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Climate Change: Hot Debate over Green Concrete

Posted by: John Carey on March 26, 2009

Climate savior or potential green-washer? Los Gatos, Calif-based startup Calera Corp has a bold idea to do something concrete about climate change—literally. With venture funding from Vinod Khosla, the company plans to take carbon dioxide emitted by coal-fired powerplants, and combine it with seawater to make a ‘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 am not sure whether Calera is ignorant or intentionally misleading, or whether they actually have a basis for their claims. If they do have a basis for their claims they should state them now. If not, the California Academy of Sciences should remove their exhibit from the museum…. I believe Calera and the Academy of Sciences are now misinforming schoolchildren, and that is not a good thing to do.”

The Academy promptly responded by saying “we are looking into the issue, and if we have incorrect information on display, we will certainly remove it.”

Calera’s accused CEO, Brent Constantz, wasn’t so accommodating. In an angry email response, he accused Caldeira of trying to force his company to license a patent that Caldeira holds. “Based on this thinly covered, transparent attempt to disguise a need to get information for a greedy hope of a royalty stream as a concern for schoolchildren, I would question your personal imtegrity (sic), and tell you Callera (sic) wants nothing to do with you [and] your bogus science,” Constantz wrote.

Ouch. “Instead of providing facts, they launched an ad hominem attack on me,” Caldeira says. “Maybe they have an explanation. Maybe they have figured out some chemistry that’s magical I don’t know about, But they should come back explaining that, and not attacking me.”

What’s really going on? Caldeira suspects that “there is still a bubble in the clean tech world. The bubble may have popped in the rest of the economy, but it looks to me that, in this segment, there still a bubble and things that not seem to make much sense are attracting venture capital funding.”

As for Constantz, he downplayed the tiff in an email reply to BusinessWeek: "I think this has already blown over, as many people have forwarded Calera's published patent applications and water discharge permits to Caldeira, who apparently was unaware of all the detailed public information about our process. Needless to say, Caldeira has no leg to stand on, as our process has been in the public domain since last year, and the detailed information about inputs and outputs are in public records related to operating permits as well as the published patent applications."

"The chairman of our Scientific Advisory Board is willing to write a response to Caldeira's ungrounded claims - but we can't decide if it's really worthy of a response. I guess you have to ask yourself what the guy's motivation has been on attacking Calera - I certainly have no idea."

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Reader Comments


March 28, 2009 05:00 PM

So what's Caldeira's patent that Constanz refers to?

And can't BW inquire among some independent chemists to find out if Calera's formula for H2O + CO2 = Cement holds water? (And just in case it needs saying, that is not intended to represent Calera's actual claims about the chemistry of their proposal.)

And can't all the involved parties agree to use pseudonyms given that their names are too nearly identical and too easy to flub?


March 31, 2009 01:27 AM

The validity of Calera's technology is clearly illustrated by Brent Constantz's rabid response to Caldeira's comments.

Clearly, Constantz has no substance to offer - hence the vitriol and the nonsensical ravings.

Caldeira is an internationally renowned climate scientist who brooks no nonsense and is known for his integrity.

Meanwhile, Constantz appears more and more to be a pseudo-scientist come snake oil salesman.


March 31, 2009 01:29 AM

Caldeira's analysis is spot-on - Calera is an excellent example of Greenwash.

The nonsense of their technology is clearly illustrated by their patent application (US 20090020044 -available at ).

The patent describes taking dolomite/dolomitic limestone and calcining it (releasing CO2) and then reacting this with seawater and CO2 to regenerate calcium/magnesium carbonate!!!

There is very little if any capture of CO2 by their process since the calcining step generates so much CO2, and there is CO2 generated from the processing operations - it is highly probable that the technology has a net positive carbon footprint.

And where is the claimed biological process for capturing CO2? Calera is using decades old and very dirty and polluting chemistry for extracting magnesium/calcium from seawater - a very similar process was used by the magnesium factory which formerly occupied the Moss Landing site.

There is nothing Green about Calera's process - basically, they are burning/calcining limestone, and using it to make artificial limestone from seawater. All smoke and mirrors.

And, Calera's product is no cement - judging by the results they cite in their patent, their product is at best a poor mineral admixture. The patent shows that at a 20% replacement of Portland cement, the strength is only 50%(3000 psi) of that of 100% Portland cement - to get close to the strength development of Portland cement, the replacement level has to be reduced to 5%. Also, the drying shrinkage is doubled at 20% replacement of Portland cement.

Not only is Calera's product not a cement, but its addition to Portland cement is very deleterious - it greatly reduces strength and increases shrinkage - and almost certainly decreases long-term durability, corrosion resistance, freeze-thaw resistance, etc..

This is very disappointing - first Calera said that it had a 100% replacement for Portland cement that would capture 1 ton of CO2 per ton of cement (impossible, unless the cement is pure CO2!)- they then amended this to a 50% replacement for Portland cement and a carbon neutral cement. Now what - 5% replacement for Portland cement?

Total Greenwash!

Mark Celebuski

April 21, 2009 08:26 AM

The greenest concrete on the planet (no green washing needed).


April 23, 2009 01:27 PM

Mark Celebuski

You are advertising your product/machinery - Portland cement concrete made with recycled aggregate and supplementary materials.

This is nothing new - such “Low-CO2/Recycled Content Concretes” have been around and promoted for decades. These products have certain advantages, but many drawbacks - including scale-up, formulation, performance and durability concerns, and substantial secondary C/E footprints.

Your product is most certainly not “The Greenest Concrete on the Planet” - that claim is Greenwash.

