Process emissions from calcination

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The reason cement is the hardest common material to decarbonise, expressed in one equation. Making clinker, the active component of Portland cement, requires heating limestone so that calcium carbonate decomposes: CaCO3 becomes CaO plus CO2. The carbon dioxide comes out of the rock. It is released whatever fuel heats the kiln, whether that fuel is coal, gas, waste, biomass, hydrogen or an electric arc, and it amounts to roughly half a tonne of carbon dioxide per tonne of clinker before any combustion emissions are counted — commonly around sixty per cent of the total for a modern plant, with fuel supplying the remainder. This is what a *process* emission means, and it is categorically different from the emissions the electricity transition has been solving. Zero-carbon electricity does not touch it. The genuine options are to capture the stream at the kiln, which is expensive but is at least a comparatively concentrated flue gas; to make less clinker per tonne of cement by substituting supplementary materials such as calcined clays, slag or fly ash, which works today and is limited by availability; or to change the chemistry of the binder itself, which is a materials research problem with a long qualification cycle in a conservative, standards-bound industry. Not a place.

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