Carbon-Negative Cement

The world consumes 4 billion tons of cement each year — accounting for 7 to 8 percent of the global carbon emissions. Learn which solutions, old and new, are lessening cement’s climate impact.

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by Pixabay/Jason Goh
About half of all cement in the world goes to creating concrete, used in infrastructure.

Author’s note: This article draws heavily on the July 2021 Rocky Mountain Institute report “Profitably Decarbonizing Heavy Transport and Industrial Heat: Transporting these Harder-to-Abate Sectors is Not Uniquely Hard and Can Be Lucrative.” I salute Amory Lovins and his colleagues for this work and its rich insights.

For those of us concerned about the climate, cement is a big deal. But as this article attests, reforming the cement industry could result in fantastic carbon dioxide reductions.

The world consumes 4 billion tons of cement each year, comparable to global food production. In other words, humanity produces a lot of cement. Each ton produced releases 0.5 to 0.6 tons of carbon dioxide, or CO2. Cement’s CO2 emissions are on par with steel — they account for 7 to 8% of the global total. Like steel, this amount of CO2 production exceeds the emissions of every country in the world except the United States and China.

How is Cement Made, Anyway?

Cement is manufactured through a chemical combination of calcium, silicon, aluminum, iron, and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk, combined with shale, clay, blast furnace slag, silica sand, and iron ore.

There are four steps in its conventional manufacture:

  1. Crushing and grinding the raw materials.
  2. Blending the materials in correct proportions.
  3. Burning the prepared mix in kilns. This is where CO2 is released in a process where calcium carbonate is “calcinated” (its CO2 is burned off) and converted to lime, the primary component of cement.
  4. Grinding the burned product, known as “clinker”, and adding gypsum used to control the time of set of the cement.

Half of the cement manufactured each year is used to bind sand and gravel to make concrete. The rest makes mortar, plaster, and blocks. Of all the cement made and used, half ends up in buildings, the other half in roads and bridges and other infrastructure.

How Can We Lessen Cement’s Climate Impact?

Cement’s CO2 emissions are formidable. At least 70% of the CO2 that results from the production of concrete comes from the cement used, roughly 10% by weight. Simply optimizing the cement mix could save up to 50% of the CO2 associated with concrete infrastructure. This is possible because concrete’s cement content can vary up to threefold without affecting performance. So, using less cement in the manufacture of concrete reduces emissions dramatically.

Novel cement chemistries there are. Some manufacturers are cutting emissions in the manufacture of the cement. Changes in the composition of concrete, such as using fly-ash, can cut CO2 emissions by 52 to 70%. LaFargeHolcim sells a portfolio of 23 green solutions, including EcoPact, which offers 30 to 100% carbon reductions. Solidia makes a patchable cement that claims 50 to 70% reductions in CO2.

Carbon-Negative Cement Absorbs CO2

Other manufacturers are striving for net negative emissions — meaning developing cements that actually absorb CO2. A potential “gamechanger” in the works by Solidia is the use of carboxylates that could make cement production a carbon sink. This process cures the cement not with water or CO2, but with “carboxylates”, such as “oxalates.”

Bottom line: Instead of emitting 1.5 carbon atoms per calcium atom, cement could sequester two to four atoms gross, or 0.5 – 2.5 net, making cement a carbon sink. Another company, Carbicrete, is manufacturing “bio-cements” made at room temperature. Others are working on electrochemical cement-making processes.

Ways to Use Less Cement

Where else might there be gains in the carbon efficiency for cement? How about less infrastructure? For instance, permeable landscapes result in less demand for massive stormwater drain pipes. Modern manufacturing of building materials enables “airy bonelike” structures that use far less cement and steel.

Half of the CO2 solution may be rooted in the excessive way cement is specified and used. A study found that optimizing the European Union’s cement use could result in the use of 65% less cement.

Saving concrete means less steel rebar too, cutting steel use for reinforcement by up to 50%. The Freedom Tower in New York City cut its cement use by 40% using a concrete mix that provided six times the strength of conventional concrete. The 128-story Shanghai Tower cut its wind loads and their structural systems by 24% by twisting the building’s rectangular frame.

Natural composites, notably wood and bamboo that absorb CO2, can also provide worthy replacements for concrete and steel. Cross-laminated timber is making a carbon-negative contribution to buildings. Many designs that cut carbon display what Amory calls “revolutionary or rediscovered ancient design frugality.” There’s a plethora of solutions to dramatically reduce cement’s CO2 emissions.

Ted Flanigan runs EcoMotiona California-based company with the mission of the cost-effective greening of cities, corporations, and campuses. He has dedicated his career to finding win-win solutions that create financial and environmental benefits while fostering a sustainable society. Connect with Ted on Facebook and Twitter, listen to The NetPositive Podcastand read all of his MOTHER EARTH NEWS posts here.

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