Cement and steel together dominate industrial carbon dioxide emissions, and each does so for a distinctive reason.

Cement. Its emissions are unusual because more than half do not come from burning fuel at all — they are released by the chemistry itself. Making clinker requires calcination of limestone:

\(\mathrm{CaCO_3} \xrightarrow{\ \sim 1450^{\circ}\mathrm{C}\ } \mathrm{CaO} + \mathrm{CO_2}\)

Every tonne of clinker liberates roughly 0.5 tonne of CO₂ from the rock, before any account is taken of the fuel needed to reach 1450°C. These process emissions are unavoidable by fuel switching, which is why cement is regarded as hard to decarbonise. Cement alone accounts for around 7-8% of global CO₂.

Steel. Here carbon is the reducing agent, not merely the fuel. In the blast furnace, coke reduces iron ore:

\(\mathrm{Fe_2O_3} + 3\mathrm{CO} \rightarrow 2\mathrm{Fe} + 3\mathrm{CO_2}\)

Producing a tonne of steel by this route emits roughly 1.8 tonnes of CO₂, and the industry contributes about 7-9% of the global total. Avoiding it needs a different chemistry altogether, such as hydrogen direct reduction.

The other industries are far smaller contributors. Paper and textiles are energy intensive in places but operate at much lower temperatures and much smaller tonnages, and pharmaceuticals are a high-value, low-volume sector whose absolute emissions are minor.

Hence the major post-industrial source is the cement and steel industry.