Cement is one of the most used substances on earth after water — and the source of 5 to 6% of global CO₂ emissions. While cement makes up only about 10% of concrete’s volume, it accounts for roughly 95% of its emissions.
Producing Portland clinker splits limestone into calcium oxide and CO₂ at high temperatures; that decarbonisation alone causes about 60% of cement’s emissions. The rest comes from energy use — which is why clinker substitution is the leverage point.