How much CO2 is embodied in concrete?
Well first of all you have to understand the typical mixture of concrete. Its 6% air (yes that’s right), 10% cement, 18% water, 25% sand, and 41% gravel. Its the manufacturing of the cement that consumes most of the energy. The other materials are mined, crushed, processed, stored, and delivered.
Concrete has anywhere between 7% and 15% cement by weight, depending on the performance requirements for the concrete – super strong or just normal. The average quantity of cement is around 250 kg/m3 (420 lb/yd3). One cubic meter (m3) of concrete weighs approximately 2400 kg (1 cubic yard weighs approximately 3800 lb). As a result, approximately 100 to 300 kg of CO2 is embodied for every cubic meter of concrete (170 to 500 lb per yd3) produced or approximately 5% to 13% of the weight of concrete produced, depending on the mix design.1
We picked the mid-point – 350 pounds per cubic yard (yd3). The size of the CO2 cube for 350 pounds is 15x15x15 cube. Or in terms of a typical room thats a 20 x 20 room, a good size bedroom or living room. In terms of how large of a concrete walk that produces, that’s about 16 running feet of a typical 4 foot wide walkway.
So a 16 foot stretch of sidewalk produces enough CO2 to fill a 20 x 20 room and the earth over the next several years is able to absorb about 40% of this emission, the rest just accumulates and builds-up in the atmosphere.
1Concrete CO2 Fact Sheet – National Ready Mixed Concrete Association – Marceau, Medgar L., Nisbet, Michael A., and VanGeem, Martha G., Life Cycle Inventory of Portland Cement Concrete, SN3011, Portland Cement Association, Skokie, IL, 2007, 121.