While clinker makes less than 15% of the total mass of concrete, it is responsible for over 90% of the carbon footprint of concrete. For every tonne of clinker produced, nearly a tonne of CO2 is emitted. Clinker is made when limestone and clays are heated together in a kiln at 1,450oC. Only one third of the CO2 emitted comes from the energy to heat the kiln and other energy intensive processes. In other words, even if clinker was produced using entirely green energy, the carbon footprint of clinker would only reduce by one third.
The remaining two thirds comes from the chemical reaction when the limestone breaks down inside the kiln from CaCO3 to CaO (quicklime) and CO2. These are known as process emissions.
Carbon capture technology is widely promoted as the solution to process emissions. While it will have a role to play in the future, this technology is as yet unproven at scale and is unlikely to make a significant impact in the global cement industry before 2040. It will require dedicated infrastructure, enormous investment, and substantially increase the energy demand of cement manufacture. It cannot align the industry with a 1.5°C warming target.
If we are to achieve deep and rapid decarbonisation in the cement and concrete industries in the short term, major reductions in the amount of clinker used in cement are essential.