Key ideas behind C3:
- Offsets occur at the point SCMs are substituted for portland cement during concrete mixing.
- For every metric ton of portland cement produced, 0.81 metric tons of CO2 is produced.
- While portland cement reductions are dependent on SCM use, offsets generated are linked to portland cement not used rather than SCMs used.
Concrete is one of the most widely used construction materials in the world – global consumption is currently estimated at 13.5 billion cubic yards (cy)1. Concrete’s plasticity, or the ability to be shaped before it hardens, is derived from cement. Depending on the strength of concrete, cement usually accounts for 10%–15% of a concrete mix. Even though the amount of cement in concrete is relatively small, it represents approximately 96%2 of the total CO2 emissions associated with concrete production.
The core concept of the C3 Program is that by reducing the amount of portland cement used in the production of concrete, CO2 emissions are reduced and offsets are created. Due to the magnitude of the concrete industry and the significant CO2 emissions associated with portland cement use in concrete, the Program can significantly reduce associated CO2 emissions.
Under the Program, offsets are created at the point SCMs are substituted for portland cement during concrete mixing. C3 attributes offset ownership to the party carrying out the replacement of portland cement with SCMs. Both ready mixed concrete manufacturers and concrete products manufacturers who purchase portland cement and SCMs from suppliers and blend their own cement during concrete production are eligible for participation.
CO2 Emissions During Portland Cement Production
Portland cement is produced through an energy intensive process using cement kilns, resulting in CO2 emissions. According to the Energy Information Administration, cement manufacturing is the largest source of CO2 emissions in the U.S. after fossil fuel consumption, and contributes just over 6% of total U.S. GHG emissions3. Approximately half of cement kiln emissions are from fossil fuels used for crushing and heating raw materials and the other half are the result of calcination, a chemical reaction.
The product emerging from cement kilns, portland cement clinker, typically comprises at least 90% of modern portland cement. For every metric ton of portland cement clinker produced, approximately 0.90 mt of CO2 are emitted (USGS)4. Therefore, for each ton of portland cement produced, 0.90 * 0.90 = 0.81 mt of CO2 are emitted. This value, known as the cement emissions factor, is used when determining a participant’s baseline and actual emissions for the Program. See sample Offset Calculations for how emissions are determined.
SCMs eligible under C3:
- Ground Granulated Blast Furnace Slag (‘slag’, ‘GGBFS’), ASTM C989 Grade 100 or 120
- Fly ash, ASTM C618
- Silica fume, ASTM C1240
- Rice hull ash
- Cement Kiln Dust (CKD)
These five SCMs qualify under C3 because they are byproducts of other processes and are therefore not associated with an increase in GHG emissions (except in the case of GGBFS, where associated emissions have been accounted for in Program calculations).
Why SCM Concrete?
With sound technical advice based on the specific project and location, a portion of standard portland cement (ASTM C150) in a concrete mix may be replaced with other similarly functioning materials – SCMs. The most commonly used SCMs are ground granulated blast furnace slag (GGBFS) and fly ash, byproducts of blast furnace steel production and coal burning in power plants, respectively. When these byproduct materials are used to decrease the amount of portland cement in a concrete mix, a concrete producer is effectively avoiding CO2 emissions, thereby creating offsets.
Because SCMs may also be used as a strength additive or a fine aggregate, the amount of SCMs used does not necessarily equal portland cement not used. Therefore, offsets generated are calculated based on portland cement not used rather than SCMs used.
- USGS 2009 http://minerals.usgs.gov/minerals/pubs/commodity/cement/mcs-2009-cemen.pdf
- Marceau, Medgar, Michael Nisbet, Martha VanGeem, Life Cycle Inventory of Portland Cement Concrete, Skokie IL: Portland Cement Association, 2007, p16.
- EPA http://www.eia.doe.gov/oiaf/1605/ggrpt/carbon.html
- USGS 2007 http://minerals.usgs.gov/minerals/pubs/commodity/cement/myb1-2007-cemen.pdf