acid
“Carbonation by Contact with Water
Natural waters usually have a pH of more than 7 and seldom less than 6. Even waters with a pH greater than 6.5 may be aggressive if they contain bicarbonates. Any water that contains bicarbonate ion also contains free carbon dioxide, which can dissolve calcium carbonate unless saturation already exists. Water with this aggressive carbon dioxide acts by acid reaction and can attack concrete and other portland cement products whether or not they are carbonated. A German specification, DIN 4030, includes both criteria and a test method for assessing the potential of damage from carbonic acid-bearing water. ”

steven f
Reply to  jmorpuss
November 22, 2016 7:23 pm

“Natural waters usually have a pH of more than 7 and seldom less than 6.”
This statement is correct for surface water. However it is often wrong for well water. PH levels for well water can be as low a 4. IF there is a volcanic source of CO2 PH can reach 3. This drop is mainly from CO2 absorbed by rain. If the rain water moves quickly down through the soil all the CO2 it picked up from the air makes it to the aquifer. This acidic water picks up a lot of minerals from the soil and rock forming hard water. When the water reached the surface the excess CO2 is quickly lost costing the PH to up. The water then becomes supper saturated with minerals which then deposits on faucets,sink and and other things

November 21, 2016 3:57 pm

i thought it mattered how the cement was made. what ratios used, before there would be any carbon dioxide getting locked up in it over years. got enough lime? check. is this study saying there’s a net gain? more back than spent? how are they making their concrete? using an efficient process? they’re not using hempcrete are they? because that’s cheating. ;D sucks up carbon dioxide to grow the stuff, and sucks up more as it ages and hardens.

November 21, 2016 4:07 pm

Concrete is calcium hydroxide and as long as it is “in use”, i.e. serves as structural element of buildings, most of it stays as calcium hydroxide with only few mm on the surface getting carbonated by atmospheric CO2. The depth of carbonation is proportional to square root of time, such as 1 mm in a year, 3 mm in nine years, 5 mm in 25 years. Given usual thickness of concrete walls and slabs, it’s negligible part of the volume in any reasonable timeframes.

Gunga Din
November 21, 2016 4:45 pm

CO2 molecules are released into the air when limestone (calcium carbonate) is converted to lime (calcium oxide), the key ingredient in cement.

Lime is also a key ingredient in many, many water treatment plants.
We add lime to water to change the calcium bicarbonate to calcium carbonate.
Calcium bicarbonate is very soluble in water and causes “hardness”. Calcium carbonate is not very soluble in water and settles out. (Making rock from water?8-)
We then add, you guessed it, CO2 back into the water to change the remaining, soluble calcium carbonate, back into the very soluble calcium bicarbonate so that it doesn’t settle out in the distribution system.
We currently get our CO2 from ethanol production.
I suppose we could get it from lime production if those older plants were set up to collect it, but then, what would the ethanol producers do with all their CO2? They are “saving the planet” after all.

November 21, 2016 5:13 pm

I didn’t deep read the article, only skimmed the abstract, but the impression I got is previously studies under-estimate the CO2 reduction in the conversion from silicates to carbonates in both the cement and the concrete aggregate. This makes sense to me as concrete is much more porous than your typical silicate rock.

November 21, 2016 5:58 pm

Well, duh! (What else can be said about something so obvious?)

tony mcleod
November 21, 2016 6:31 pm

Whoops indeed.

Hank Yancy
November 21, 2016 8:17 pm

There’s a huge error in the article. CO2 can’t contribute any alteration to climate. There’s a law of thermodynamics written specifically for gases. CO2 can’t affect climate, and that’s why the thermo-billies who believe in it show such allergic reaction to the real science of atmospheric chemistry.

November 21, 2016 10:41 pm

So something analogous to silicate weathering – the process by which newly exposed rock from tectonic activity absorbs CO2 – also happens in concrete? These weathering processes, if unopposed by processes like volcanic eruption and human burning of fossil fuel, will eventually kill off life on earth through CO2 starvation.

Non Nomen
November 22, 2016 2:55 am

The scientists concluded, however, that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were gradually reabsorbed during that time frame.
What about the remaining 57%?

Non Nomen
Reply to  Non Nomen
November 22, 2016 2:56 am

I put the blockquote wrong…
Question was: what about the remaining 57 %?

November 22, 2016 10:55 am

The folks at Biosphere II figured this out a long time ago…

steven f
November 22, 2016 7:08 pm

It is not just cement that absorbs CO2. When alumni oxide is converted to metal, a waist byproduct called red mud is produced. This mud contains iron,, sodium, calcium, and magnesium that was originally trapped in the aluminum oxide ore. This sodium, calcium, and magnesium also absorbs CO2. Possibly much faster than in concrete. Also mining breaks up rock into fine particles And atoms in the mine tailing starts to react with air pulling more CO2 out of the air.
As far as I know no one has calculated how much CO2 is removed by these activities.

Mary Brown
November 22, 2016 7:38 pm

Strikes me that calculating the net sink or source of things like this is nearly impossible. The errors and feedbacks and unknowns dwarf the results.

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