The clinker factor is the mass proportion of Portland cement clinker within a finished cement, expressed as a percentage or decimal fraction (e.g. 0.65 or 65%). Because clinker production is the main source of CO₂ emissions in cement manufacturing, the clinker factor is widely used as a proxy indicator for the carbon intensity of a given cement or concrete mix.
Calculation
The clinker factor is calculated as the mass of clinker divided by the total mass of the cement, including all supplementary cementitious materials (SCMs), calcium sulfate (gypsum/anhydrite), and minor additional constituents. It is typically stated on cement data sheets, declarations of performance, and Environmental Product Declarations (EPDs).
Clinker content by cement type (EN 197-1)
EN 197-1 defines common cement types by clinker content ranges:
- CEM I (Portland cement): 95–100% clinker
- CEM II (Portland-composite cement): 65–94% clinker
- CEM III (Blastfurnace cement): 5–64% clinker, with subtypes A (35–64%), B (20–34%), and C (5–19%)
- CEM IV (Pozzolanic cement): 45–89% clinker
- CEM V (Composite cement): 20–64% clinker
Lower clinker factors generally correspond to higher proportions of SCMs such as ground granulated blast-furnace slag (GGBS), fly ash, natural pozzolans, or limestone.
Relevance to CO₂ emissions
Clinker production involves the calcination of limestone, a chemical reaction releasing CO₂, combined with the fuel combustion required to reach kiln temperatures above 1400°C. Published industry figures place the emissions associated with clinker production at approximately 0.80–0.90 tonnes of CO₂ per tonne of clinker, depending on fuel mix and kiln efficiency. Since SCMs used to replace clinker typically carry a substantially lower embodied carbon footprint, reducing the clinker factor is one of the most direct levers for lowering the CO₂ intensity of cement and concrete.
Reduction strategies
Common approaches to lowering the clinker factor include substituting clinker with GGBS, fly ash, calcined clays, natural pozzolans, or limestone powder, as permitted by the relevant cement type. The achievable reduction depends on blend ratios and the availability of suitable by-products; GGBS and fly ash availability, for instance, is linked to steel and coal-fired power generation output and can vary regionally. Clinker factor reduction is also pursued through process changes such as increased use of alternative fuels in kilns, though these primarily affect fuel-related emissions rather than the clinker factor itself.
Coverage in this publication
The clinker factor has been addressed in the context of UK building regulations and sustainability measures in the article "Wienerberger UK Advances Sustainability Push in Response to UK Building Regulations", which notes that CEM III cement formulations substituting clinker with GGBS or fly ash can reduce the clinker factor — and thus CO₂ intensity — by 20–50% depending on blend ratios, while highlighting supply-chain constraints on the availability of these by-products in the UK.