I am trying to identify the plausible CO2-mitigation strategies that are scalable — that can comprise at least a half a wedge (see “How the world can stabilize at 350 to 450 ppm: The full global warming solution).

So when a new process gets this much hype — as in Scientific American’s, “Cement from CO2: A Concrete Cure for Global Warming?” — it deserves scrutiny. Wired magazine’s “The Top 10 Green-Tech Breakthroughs of 2008,” provides both a good summary of the process and more evidence of the hype:


Cement? With all the whiz bang technologies in green technology, cement seems like an odd pick for our top clean technology of the year. But here’s the reason: making cement — and many other materials — takes a lot of heat and that heat comes from fossil fuels.

Grist thanks its sponsors. Become one.

Calera’s technology, like that of many green chemistry companies, works more like Jell-O setting. By employing catalysis instead of heat, it reduces the energy cost per ton of cement. And in this process, CO2 is an input, not an output. So, instead of producing a ton of carbon dioxide per ton of cement made — as is the case with old-school Portland cement — half a ton of carbon dioxide can be sequestered.

With more than 2.3 billion tons of cement produced each year, reversing the carbon-balance of the world’s cement would be a solution that’s the scale of the world’s climate change problem.

In August, the company opened its first demonstration site next to Dynegy’s Moss Landing power plant in California, pictured here.

As the sage once said, “Amazing, if true.”

Grist thanks its sponsors. Become one.

Yet whether Calera’s process can actually sequester significant amounts of net CO2 and whether it is scalable has been called into question by some of the country’s leading climate scientists, including Ken Caldeira, a widely published expert on the carbon cycle whom I have known for many years.

Emails on this subject have been racing around the Internet, and I have communicated with both Calera and Caldeira (yes, I know, the kind of strange coincidence that makes reality so much less plausible than fiction).

While this is a long post with a lot of unavoidable chemistry it, the bottom line is that I think Caldeira has made a strong case that

  • The scalability of the process is in doubt
  • We won’t know if net CO2 is saved unless Calera is much more forthcoming on all of the inputs and outputs

Questions surrounding Calera’s process — and the too-hot e-mail exchange — became public when John Carey of Business Week wrote about the “Hot Debate over Green Concrete“:

The process is similar to the formation of coral reefs, the company says. It even arranged for an exhibit showing the process at the California Academy of Sciences.

Not so fast, says Ken Caldeira, climate scientist at the Carnegie Institution of Washington at Stanford University. “Their claim that they can put CO2 in sea water and create minerals makes no sense to me at all.” When coral does make reefs, Caldeira points out, CO2 is actually released to the atmosphere. Making concrete-like minerals through the process “is backwards to the chemistry the rest of the world is accustomed to,” Caldeira says.

So in an email message on March 22, Caldeira took on Calera, company founder and CEO Brent Constantz (also an earth sciences professor at Stanford), and the California Academy of Sciences.

He wrote: “From the publicly available information it seems that Calera’s process goes in the wrong direction and will tend to increase and not decrease atmospheric CO2 content. Furthermore, when I raised these concerns to Calera, they would not respond openly to my critique, asking me instead to sign a non-disclosure agreement.”

“I call upon the California Academy of Sciences to withdraw the Calera exhibit until such time that Calera demonstrates (i) that its process does not remove cations from the ocean in a way that will ultimately drive a CO2 flux from the ocean to the atmosphere that exceeds the amount of fossil fuel stored in the carbonate mineral and (ii) that its process does not acidify the ocean.”

I asked Calera for a response to what Caldeira (and other scientists) have said in emails. Brent Constantz replied with a forwarded email:

Dear Dr. Pope,

Brent Constantz informed me yesterday of the negative comments about the Calera Corporation made by Ken Caldeira on a blog site. I judge these comments to be fatuous and indeed insulting and question Caldeira’s motivation for writing them.

