Carbon Negative Concrete

[Dave Jones]

with the government pushing hard to decarbonise house building, sorting the concrete would be a massive step.

 

https://www.popularmechanics.com/science/green-tech/a40785162/microalgae-carbon-neutral-cement/

 

[75darren]

Wow, I never knew concrete accounts for over 7 percent of the world’s annual greenhouse gas emissions. Interesting article.

[joe90]

Very interesting, especially after watching blue planet 3 last night !

[SteamyTea]

There is a difference between cement and concrete.

In 2021, cerement accounted for 1,672,592,400 tonnes of CO₂

In the same year, the world produced 37,123,850,000 tonnes.

Concrete was 4.5% of global CO₂ emissions.

https://ourworldindata.org/grapher/annual-co2-cement?tab=table&time=latest

https://ourworldindata.org/co2-emissions#co2-emissions-by-region

 

There is around 750 kg CO₂/tonne for cement and around 1000 kg CO₂/tonne for concrete emitted.

Cement looks very bad in isolation.

Concrete can last several hundred years and can be easily reused.

 

CO₂ from cement is not just the energy used (usually natural gas burners) but is also produced as part of the chemical change during roasting.

 

If a house has a slab that is 10 tonnes of concrete, that is 10 tonnes of CO₂.

A good car these days produces around 0.1kg/CO₂.km.  So 100,000 km in a car is about the same.  That is around 62k miles.

My 16 year old car, newly MOTed at 240k miles, produces 0.143kg CO₂/km, last year I drove 29,678 miles, that is 47,762 km, so it produced 6.83 tonnes of CO₂ at the tail pipe.  That is a mean of 3,452 tonnes/year so far over the life of the car.

Another way to look at that is to compare it to my house, which uses around 4 MWh/year.  It is all electric and CO₂ emissions for electricity last year were around 200 kg CO₂/MWh, so my house produced around 0.8 tonnes (maybe a little less as I think last year I only used 3.3 MWh and CO₂ emissions are dropped quite rapidly last year).

 

[saveasteading]

Interesting but perhaps academic.

2 million acres of open ponds, in a warm climate.

Thinks...that's...a lot of shallow, level, warm ground, and a lot of water.

 But if it takes that to cancel out the emissions, let's do double that and pile up the surplus lime.

 

And hang on, my brain's not working properly and I'm sure I used to know this.

Lime is roasted, using energy and forcing out CO2.

When we hydrate it back to a hard lump,  aren't we taking back carbon?

 

Or is that just 50 years later when it is crushed and exposed to air?

 

Anyway, my policy in design was always simple. Design well and use less stuff. Accept fly ash in the mix even if it isn't much cheaper. Don't overspecify the concrete mix.

No waste allowed: have a ready use for excess concrete, offcuts etc. Half and faulty bricks collected and used.  No skips on site...or they get filled with stuff you bought.

 

In other words, what you would do if you were paying for it yourself , must be applied whatever the job size.

If the whole construction industry did that..?

[SteamyTea]

58 minutes ago, saveasteading said:

Lime is roasted, using energy and forcing out CO2.

When we hydrate it back to a hard lump,  aren't we taking back carbon

No.

I have been reading up on it.

It is a complicated process, both the roaring (4 stages) and the hydration a multi stage process depending on type.

The typical makeup of Portland cement is.

Cement Compound Weight Percentage Chemical Formula

Tricalcium silicate 50 % Ca3SiO5 or 3CaO.SiO2

Dicalcium silicate 25 % Ca2SiO4 or 2CaO.SiO2

Tricalcium aluminate 10 % Ca3Al2O6 or 3CaO .Al2O3

Tetracalcium aluminoferrite 10 % Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3

Gypsum 5 % CaSO4.2H2O

 

It is all about calcification and hydroxides which cause the crystallization.

The curing is very interesting as time is important. Initially there is a rapid rise in temperature and the tricalcium silicate cures to give initial strength, then, over time, the dicalcium silicate comes into play and starts to cure, contributing to the final strength.

While that is happening, some of the other compounds in it are reacting.

 

As you know, concrete does not dry, it cures. The word drying should stopped being used in conjunction with cement products as it is misleading and causes misunderstanding of the processes (note plural).

 

I grabbed the above from here, which is a long, but easy to follow description.

 

http://matse1.matse.illinois.edu/concrete/prin.html

 

You will see that energy units (kJ) appear often in the formula. That energy is what is doing the work of forcing molecules to bond. We often think that things getting hot is a nuisance, but in reality it is a vital part of the process.

 

If you stop and think about it, a builder's labourer, who may have no experience in chemistry, should not really be allowed to mix up concrete, ever.

Edited October 23, 2023 by SteamyTea

[Adrian Walker]

On 30/04/2023 at 06:41, Dave Jones said:

with the government pushing hard to decarbonise house building, sorting the concrete would be a massive step.

 

https://www.popularmechanics.com/science/green-tech/a40785162/microalgae-carbon-neutral-cement/

 

You can have low carbon and zero carbon, although the latter needs some offsetting.  Some concrete has liquid CO2 added at the final stage.

 

Please note: China has emitted more carbon dioxide over the past eight years than the UK has since the start of the Industrial Revolution. Between 1750 and 2020, the UK emitted 78 billion tonnes of carbon dioxide into the atmosphere, compared with China's emissions of 80 billion tonnes since 2013.