The construction industry is (slowly) beginning to see increased focus on the environmental impacts of buildings and construction. One area that seems to be getting a lot of attention recently is the carbon emissions due to cement and concrete. This seems at least partly due to Bill Gates’ book “How to Avoid a Climate Disaster”, which talks about emissions from cement (and I’ve been told everyone in Silicon Valley read.) Cement production is a huge industry, and is responsible for somewhere around 5-10% of global CO2 emissions [0]. Because concrete is such a fundamental part of the built environment, it’s worth taking a look at this problem in the context of how, as a civilization, we use concrete.
Most horizontal concrete (pavement, sidewalks, curbs etc.) is unreinforced - I would guess somewhere around 1/3 of all concrete placed in the US is unreinforced.
Concrete mix design is all about tradeoffs - strength, workability, set time, durability, cost, etc. Embodied CO2 is one of those tradeoffs. Each of these tradeoffs has a central value as well as risk. The easiest way to reduce CO2 is to reduce the cement content (i.e. more rock, less glue). This generally also increases durability, but also brings in risks on the strength and workability fronts. Mitigating this risk requires thought/engineering. In general cement is cheap and thought is expensive.
(As a note, when comparing concrete costs you need to be careful about where that cost is assessed - at the batch plant, or delivered to the site (including transport), or in place (including formwork, screeding, finishing, curing etc.).
With regards to carbonation, carbonation only applies to the lime component (calcium hydroxide aka CH). An excess of CH is generally necessary to get an acceptable set time, but is in general detrimental to durability. CH can get soaked up by supplementary cementitious materials (flyash, slag, natural pozz, CO2 injection (trivially in that case) etc), but then that CH is not available for future carbonation. In a geologic timeframe all of the available CH will get soaked up by something, but good quality concrete doesn't have a lot of excess CH, and the penetration of carbonic acid is very slow.
Before we start coming up with innovative solutions, we can start with:
1. actually designing structures that are efficient, instead of utilizing 75% of the capacity because 95% 'feels' too dangerous.
2. notifying the client that the architect's design could've been a lot more sustainable if it is not a 10m multistory cantilever that would require a transfer structure and a heavily reinforced core wall.
3. not giving in to seemingly sustainable design choices e.g. sky garden, which adds an insignificant amount of SDL.
In general... do you job well and less regulations
I would have thought that carbon capture and storage (CCS) is one good solution. What it needs to be cost-effective is a) a concentrated source of carbon emissions, which cement production sites can be, and b) existing pipeline infrastructure so you can pump carbon back down into depleted oil or gas fields to store underground for the long term. So some, but not all, cement production sites would seem to fit the bill. I'm not an expert in cement or CCS but am making an educated guess.
I came across this article quite by accident but it's fascinating.
"In a pure mass-flow sense, human civilization is basically a machine for producing concrete and gravel (and to a lesser extent bricks and asphalt), with everything else as just a byproduct."
This is an interesting article. What I am wondering is what about using stone for foundations. I grew up in a building in Pullman on the south side of Chicago that had Lanon stone foundations; it was built in 1880 and it is still there. Furthermore, stone arches could replace steel and concrete in small span areas. The Pont du Gard aqueduct over the Gard River in southern France has a span of 80 feet and it is made of dry stone (no cement) and was built in the first century. In such buildings, stone goes directly from the quarry directly to the building site. Isn't this a possibility?
If you'd use CCS to capture all the CO2 from calcination during cement production, could carbonation actually make concrete into a material with negative CO2 emissions?
Using 50% of your ballpark estimate, 0.46 tons of unavoidable CO2 emissions for each ton of cement, means that you can have net zero cement for an extra $27 per ton using CCS with deep geological storage [1]. Or even less, considering the absorption. This amounts to an extra $8 per cubic meter of concrete, roughly 1/20 typical prices.
So there's no need to replace or reduce concrete, just create adequate carbon pricing incentives and push clean energy on the whole chain (that will cost as well, but it will affect alternatives the same or more).
That's good news since concrete is such a great material for flexibility, mass effects, durability in direct contact with the environment, availability, price, fire resistance, existing infrastructure and skill base etc. For example, I doubt wood foundations are a good idea in all but the most arid locations.
