As the World’s Largest Single Source of Carbon Dioxide, Can Cement Ever be Green?

There are few things as impactful to the modern world as concrete. Usage of different types of cement and concrete predates the Roman Empire. Roads, buildings, bridges and more would not be possible as we know it without concrete. Modern civilization as we know cannot exist without it.

However, there are few things in the world with as much negative environmental impact. For starters, nearly all applications of concrete deliberately disrupt the environment — namely, in roads and buildings that require the destruction of trees, grass and natural habitats. Even if we disregarded this fact, the production of cement is still one of the single highest sources of CO2 in the world. It is often estimated that 5 percent of the global CO2 production comes from the manufacture of cement.

(Vocabulary Time: “cement” is the binding substance that makes “concrete.” Concrete also includes a mixture of coarse and fine aggregates, such as gravel and sand respectively.)

This is a problem that the construction industry and the manufacturers that supply to it have been well aware of. As we reported last year, green building products are all the rage. One study estimated that more eco-friendly building products would see a 30 percent increase throughout 2012, and continue to grow at an average of 7.3 percent per year for the next five years. This includes sustainable replacements for plywood, LED lighting, and more.

But cement — Portland cement, in particular, a binder made from limestone used in most concrete — remains a dirty thorn in the side of the construction industry. Every 10 pounds of cement produces approximately 9 pounds of CO2. A single cubic yard of concrete produces the same amount of CO2 as burning 16 gallons of gasoline in your car.

The bulk of the CO2 emissions come from the manufacture of cement. Portland cement is made by de-carbonating limestone by heating it to 2,300 degrees F. Cement manufacturers estimate that 50 to 60 percent of the CO2 from cement comes from the de-carbonation. Fossil fuels required to power the kilns contribute another 20 to 30 percent. Transportation of product contributes the remaining amount. It should be noted that the energy usage of the plant is rarely calculated, as this varies too much from facility to facility. Nor do estimates include emissions from digging up and transporting limestone.

The amount of CO2 produced by the cement industry alone is so vast that a Massachusetts Institute of Technology (MIT) study estimates that a 10 percent reduction is global cement CO2 emissions would contribute one-fifth of the Kyoto Protocol’s goal of a 5.2 percent reduction in total carbon emissions. The industry is taking steps to help make that happen.

Some of the oldest and best known solutions are geopolymers. These are cements made by substituting Portland cement with industrial waste, such as fly ash (mineral residue left over from the combustion of coal) and slag (a byproduct of melting ore to purify metals). It’s commonly believed that this is how the Romans and other early civilizations made concrete. Most modern cement producers sell geopolymers in a mixture with traditional Portland cement. Ratios can be anywhere from 25 to 80 percent depending on the company and the application.

In Nova Scotia, the company CarbonCure has begun to apply a process known as STEP (Solar Thermal Electrochemical Photo) to ionize CO2 emitted during the conversion of limestone to cement, separating it into solid graphite and oxygen. At higher temperatures, it can create carbon monoxide, which can be further synthesized to create jet fuel or diesel fuels.

But other companies are going much further, with several investigating production methods that don’t just reduce carbon emissions, but actually capture carbon, sequestering it within the concrete.

Magnesium oxide used to create a carbon-negative cement. Credit: Novacem.

Novacem, based in London, has developed a process for making a replacement for Portland cement using magnesium oxide and hydrated magnesium carbonates. The process requires less heat (up to 1,300 degrees F). The CO2 emitted from the heat is captured and undergoes its own process to become magnesium carbonates. The creation of the carbonates actually requires more CO2 than is produced from the overall production, which means additional CO2 must be absorbed from the air as well as from other industrial sources (such as nearby factories). In total, Novacem estimates that every ton of cement produced absorbs a little over 200 pounds more CO2 than it emits, making it carbon negative. California-based Calera uses a similar process to create bicarbonate materials, which can be combined with calcium in the form of hard water or calcium chloride to create cement.

This absorption of carbon by calcium and magnesium is similar to a process that takes place in nature. The process whereby marine organisms make shells involves absorbing calcium and magnesium ions from the ocean and carbon dioxide from the air. Ironically, when these organisms die, the shells disintegrate into carbonate sediments such as limestone.

Novacem and Calera are both startups. The commercial, large-scale viability of their products has yet to be proven. In addition, there are plenty of skeptics who doubt whether any substitute for Portland cement will perform comparably and at a competitive price.

But there’s a bigger question floating around the construction and cement industries: Is all this even necessary? Studies have shown that dried concrete has the potential, over time, to absorb CO2 from the air without any additional help from people. The process of carbonation, studies have found, takes places even if concrete is not exposed to the atmosphere (i.e. underground structures).

But the problem, as the Portland Cement Association (PCA) notes in a report on its website, is that no one is sure just how much CO2 is absorbed. One experiment conducted in Norway in 2005 concluded that up to 57 percent of the CO2 emitted during creation of the cement will be absorbed by the concrete — over the course of 100 years, that is.

All we know for sure is that cement is a $170 billion industry globally, and the amount of CO2 released in the creation of cement has created a golden opportunity for green-minded companies who are confident they can unseat Portland cement as the staple of the industry.

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  • Michael H. Weber
    January 23, 2013

    Dear Christian,

    I’ve been in the concrete and/or cement industry since 1981 and have seen a lot of advances when it comes to sustainability. There are cements are made of 95% recycled fly ash and 5% renewable liquid activators. For every ton of portland cement that is displaced, one ton of CO2 is eliminated from being released into the atmosphere.

    I believe some of the other “green” technologies you have mentioned in the article are still in the “research and development” phase and/or out of business. These new cements are the first commercialized zero carbon cements and are currently available in the United States to concrete producers.


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