CDR geoengineering challenge: low-carbon cement, high-carbon concrete

CDR geoengineering cement concreteCurrent practice is to grind and burn enormous amounts of limestone, releasing equally enormous amounts of CO2 to the atmosphere. But isn’t there some way to reverse the chemical process and still end up with building material?

First step is to reduce cement emissions; second step is to find a means to actually turn the world’s countless concrete construction sites into carbon sinks, reabsorbing some of that CO2 we emit and safely storing it in sturdy molecules that allow us to live in houses with walls that can carry many floors and a roof.

Let’s be optimistic and say new research shows we’re almost halfway.

Under 450 Scenario we are all essential

In the global effort to stabilise atmospheric greenhouse gas concentrations at no more than 450 ppm CO2 equivalents the first thing to do is lower emissions as far as possible. To achieve this UN climate target the energy sector deserves prime focus, as both energy efficiency improvements and maximised transition to zero carbon sources are essential. Of course that big chunk of deforestation and other land use carbon emission sources will have to abated as well.

But there is one other industry we shouldn’t ignore – but mostly are: cement. It is said to be responsible for over 5 percent of all anthropogenic emissions – and for a single industry that is more than perhaps it may sound.

You want a building block, start with stone – and end up with gas…?!

Let’s acknowledge there are clear benefits to the use of concrete. The one unfortunate thing about it though is the way we use it today. Most of it is a blend based on Portland cement, and Portland cement is next to some added gypsum and clay just plain quicklime – calcium carbonates (CaCO3) heated to the point that all CO2 evaporates and CaO remains.

To illustrate how carbon intensive the process is: to create one kilogramme of Portland cement, 0.9 kilogrammes of CO2 are released to the atmosphere, 50 percent from unlocking the CO2 in the carbonates and 40 percent from the high energy requirements.

Together with the other mentioned ingredients and mixed with water, in this conventional cement calcium silicate hydrate (C-S-H) is formed, which gives concrete its strength and durability.

Improving concrete with other C-S-H source

But this conventional Portland C-S-H is essential to the mix, says a professor of civil and environmental engineering at the University of California at Berkeley. Together with researchers of the Lawrence Berkeley National Laboratory Paulo Monteiro has investigated an alternative form of calcium silicate hydrate, the mineral tobermorite.

In nanoscale lab tests tobermorite proved to have excellent properties to use it as a concrete ingredient, the researchers write [publication in Cement and Concrete Research].

Compared to Portland cement the big climate advantage is that this source of calcium silicate hydrate is already there, as tobermorite occurs naturally in metamorphous limestone (and in skarn) – and grinding it will not release any chemically bound carbon as CO2.

It’s not the sole option to decrease the carbon intensity of the concrete mix. Depending on use concrete can actually improve when you add ample amounts of fly ash to the mix. Ironically that is the solid state left-over stuff of the coal industry. But in any case better make good use than to simply dump in enormous landfills as is current practice.

Are you good at chemistry?

The biggest open challenge is however for that one smart chemist that would really like to win a well-deserved Nobel Prize. Try and think of a concrete that – as it hardens, or ripens, includes some actual carbonates in or in between its molecules, perhaps as an organic means to cure concrete rot. It would count as a means of carbon dioxide removal geoengineering that we feel no one could oppose – and thereby deliver a valuable contribution to keeping atmospheric CO2 levels within the utmost margins.

If however creative cleverness fails us or if such a genius invention would fail to reach construction markets – there’s an attractive back to basic alternative at hand: wood building.

© Rolf Schuttenhelm |

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