Emerging Concrete Technologies
Concrete is one of the world’s most ubiquitous and oldest materials and the second most used substance after water. Unfortunately, it is also one of the most impactful materials to our environment. Typically made from Portland cement, coarse and fine aggregates, water, and a variety of admixtures, it is the Portland cement component in concrete that accounts for its large carbon footprint. Portland cement creation requires intense heat and also creates carbon dioxide as a natural part of the chemical reaction. The result is an extremely carbon-intensive process that accounts for 4.4 billion tons of carbon dioxide or 8% of the world’s total global carbon emissions and the distinction of being the world’s second-largest CO2 emitter.
The first step to reducing the carbon impact of concrete that should be done on every project, every time, is to only use the cement that is actually needed. Concrete specifications should be performance based and written to state what strength is needed when—and this may not always be 28 days. Considering longer cure times for elements such as foundations, columns, and shearwalls can also lead to lower cement mix designs. Additionally, cement may be reduced in some regions by specifying higher quality aggregate or using less water. Since cement production has been identified as being so impactful to the environment, the industry has been looking to cement alternatives to reduce concrete’s carbon impact. In fact, Project Drawdown, a comprehensive plan to reduce global warming, identified Cement Alternatives as strategy #36 and estimated a potential carbon savings of 440 million tons of carbon dioxide emissions annually if the strategy were implemented. Several well-known cement alternatives such as fly-ash (a byproduct of the coal industry) and ground-granulated blast furnace slag (a byproduct of the steel production industry) have been successfully used for decades and are quite commonplace in modern mix designs. In the short-term while other cement alternatives are being researched and introduced into the market, maximizing the industry’s use of readily available cement alternatives such as fly ash and slag is the most important step engineers can take to reduce concrete’s carbon impact.
Meanwhile, other cement alternatives are emerging to bring even more options to the market. Metakaolin is a pozzolan produced from the calcination of kaolin clay at much lower temperatures than Portland cement. However, metakaolin is expensive and only used to replace up to 10% of cement and thus has not been widely used. Research in this area did lead to the development of another cement alternative—Limestone Calcined Clay (LC3). LC3 is a new ternary blended cement comprised of Portland cement with calcined clay and limestone. Preliminary studies in Cuba have shown that LC3 is an extremely promising option to achieve lower C02 emissions, increase in supply capacity, higher return on investment, and potentially lower price on the construction market . Other possible emerging cement alternatives include recycled glass and volcanic ash.
Several other materials such as plastics, glass, foams, and paper have been proposed as aggregate substitutes in concrete, but most cannot be used without compromising strength and durability. Additionally, since 95% of the carbon impact of concrete is due to the cement, there is not much efficiency in focusing on aggregate substitution.
Another emerging technology in concrete production is to utilize carbon sequestration and injection. Technologies such as CarbonCure ®, CarbiCrete®, and Solidia® have been emerging in markets around the country. Walter P Moore recently specified the use of CarbonCure ® for a commercial office building development in Atlanta, Georgia. On this project our team was able to work with the concrete supplier for the drilled pier foundations to inject CO2 into the concrete mixture at the batch plant and reduce the cement content by 7%. This strategy when used in conjunction with cement alternatives saw a 55% reduction in the embodied carbon from a straight cement mixture.
One of the issues with any emerging technology is availability and acceptance in the market. However, consistent requests from specifiers for new materials may hasten their availability and ongoing education. Another concern is creating “franken-concrete” that may combine multiple new technologies and materials thus resulting in concerns about long-term durability and potential end of life issues. More research will be required in this area. In order to tackle embodied carbon, the carbon emissions from concrete must be considered as it can be found on every single project—even projects using a steel, wood, or alternate material for the structural system. It is ubiquitous and extremely impactful. In order to make a meaningful impact, most projects will need to take a multi-faceted approach by incorporating cement reduction, cement replacement, and a variety of new technologies. As concrete designers and specifiers we need to be nimble and willing to think outside the box and consider new technologies as they arise. We cannot make a dent in embodied carbon without reducing the embodied carbon in concrete.