Q: I’m a designer who’s concerned about carbon emissions. Are alternative cements more environmentally-friendly than portland cement?
A: It depends. The best way to find out if an alternative cement produces less environmental impact than portland cement is to do a life cycle assessment (LCA). An LCA includes the environmental impacts from raw material extraction through material processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling.
For example, an alternative cement may require less energy but have a higher total CO2 impact than portland cement. This could be due to higher maintenance requirements of a structure made with alternative cements, a shorter service life, or less reabsorption of CO2 over the structure’s lifetime.
Q: Shouldn’t we try to move away from using portland cement because of the high CO2 emissions it produces?
A: Many companies are exploring alternative cements. However, I don’t see the construction industry moving away from portland cement as the first choice for most projects.
It’s true that the cement industry is responsible for almost 8% of the manmade CO2 in the world. Portland cement is manufactured at a very high temperature, using a lot of energy. That and the breakdown of the limestone (CaCO3) release a lot of CO2. But guess what? Manufacturing some alternative cements can also require high temperatures and emit as much CO2 as portland cement—or more.
Second, a lot of alternative cements require additional chemicals to activate them in concrete. These chemicals can be very harmful to the environment and potentially harmful to those who are using them.
Further, some of the chemicals and materials for alternative cements aren’t widely available globally. Transportation of these materials can offset the benefits of using the alternative cements.
Finally, even if an LCA shows that an alternative cement produces less CO2 than portland cement, it may not be as economical to manufacture. It is unrealistic to expect that owners and contractors, who are very price-conscious, will pay two or more times the typical cost per yard of concrete in comparison to portland cement.
In fact, over the last few decades the use of portland cement has increased because emerging countries can afford it.
Q: So, is there any good news about alternative cements and sustainability?
A: Yes. Several alternative cements produce less environmental impact and have comparable service life and economics to portland cement. However, these materials may not be commercially available or may not meet current building codes. At Beton, we love exploring the use of alternative cements with clients. And we can help them bring new materials to market. New standards are being developed to make this easier. In addition, building codes that incorporate CALGreen encourage more use of alternative cements.
I believe that the concrete industry can do more to lower the environmental impacts of portland cement concrete. Both designers and the concrete industry need to move from prescriptive- to performance-based concrete mixture design. Performance-based design allows more freedom to lower the portland cement content. Designers can use supplementary cementitious materials, more efficient aggregate grading, or recycled concrete aggregate. And they can reduce water demand through the use of admixtures to create durable and environmentally-friendly concrete.