Riding in a mini-bus in Guatemala, Mary Gilbert and I were noticing that most of the construction used concrete blocks as building materials. I commented that was because many of the trees had already been cut down, and using concrete saved trees. Mary said the manufacture of cement had a big carbon footprint. I was upset because that meant concrete was not as good a as I had thought, so I investigated.
The manufacture of cement accounts for five per cent of global carbon dioxide (CO2) emissions. Although the two words are sometimes used interchangeably, cement is not concrete; rather, cement is the binding material used to make concrete when combined with sand and water. Concrete is the second most consumed substance on Earth after water, with three tons used per person per year (global population).
Cement is made by heating limestone and other clay materials in a kiln at 1,400 degrees C to form a “clinker,” which is ground and combined with gypsum to form cement. This process releases CO2 directly from the limestone which is converted to calcium oxide and CO2 when it is heated and indirectly from the fuel used to heat the kiln to such an extremely high temperature. Finally, the electricity to run the manufacturing plant and transportation account for up to 10 per cent of CO2 emissions from cement manufacture.
The good news is that because the manufacture of cement is so abundant on Earth, here is an opportunity to reduce greenhouse gas emissions. Reductions could take place several places in the process where. The fossil fuels used to heat the kiln could be changed to biomass or waste such as tires and other solid waste. By 2050, that change could reduce by 18 to 24 percent overall CO2 emissions from cement manufacturing.
Some analysts suggest that emissions could be reduced by increasing the efficiency of the kiln operation, but others say that the industry has already increased the efficiency of operation to the maximum. Another option is to reduce emissions by substituting other materials for some of the limestone, producing what is called, “blended cement,” but there are disadvantages. The coal ash or blast furnace slag used to substitute can have toxic heavy metals and the resulting cement takes longer to set.
Carbon emissions during the process can be captured by a process called “accelerated carbonation,” in which concrete is used a sink for CO2. CO2 is pumped into the concrete along with water and reacts with calcium hydroxide to form calcium carbonate, a stable, long-term CO2 storage.
There is an interdisciplinary research group at Massachusetts Institute of Technology (MIT) called the “MIT Concrete Sustainability Hub that “brings together leaders from academia, industry, and government to develop breakthroughs using a holistic approach that will achieve durable and sustainable homes, buildings, and infrastructure.” You can read more at cshub.mit.edu.
Because concrete is so ubiquitous in our lives, those involved in research and manufacturing have a major contribution to make to CO2 emission reductions. I hope they realize the importance of shepherding this industry into a low-carbon and then no-carbon future.
For more on statistics and process, see Emissions from the Cement Industry, by Madelein Rubenstein. In “State of the Planet, Earth Institute,” Columbia University (2012): blogs.ei.columbia.edu.