More to the point, this blog is not for self-promotion.

Mark Celebuski

April 24, 2009 11:19 AM

If you will dig a little deeper you will see that the binder we used is fly ash based and contains very little if any Portland. The binder contains 94% recycled content. The key to everything working is the fact that the binder also mitigates ASR.

Here is a link the manufacturer’s website for further investigation:

My reason for posting this and the video is to get more people to investigate what I see as a giant step in solving a few problems at once: Recycled glass (60% land filled) fly ash (75% land filled) and CO2 emissions from making Portland and CSA cement. The answer exists, people are looking in the wrong places, and we investigated and found it.

We are a very small company without resources time or money to investigate or pursue this discovery further. We will make some green concrete countertops but that’s about it.

There should be a green concrete plant next to every recycling plant in the country. This could literally be done next week.

Sorry about the commercial aspects. You can call me with any questions.

Mark Celebuski


April 30, 2009 12:42 AM

Mark Celebuski

Hardly a discovery - the manufacturer and the technology are well known.

The company copied, or rather plaigarized the technology from patents dating back to the 1970s. Two major cement manufacturers patented and tried to commercialize the cement in the 1970s-1980s, and both failed.

They were unsuccessful because concretes made with this system failed during large-scale pours - the three outstanding issues were highly variable heat of hydration, shrinkage leading to major macro-crack defects, and rapid deterioration on wet-dry cycling.

The underlying chemistry, and the problems it causes are well known. Of course, this does not appear to have stopped the company from selling this defunct technology and claiming it as their own innovation.

In summary - a plaigarized technology - a bad cement and a worse concrete.


Mark Celebuski

April 30, 2009 07:38 PM

If you would have investigated a little further you would have found that the Soviets (Ukraine) where building entire buildings out of a similar cement technology in the 1960s.

The concrete used in these structures was recently tested and found to be as sound as Portland cement based concrete.

The “new old” cement has been and is being used in large scale pours without drying shrinkage problems (unless the W/C ratio was too high, which induces drying shrinkage problems in Portland based mixes as well) or variable heat of hydration.

The porosity of concrete made with this cement is actually lower than a Portland based mixes due to the fineness of the binder. The resistance to deterioration due to wet dry or freeze thaw cycles is actually better than Portland based products.

Mark Celebuski


May 2, 2009 12:54 AM


The Ukranian cements have nothing to do with the cement that you are using.

We are very familiar with the Ukranian soil cements/geopolymer cements - these are all based around alkali-catalyzed ash-slag formulations and produce unique network silicate structures. Yes, these cements can be very durable and when properly formulated, can replace Portland cement. These cements are enjoying a resurgence and are being investigated around the world. But, apart from decades-old constructions in the former USSR, a number of questionable operations in China, and one company in Australia attempting pilot-scale production, there are no present-day examples of geopolymer construction. Geopolymer cements require considerable expertise, and few people know how to make them. Witness the number of companies which tried to commercialize geopolymer cements in the 70s and 80s - they all failed due to cement variability and massive failure of the concretes.

However, this is an aside - since the cement you are using is not a geopolymer cement - its formulation, mechanism of set and the final structure are completely different. Also, it does not deliver anywhere near the robustness of a properly formulated geopolymer cement.

Mark Celebuski

May 5, 2009 07:52 AM

I believe that this requires more investigation prior to declaring “a bad cement and a worse concrete”. So far I’ve found the opposite to be true (I realize I’m a tiny fraction of the sample size currently using this cement). With the potential to solve multiple problems at once it would be a shame to dismiss it so easily.

I’d like to see a pilot green concrete plant (a volumetric mobile mixer would work) utilizing waste glass and green cement set up in the near future. It could be done next week.

Every 1,000 lbs. of concrete produced would save about 980 lbs. of material from a land fill and about 140 lbs. of CO2 from being released. Cast “safe” things at first such as walks, curbs, driveways, roads, slabs, countertops, etc. if people are worried about long term durability.

I would suggest contacting the cement manufacturer directly (the cement seems to be at the center of the debate) and investigating further before dismissing this technology as “greenwash”.

Mark Celebuski


May 5, 2009 12:11 PM


More investigation? Yes - you should investigate further to determine what type of cement you are using.

We are very familiar with the company and have obtained and analyzed several of its products:
1. None is "Green" by any stretch of the imagination.
2. None is novel as claimed - all the formulations are based upon other manufacturers patents/technologies.
3. Not one has worked as claimed.
4. The performances of all has been unacceptable for even very small projects, never mind large pours.

Low-CO2 footprint concretes utilizing BCs with fly ash/slag/CKD at up to 100% replacement of Portland cement, and recycled aggregate contents up to 88% have been around for decades. These are highly durable, have low CFs/EIFs, and are extensively used in Europe, China, Japan, S America, etc.

These are by far the best "Green" cements/concretes to date.

This is my final comment on this topic.



August 4, 2009 12:01 PM

Calera must be in dire straits - now they are dissolving olivine in hydrochloric acid and using the Mg/Ca stream from this, along with sodium hydroxide as base, to precipitate CO2.

They have also re-invented the electrolytic cell for the production of sodium hydroxide from brine.

Calera would have people believe that these energy/resource-intensive processes are "Green".

Yet more proof of the Calera Scam.

See their patent application WO/2009/086460 (METHODS OF SEQUESTERING CO2)

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BusinessWeek correspondents John Carey and Mark Scott, cover the green scene, keeping on top of the business aspects of energy, the environment and climate change, as well as the technologies, policies, markets and people that are shaping how the earth's resources will be used in the century ahead.

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