[1] Total cost of carbon capture and storage implemented at a regional scale: northeastern and midwestern United States William J. Schmelz, Gal Hochman and Kenneth G. Miller
A couple of minor comments:
Most horizontal concrete (pavement, sidewalks, curbs etc.) is unreinforced - I would guess somewhere around 1/3 of all concrete placed in the US is unreinforced.
Concrete mix design is all about tradeoffs - strength, workability, set time, durability, cost, etc. Embodied CO2 is one of those tradeoffs. Each of these tradeoffs has a central value as well as risk. The easiest way to reduce CO2 is to reduce the cement content (i.e. more rock, less glue). This generally also increases durability, but also brings in risks on the strength and workability fronts. Mitigating this risk requires thought/engineering. In general cement is cheap and thought is expensive.
(As a note, when comparing concrete costs you need to be careful about where that cost is assessed - at the batch plant, or delivered to the site (including transport), or in place (including formwork, screeding, finishing, curing etc.).
With regards to carbonation, carbonation only applies to the lime component (calcium hydroxide aka CH). An excess of CH is generally necessary to get an acceptable set time, but is in general detrimental to durability. CH can get soaked up by supplementary cementitious materials (flyash, slag, natural pozz, CO2 injection (trivially in that case) etc), but then that CH is not available for future carbonation. In a geologic timeframe all of the available CH will get soaked up by something, but good quality concrete doesn't have a lot of excess CH, and the penetration of carbonic acid is very slow.
Before we start coming up with innovative solutions, we can start with:
1. actually designing structures that are efficient, instead of utilizing 75% of the capacity because 95% 'feels' too dangerous.
2. notifying the client that the architect's design could've been a lot more sustainable if it is not a 10m multistory cantilever that would require a transfer structure and a heavily reinforced core wall.
3. not giving in to seemingly sustainable design choices e.g. sky garden, which adds an insignificant amount of SDL.
In general... do you job well and less regulations
I would have thought that carbon capture and storage (CCS) is one good solution. What it needs to be cost-effective is a) a concentrated source of carbon emissions, which cement production sites can be, and b) existing pipeline infrastructure so you can pump carbon back down into depleted oil or gas fields to store underground for the long term. So some, but not all, cement production sites would seem to fit the bill. I'm not an expert in cement or CCS but am making an educated guess.
I came across this article quite by accident but it's fascinating.
"In a pure mass-flow sense, human civilization is basically a machine for producing concrete and gravel (and to a lesser extent bricks and asphalt), with everything else as just a byproduct."
!!!
This is an interesting article. What I am wondering is what about using stone for foundations. I grew up in a building in Pullman on the south side of Chicago that had Lanon stone foundations; it was built in 1880 and it is still there. Furthermore, stone arches could replace steel and concrete in small span areas. The Pont du Gard aqueduct over the Gard River in southern France has a span of 80 feet and it is made of dry stone (no cement) and was built in the first century. In such buildings, stone goes directly from the quarry directly to the building site. Isn't this a possibility?
If you'd use CCS to capture all the CO2 from calcination during cement production, could carbonation actually make concrete into a material with negative CO2 emissions?
Using 50% of your ballpark estimate, 0.46 tons of unavoidable CO2 emissions for each ton of cement, means that you can have net zero cement for an extra $27 per ton using CCS with deep geological storage [1]. Or even less, considering the absorption. This amounts to an extra $8 per cubic meter of concrete, roughly 1/20 typical prices.
So there's no need to replace or reduce concrete, just create adequate carbon pricing incentives and push clean energy on the whole chain (that will cost as well, but it will affect alternatives the same or more).
That's good news since concrete is such a great material for flexibility, mass effects, durability in direct contact with the environment, availability, price, fire resistance, existing infrastructure and skill base etc. For example, I doubt wood foundations are a good idea in all but the most arid locations.
[1] Total cost of carbon capture and storage implemented at a regional scale: northeastern and midwestern United States William J. Schmelz, Gal Hochman and Kenneth G. Miller