The production of cement-clinker is an energy and carbon emissions intensive process.

The main component of clinker (the semi-finished product that is ground to obtain cement) is calcium oxide.
In nature, this element is found linked with CO₂ in the form of calcium carbonate (usually as limestone) which makes up about 78% of the raw materials used to produce clinker.

60% of the direct CO₂ emissions from cement manufacturing are generated by the limestone decarbonation, while 40% come from the combustion of fossil fuels used to reach the temperatures required for decarbonation to take place.

The most widely used cement type, Portland cement, is 95% cement clinker. However there is a range of other types of cement (called blended cements) which contain a variety of clinker substitutes to reduce the amount of clinker used, and therefore reduce carbon emissions per tonne of product.