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Cement and Concrete Greenhouse Gases

Concrete accounts for 5% of all greenhouse gases. There are new techniques to reduce this impact. Are they being used?

Concrete is a building material first used by the ancient Romans, and consists of around 14% cement, the rest being variable proportions of fly ash, aggregate, and pozzolan.

Road vehicles are responsible for around 13% of global CO2 emissions. However, the production and use of cement, mainly for construction concrete, is alone producing 5% of the world's total CO2 emissions! This comes from two sources: a little less than half of the carbon dioxide is produced during the consumption of fossil fuel in the cement production process, and most of the rest is released while calcium carbonate undergoes thermal decomposition as the concrete sets in-situ. 900kg of CO2 is released for each tonne of cement.

Each year, more than one cubic metre of concrete is made for every person on Earth. This corresponds to around 5 tonnes of CO2 emissions per person per year.

Portland cement production involves high temperatures, 1500°C, to produce clinker from limestone, which accounts for 40% of the CO2 emissions of concrete

Limestone is burnt at temperatures as high as 1500°C for a few hours to form clinker. This is due to the fact that alite (Ca3SiO5), the mineral component which provides concrete with its initial strength, needs to be heated to 1500°C during the clinker-forming process. The substitution of this function by belite (Ca3SiO4) would save on fuel, since this requires a lower temperature of 1200°C. Belite is even stronger than alite in fully cured concrete, although this takes up to months to reach maximum, creating the difficulty of weaker concrete for the intervening period, which may make it unfeasible for certain applications. Additives are being investigated, which may be able to speed up the curing time. A final problem is that belite requires more energy in processing, mainly grinding, so this offsets the energy saved in heating fuel.

Concrete consists of c. 14% cement, a serious greenhouse gas emitter
Concrete consists of c. 14% cement, a serious greenhouse gas emitter

Concrete consists of around 15-20% cement, the rest being variable proportions of fly ash, aggregate, and pozzolan. Normally, only the cement needs to be imported from remote suppliers, making the embodied energy (the energy budget for providing and using the materials, also known as grey energy) of concrete favourable in comparison to other materials, such as wood and steel. The embodied energy of concrete is typically 7% transportation and 70% for cement production. The admixture of fly ash reduces the cement production embodied (grey) energy account proportionally by 70% of the replaced weight.

Equation: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O) + 3Ca(OH)2. During curing, cement powder is hydrated to produce a cementiferous paste, or calcium-silicate hydrate, and calcium hydroxide.

Concrete has a natural tendency to absorb some CO2 from the atmosphere over time. And concrete can be made CO2 emission neutral by the addition dicalcium silicate during the curing phase, which absorbs CO2 and offsets up to 100% (or more) of the CO2 emissions from heating fuel. This could save the 400kg/m3 in the final concrete, which is the normal CO2 cost of the curing phase.

CO2 sequestration by concrete

Concrete can have negative CO2 emissions, absorbing the greenhouse gas from the air around it, if it is designed and applied correctly.

It has always been assumed that admixtures which change characteristics of concrete, such as fire retardation and adsorption/absorption, would be paid for by a loss in strength, hardness or elasticity. New research has revealed a technique through which concrete can be a net sink of CO2, and yet have excellent characteristics for a broad range of applications.

When organic matter is heated in the absence of oxygen, a process known as pyrolysis, it produces a substance which is around 70% pure carbon, but in a structure which is very porous, with an enormous surface area. Such a substance has the ability to draw molecules from the air and lock them more or less permanently onto its surface. This is is known as adsorption.

The activated carbon, or biochar, produced through pyrolysis has long been used in filters to remove pollutants from liquid and gas streams. However, no-one knew that when it is included in porous concrete, the air which comes into contact with it will be cleansed of its dangerous pollutants.

Concrete containing biochar can sequester so much CO2 and NOx, it provides a net sink for the pollutants. Employing concrete of this type not only reverses the environmental impacts of the manufacture of the cement itself, but can continue to cleanse the air of NOx and CO2 from other sources for decades. And the new concrete shows no loss in strength, durability or longevity. It also matches PP (polypropylene) as a fire resistant admixture, preventing spalling during intense heat episodes.

Something of the order of 5.5% to 6.5%1 of anthropogenic greenhouse gases are the result of cement production for concrete alone. Turning the over 4 billion tonnes of cement produced annually into a net sink, would save more than 3 billion tonnes of CO2.

A technique has been developed and tested successfully by:

IMM SA is in corporate member of UmweltScience, and more details about its research may be read on its member page: Corporate Homepage: umwelt.science/IMM.

A profile about the researcher Michel Di Tommaso is also available: Profile: Michel Di Tommaso.