Concrete alone produces 5% of all greenhouse gas emissions
We often hear that 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.
Limestone is burnt at temperatures as high as 1500°C for a long period 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, 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 around 14% 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) 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 energy account proportionally by 70% of the replaced weight.
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.
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