[PradeepIV]

Recently, Calera proposed a process suitable for power plants and other major CO2 emitters which converts CO2 to calcium carbonate (CaCO3). From the Scientific American:
"...The Calera process essentially mimics marine cement, which is produced by coral when making their shells and reefs, taking the calcium and magnesium in seawater and using it to form carbonates at normal temperatures and pressures. "We are turning CO2 into carbonic acid and then making carbonate," Constantz says. "All we need is water and pollution...
....Calera hopes to get over that hurdle quickly by first offering a blend of its carbon-storing cement and Portland cement, which would not initially store any extra greenhouse gases but would at least balance out the emissions from making the traditional mortar. "It's just a little better than carbon neutral," notes Constantz, who will make his case to the industry at large at the World of Concrete trade fair in February. "That alone is a huge step forward.".."

In the following, I discuss some of the critical challenges to make this a feasible operation. The roles of the abundance of calcium and magnesium in sea water and its pH are discussed in detail. The formation of calcium/magnesium carbonates from sea water requires energy to proceed. This energy could be supplied either in the form of a pH shift (by adding strong base) or by coupling the carbonate formation to other processes (algal photosynthesis).

Comments on a marine cement production process

[Gerben]

Chalk is not a scarce resource atm. If we wanted to add chalk to concrete or cement to save energy, we can. No new technology is required for that. Just the will to make lower quality cement and concrete.

[PradeepIV]

Chalk is not a scarce resource atm. If we wanted to add chalk to concrete or cement to save energy, we can. No new technology is required for that. Just the will to make lower quality cement and concrete.

I agree. This is where the cement industry will have to play a role in coming up with newer blends and strength & durability studies for these carbonate-cement blends.

BTW, if we just blended chalk with cement (clinker+fly ash), it does not contribute to additional CO2 emission reductions, apart from that saved by not using the equivalent amount of clinker.
Each T of cement produces ~1.3 T CO2.
Each T of chalk/CaCO3 replacing clinker will save 1 T clinker ~ 1.3 T CO2.
So if we consider a 5% cement-CaCO3 blend,
100 T blend = 5 T CaCO3 = 5*1.3 T CO2 reduced
So, the CO2 reductions are 6.5 T CO2/100 T cement blend.(not accounting for GHG emissions during chalk mining and comminution)

On the other hand, if we converted the CO2 back to CaCO3:
5 T CaCO3 saves 2.2 T CO2, so, total savings are 8.7 T CO2/100 T of th cement blend.

Therefore, converting CO2 to CaCO3 and blending into cement has higher GHG reduction potential (34% more) compared to blending chalk.

[Tanada]

How different really is it from THIStype of project?

Frankly, it sounds too good to be true, given the cement and concrete industry represent more than 5 per cent of global CO2 emissions and something has to be done about it. If all precast operations used Carbon Sense’s process, it would sequester as much as 20 per cent of those emissions in concrete, says Niven. How could this be? Because a precast plant alone wouldn’t have enough emissions to feed the process. To maximize CO2 absortion, a precast plant would have to get more CO2 from the flue stacks of neighbouring industrial facilities — assuming ideal logistics.

[PradeepIV]

How different really is it from THIStype of project?

Frankly, it sounds too good to be true, given the cement and concrete industry represent more than 5 per cent of global CO2 emissions and something has to be done about it. If all precast operations used Carbon Sense’s process, it would sequester as much as 20 per cent of those emissions in concrete, says Niven. How could this be? Because a precast plant alone wouldn’t have enough emissions to feed the process. To maximize CO2 absortion, a precast plant would have to get more CO2 from the flue stacks of neighbouring industrial facilities — assuming ideal logistics.

Disclaimer: This is not an endorsement of a specific process. Both of the two processes I compare below have their advantages and limitations. Any comments are appreciated.
Tanada,
Thanks for the link. There certainly are many processes for reducing CO2 emissions from concrete. Whereas the Calera process converts CO2 in the flue gas to CaCO3 and then blends this with the portland cement produced, the Carbon Sense Solutions puts the CO2 back into the cement chemically. Cement itself is manufactured by using calcium carbonate, bauxite, silica and other additives. Basically, it is taking CO2 out of the carbonate and combining the lime with silica and alumina. When this calcium aluminosilicate is exposed to CO2, it reacts forming CaCO3.

The Carbon Sense process likely does not lead to a reduction of CO2 in the atmosphere because the process of concrete (cement) manufacture itself produces CO2 emissions. For more info about the background chemistry, see this presentation.

10% of the US cement consumption is ~10 million tonnes of cement per annum (MT/year) (or 13 MTCO2/year). If we assume that when this 10 MT cement is used for making CO2-cured concrete, all of it reacts with CO2 to form CaCO3 (an unlikely scenario), all we are doing is putting back CO2 into the concrete, making the process CO2-neutral. Additionally, as the article mentions, it will get many donkeys to fly to get the CO2 close to the precast fab plant.

On the other hand, the Calera process needs sea water (or a source of ADDITIONAL calcium or magnesium) to form ADDITIONAL CaCO3 or MgCO3 which are blended in the cement. If someone comes up with a neat way to swing the pH, this COULD represent additional GHG reductions.

[Gerben]

Euh. Wait a minute. Carbonate is not the same as concrete. It's not as usefull as building material. You cannot compare a situation where you use carbonate with a situation where you use concrete. You cannot build the same structure with it. It's like comparing concrete with straw. Both can be used to make structures.

[PradeepIV]

Euh. Wait a minute. Carbonate is not the same as concrete. It's not as usefull as building material. You cannot compare a situation where you use carbonate with a situation where you use concrete. You cannot build the same structure with it. It's like comparing concrete with straw. Both can be used to make structures.

Gerben,
You are correct. We strictly cannot compare carbonate to concrete. Does any one have data comparing the strength and durability of a) co2-cured concrete and b) concrete made from CaCO3 blended with portland cement?

The CO2-curing of the concrete still produces calcium carbonate by reacting the calcium aluminosilicates with CO2. On the other hand, marine cement, CaCO3 is blended into the cement in the Calera process. Currently, the ASTM standard allows 5% blends of ground limestone (a calcium carbonate mineral) with Portland cement.

I guess my earlier post was not about the quality of the concrete itself, but the potential for reducing CO2 emissions from the two processes.