The credentials of Brent Constantz and those of the group of distinguished scientists who comprise his Scientific Advisory Board are beyond dispute. Let me assure you that the Calera process does not introduce carbon dioxide to the atmosphere! In stark contrast, the process is an extremely effective means of sequestering carbon dioxide that would otherwise go into the atmosphere from the stacks of power plants. The process described by Caldeira has nothing to do with the Calera process and he should know better than to suggest that it does.

The attached file is a brief explanation of how the Calera process sequesters carbon dioxide. If you have any questions, please do not hesitate to contact me.

Sincerely yours,

J. R. O’Neil, Chair

Scientific Advisory Board

Calera Corporation

Here is the attached file from Calera (see here for original with subscripts and superscripts) — my apologies for the chemistry, but it is unavoidable:

The Calera Process: An Effective Means of CO2 Sequestration

The Calera Process consists of reacting carbon dioxide (CO2) tapped from stacks of operating energy generating plants with treated seawater to produce solid carbonates of calcium (Ca) and magnesium (Mg). These solids are then used in various ways in the production of concrete. The process is a simple and effective means of sequestering CO2 that would otherwise pollute the atmosphere and contribute to global warming..

Seawater contains the following pertinent chemical species:

Ca2+, Mg2+, CO32-, HCO3-, (CO2)aq, H2CO3, H+ and OH-

At a given pH the relative amounts of the various carbonate species are all in rapidly attained chemical equilibrium. Carbonate precipitation can occur if the solubility products (Ksp) of the various possible carbonates are exceeded. The solubility product of a carbonate is given by the following expression:

[M2+][CO32] = Ksp

where [M2+] is the concentration (activity) of the metal cation and [CO32-] is the concentration of the carbonate ion. Precipitation of a solid carbonate from seawater will take place under one of two conditions.

1. The concentrations of the cation (M), in this case Ca2+ or. Mg2+ or both, are increased to the point where

[M2+][CO32-] > Ksp of MCO3.

2. The concentration of CO32- is increased to the point where

[M2+][CO32-] > Ksp of MCO3

In the Calera process the concentration of CO32- is raised (case 2) by the addition of CO2 and most or all of the Ca and Mg present in a given volume of seawater precipitates as a solid carbonate. The concentrations of Ca and Mg in seawater are relatively constant and fixed worldwide.

The concentration of CO32- in seawater is increased upon introduction of the stack CO2 because the pH of the seawater (normally around 8 ) has been raised to the point where CO32- is the dominant and stable species of dissolved carbonate. Alkaline solutions like this are very effective sinks for gaseous CO2. Calera methods for making seawater appropriately alkaline are proprietary, but it can be done simply by addition of a base like sodium hydroxide.

I then shared this document with Caldeira (who in turn shared it with others).

This was Caldeira’s reply:

The document you send gives away the piece of information missing from the museum exhibit. They need to add alkalinity to the system and that is not mentioned in their museum exhibit.

They need to add a base like sodium hydroxide. How much sodium hydroxide is available in the world? The answer is not much.

Kheshgi 1995 discussed the availability of alkaline resources in the world and his conclusion was that there was not enough to make a substantial dent in global emissions. (I sent this paper to the google discussion group.) For example, Kheshgi estimates that if you mined all of the available sodium hydroxide in the world, you would be able to offset about 5 GtC of CO2 emissions.

They claim publicly that their process requires only seawater and CO2, both of which are abundantly available, and then it turns out that their process depends on relatively rare alkali deposits.

Anybody can reduce net emissions with a good supply of alkali materials, so if that is their process, it is a non-event. They promised a scalable solution and provide a solution with very limited applicability.

So, they did misrepresent their process to schoolchildren. They neglected to mention their most important ingredient — relatively rare alkali materials.

By the way, if you do have alkali materials, it is much more effective to dissolve it in the ocean — reduce ocean acidification and store more CO2 in the ocean — than to make carbonate minerals. Dissolving it in the ocean would store about twice as much CO2.

