Cement LCA: clinker ratio, supplementary materials and carbon capture

Cement is responsible for about 7-8% of global CO₂ emissions. The bulk comes from clinker production — calcining limestone (CaCO₃) at high temperature triggers the reaction CaO + CO₂. This "process emission" differs from fuel emissions — it cannot be reduced via energy efficiency because it is purely chemical. Cement LCA therefore revolves around the clinker ratio.

CEM types and clinker ratios

• CEM I — pure Portland; clinker ratio 95-100%; highest GWP (~850-900 kg CO₂eq/ton)
• CEM II — Portland blend; clinker 65-94%, with limestone or slag substitution; GWP drops 10-20%
• CEM III — blast-furnace slag cement; clinker 20-64%; GWP drops 30-50%
• CEM IV — pozzolanic cement; clinker 45-89%; blended with fly ash; medium GWP
• CEM V — composite cement; clinker 20-64%; combines slag + fly ash + limestone; lowest GWP

SCM (Supplementary Cementitious Materials)

SCMs — fly ash, blast-furnace slag, ground limestone, silica fume — substitute for clinker and cut cement GWP significantly. Allocation is critical: fly ash is a coal-plant by-product; should we treat it as waste (no burden) or as an economic co-product (partial burden)? EN 16908 defaults to cut-off — fly ash enters A1 burden-free and only bears transport burden in A2. Same for slag. This is why SCM-rich cements show dramatically lower GWP.

Alternative fuels

Cement kilns traditionally burn coal; modern plants blend in alternative fuels (RDF — refuse-derived fuel, waste tyres, biomass) up to 30-60%. Alternative fuel reduces fuel-based emissions by 15-25% but cannot affect process emissions (calcination). With biomass and a carbon-neutral assumption, GWP-fossil falls further (biogenic carbon shifts to GWP-biogenic). EN 16908 requires documenting alternative-fuel rate and calorific content.

CCUS — carbon capture scenarios

The cement industry has begun investing in CCUS (Carbon Capture, Utilisation and Storage) as the only route to eliminating calcination emissions. HeidelbergCement's Brevik plant in Norway became the world's first industrial-scale carbon-capture cement facility (as of 2024). In LCA modelling, CCUS is added as an A3 sub-process; the capture-transport-storage energy burden is accounted for, and net emissions improve -50 to -85%. Widespread adoption could halve sector emissions by 2030+.

Connection to concrete

Cement can be a standalone EPD product, but its real use is inside concrete. A cement EPD is published per ton (declared unit); the concrete producer takes that EPD and scales it into a m³-of-concrete modulary table. clca chains this automatically — when the cement model is updated, the concrete products that use it recalculate automatically.