Our Environment
FACTSHEET #1
The future of lower-carbon concrete is already here
Concrete is increasingly a low-carbon product, and can compete favourably against other building and construction materials on that and other criteria.
Concrete is increasingly a low-carbon product, and can compete favourably against other building and construction materials on that and other criteria.
While a Portland cement-based concrete used to have embodied carbon of 410 kg of CO2 per cubic metre of ready-mixed, that figure has dropped below 300 kg/m3, and down to 100 kg/m3 at the lower end. The figure drops further as exposed concrete absorbs CO2 from the air over time (see Factsheet #4 on carbon uptake), and in view of the 100+ year design life of concrete, and its amenability to recycling, reuse and repurposing. The New Zealand cement and concrete industry aims to achieve net zero carbon concrete by 2050, and this leaves open the possibility of carbon-negative concrete.
Concrete is increasingly a low-carbon product, and can compete favourably against other building and construction materials on that and other criteria.
While a Portland cement-based concrete used to have embodied carbon of 410 kg of CO2 per cubic metre of ready-mixed, that figure has dropped below 300 kg/m3, and down to 100 kg/m3 at the lower end. The figure drops further as exposed concrete absorbs CO2 from the air over time (see Factsheet #4 on carbon uptake), and in view of the 100+ year design life of concrete, and its amenability to recycling, reuse and repurposing. The New Zealand cement and concrete industry aims to achieve net zero carbon concrete by 2050, and this leaves open the possibility of carbon-negative concrete.
A big change in the concrete scene internationally and in Aotearoa New Zealand is the partial or total replacement of cement with other materials that do the same or a better job in terms of strength and other properties in concrete.
“Supplementary cementitious materials” or SCMs take various forms: “fly ash” (a residue from coal use in power stations), granulated blast furnace slag, and natural mineral deposits, such as volcanic ash or pumice, and diatomite, the accumulated skeletons over geological time of a type of plant plankton.
All SCMs have in common a high-silica content, and reactivity as part of a cement when water and aggregate are added to make concrete. Substitution rates for cement can range from 10 percent to 100 percent, depending on the SCM, having a significant impact on embodied carbon in the resulting concrete.
In New Zealand, cement companies are already producing lower-carbon cements, many captured in Environmental Product Declarations. When brought into a concrete design, the resulting concrete can take longer to cure and harden, however, can reach any required strength, and also produce an attractive surface finish.
Among the factors influencing the uptake of cement containing SCMs are cost and supply; however, the economics are changing as concrete customers increasingly demand, and regulators incentivise lower-carbon building and construction materials.
What does this mean for specifiers and designers, and regulators and customers of materials? It is not certain that timber products, for example, have a lower whole-of-life, or cradle-to-cradle carbon footprint than concrete. Both materials have their place, depending on situation, as a February 2021 article in The Structural Engineer has shown.
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