So, they advertise to the world that they can store CO2 as cement using only seawater and CO2 as source materials, which would be a miraculously impressive invention. Then when pushed, they say they can store CO2 if you would give them an abundant supply of alkali minerals — but everyone knew this already. If they had said that from the outset, nobody would have found their process interesting.

So, it is clearly a case of public misrepresentation: They claimed they could sequester CO2 with seawater and they cannot. Now they are saying they can sequester CO2 using alkali minerals. They certainly can, but everyone knew that already … and this approach has been discounted as being unimportant to the climate-carbon problem because it is not scalable to the scale of the problem….

Ken Caldeira
Carnegie Institution Dept of Global Ecology

Caldeira adds in a separate email:

I am pretty sure that the magnesium hydroxide [Mg(OH)2] at Moss Landing was made through the process something like:

Mg2+ + CaMg(CO3)2 + 2H2O –> 2Mg(OH)2 + 2CO2 + Ca2+

If Calera is using this magnesium hydroxide in their process, they are just recovering the CO2 released during its manufacture.

And he adds:

I note that Calera still is not forthcoming in response to my question regarding what are the inputs to and outputs from their process, in a way that allows balances of mass, energy, and electric charge to be assessed.

They need to maintain acid-base balance and get the alkalinity from somewhere or dispose of acidity somewhere, and until they are forthcoming on this point there is no way their process can be assessed.

Their process can be proprietary but there is no need for secretiveness with respect to inputs and outputs.

Until such time as they present information that allows independent assessment, I will assume their process can make no quantitatively important contribution to addressing the climate-carbon problem.

I am not a chemist, but I have received emails supporting his analysis. Caldeira’s argument seems strong, especially as to scalability.

Ken sent me a further elaboration when I asked for something for a non-technical audience:

You need alkalinity from somewhere. Alkalinity is the net positive charge on the cations of the strong acids (HCl, etc) minus the net negative charge on the anions of the strong bases (NaOH, etc). This difference is available to bind with CO2 to form carbonates.

There are at least three approaches to getting alkalinity:

1. From carbonate minerals like CaCO3. Unfortunately, this comes with CO2 (CaO+CO2) so if you are trying to produce carbonates this is no help.

2. From strong bases available naturally. Unfortunately, there are no large pools of lye hanging around ready to react with CO2. Strong bases today are formed in factories, and are not generally mined. For example, most NaOH would have already reacted with CO2 to form Na2(CO3), but since they are forming carbonates they cannot afford to start with a carbonate. Also, its production, say by electrolosis of NaCl, also produces HCl, which you would need to get rid of somehow. Another example is the Mg(OH)2 at Moss Landing which was produced by heating the CO2 out of dolomite.

3. By disposing of acidity from seawater. You could, as above, electolyze seawater (with large energy input) and then make NaOH and HCl (again, cost is about $1000/ton NaOH). Then you need to do something with the HCl. If you return it to the ocean it will acidify the ocean and drive CO2 into the atmosphere. I suppose you could pump the HCl underground or something and sequester HCl instead of CO2. This is probably scalable, but unlikely to be economic.

4. By accelerating the weathering of silicate rocks. This is something that Klaus Lackner and others have been working on. The problem is that the kinetics are slow.

Recall that they will need 1 atom of Mg or Ca for each molecule of CO2. So for each ton of CO2, they will need approximately and equal mass of Mg or Ca from a strong base. These are not minor requirements that can be easily overlooked.

So, without them saying exactly what their inputs and outputs it is hard to evaluate their scheme. My guess is that they may be heading to option 3, but it is hard to see how that will be economically viable.

One way to get a handle on this is to look at prices of strong bases like NaOH, Mg(OH)2, etc. I think you will find that if you have to pay market prices for these strong bases (making sure that you are producing them by methods that do not release CO2 or acidity into the environment), their process will not be economic.



So I think at the least, Calera needs to prove the “inputs to and outputs from their process, in a way that allows balances of mass, energy, and electric charge to be assessed” independently.


This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.

Reader support helps sustain our work. Donate today to keep our climate news free.