Portland Cement
One kilogram of ordinary portland cement represents approximately:
What this means
Ordinary portland cement (OPC, ASTM Type I or Type I/II) is the binder in most concrete. About half of its lifecycle GHG comes from a chemical reaction that is intrinsic to the material — not from fuel choice or plant efficiency. When limestone (CaCO₃) is heated to make clinker, it releases roughly 44% of its mass as CO₂. That process emission cannot be eliminated by switching fuels; it is a property of the chemistry. For a different industrial-process example, see diesel.
Formal measurement basis
| Item measured | Ordinary portland cement (ASTM Type I or Type I/II) |
|---|---|
| Formal functional unit | 1 kg portland cement produced |
| Reader-facing unit | One 94-lb bag of portland cement, approximately 42.6 kg |
| Primary boundary | Cradle-to-gate: quarrying, raw meal preparation, clinker production, grinding, and cement packaging at plant gate |
| Secondary boundary | Clinker-only, calcination-only, or fuel-combustion-only where a narrower boundary is stated |
| Source review | Version 2 source review, 2026 |
Full measurement table
| Physical quantity | Working value | Literature range | Unit | Boundary note |
|---|---|---|---|---|
| Cradle-to-gate GHG | ~0.92 | ~0.78–0.95 | kg CO₂e / kg cement | U.S. industry-average portland cement, PCA/Athena EPD 2023; GWP100 basis |
| Cradle-to-gate GHG, 94-lb bag reader unit | ~39 | ~33–40 | kg CO₂e / 94-lb bag | Reader-facing conversion, 42.6 kg per bag |
| Calcination CO₂ (process only) | ~0.44 | 0.51 per kg clinker, scaled by clinker ratio | kg CO₂ / kg cement | Limestone decarbonization: CaCO₃ → CaO + CO₂. IPCC default 0.510 t CO₂/t clinker at ~0.87 clinker-to-cement ratio for OPC |
| Fuel combustion + upstream | ~0.36 | Range depends on kiln fuel mix | kg CO₂e / kg cement | Cradle-to-gate GHG minus calcination CO₂; captures kiln fuel, electricity, upstream |
| Total primary energy | ~3.5 | ~3.0–4.5 | MJ / kg cement | Includes thermal energy for kiln plus electrical for grinding; varies with kiln generation and alternative fuel share |
| Freshwater consumption at plant | ~0.30 | ~0.19–0.37 | L / kg cement | Direct plant water; range from major producer disclosures (Holcim ~0.19, Lafarge ~0.31, Cemex ~0.37 L/kg). Excludes upstream fuel-cycle water |
| Particulate matter (PM10) | Not selected | Plant-controls-dependent | varies | Modern kilns with baghouses or electrostatic precipitators emit <0.5 kg PM/tonne; unabated kilns much higher. Not a property of the material |
The two-thirds/one-third pattern
Cement CO₂ is often summarized as "roughly two-thirds from calcination and one-third from fuel." That ratio is measured per tonne of clinker with combustion-only boundary. On a cradle-to-gate per-tonne-of-cement basis — which is what a specifier or buyer typically encounters — the calcination share is closer to 48% because the cement also contains non-clinker components (limestone filler, gypsum, supplementary materials) and because electricity and upstream fuel add to the combustion side. Both framings are correct under their stated boundary; they measure different things.
What is included
The cradle-to-gate boundary covers limestone and clay quarrying, transport to plant, raw meal preparation, calcination in the rotary kiln, cooling, addition of gypsum and any allowed supplementary materials, cement grinding, and packaging at the plant gate.
What is excluded
- Concrete production (mixing cement with aggregates and water)
- Transport of cement from plant to concrete batching or construction site
- Construction, use-phase, and end-of-life effects of finished concrete
- Carbonation uptake (concrete slowly reabsorbs CO₂ during service; not credited here)
- Portland-limestone cement (Type IL), lower-clinker blended cements, and geopolymer alternatives — each has its own boundary and lower typical GWP
- Dollar conversion of climate or air-quality effects
Why values vary
Cement GHG varies with clinker-to-cement ratio (blended cements substitute limestone, slag, or fly ash for a portion of clinker), kiln fuel mix (coal, petcoke, natural gas, alternative fuels), kiln technology (dry preheater-precalciner vs. long dry vs. wet), electricity source, and geographic scope. Water consumption varies with plant cooling design and local water availability. Calcination CO₂ is the most stable quantity because it follows directly from the chemistry.
Source notes
The cradle-to-gate GHG working value uses the U.S. industry-average from the 2023 PCA industry-wide EPD prepared by Athena Sustainable Materials Institute. The calcination CO₂ value applies the IPCC Tier 1 default emission factor for clinker (0.510 t CO₂/t clinker) at a typical U.S. clinker-to-cement ratio of approximately 0.87 for Type I/II OPC. Water consumption is a range from three major producers' public disclosures rather than a single working value; a single hardened U.S. industry figure would improve the row. Primary energy comes from published cement LCA compilations and is presented as a range because kiln technology and alternative-fuel share drive most of the variance.
Sources
- Portland Cement Association / Athena Sustainable Materials Institute, Environmental Product Declaration: Portland Cement, industry-average, revised November 2023. Used for: cradle-to-gate GHG working value of approximately 922 kg CO₂e per metric ton of portland cement.
- IPCC, 2006 Guidelines for National Greenhouse Gas Inventories, Volume 3, Chapter 2: Mineral Industry Emissions. Used for: Tier 1 default calcination emission factor of 0.510 tonne CO₂ per tonne clinker.
- World Business Council for Sustainable Development, Cement CO₂ and Energy Protocol. Used for: clinker-to-cement ratio methodology and process-vs-combustion accounting framework.
- Global Cement, industry water-intensity reporting summary. Used for: water consumption range from Holcim, Lafarge, and Cemex public disclosures (approximately 0.19 to 0.37 L per kg cement).