Cement's carbon footprint has two independent sources. Process emissions come from calcination — heating limestone (CaCO₃) drives off CO₂ to form clinker's reactive lime (CaO), roughly 0.525 t CO₂ per tonne of clinker, fixed by the chemistry itself. Combustion emissions come from the fuel burned to reach kiln temperature (~3.3 GJ per tonne of clinker), and depend entirely on what's burned.
process = 0.525 × clinkerRatio
combust = 3.3GJ × EF(altFuel%) × clinkerRatio
gross = process + combust
net = gross × (1 − captureRate)
- Clinker-to-cement ratio — blending clinker with ground limestone, fly ash or slag (supplementary cementitious materials) dilutes both the process and combustion emissions per tonne of finished cement, since less clinker needs making at all.
- Alternative-fuel substitution — replacing coal/petcoke with biomass or waste-derived fuel in the kiln lowers the fuel emission factor; biogenic carbon is treated as near-zero net.
- CCUS capture rate — post- or oxy-combustion capture on the kiln flue gas removes a fraction of whatever CO₂ is still produced, gross of the first two levers, and routes it to storage instead of the stack.
The three levers compound: cutting clinker content shrinks the pie, cleaner fuel shrinks one slice of what remains, and CCUS mops up a share of whatever is left. Reaching near-zero net cement typically needs all three working together — no single lever gets there alone.