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Alternative Cement

The Pantheon was a Roman temple commissioned during the consulship of Marcus Agrippa 2,000 years ago and completed by the emperor Hadrian about 128 AD. After nearly two millennia, the dome remains the largest unreinforced concrete dome in the world. What is more remarkable is that the concrete remains intact, strong and almost ageless. Standing in what is now a church, the oculus at the center of the dome rises 142 feet. Six million people visit it every year.

Cement is a vital source of strength in infrastructure, second only to water as one of the most used substances in the world. It is also a source of emissions, generating 5 to 6 percent annually.

To produce Portland cement, the most common form, a mixture of crushed limestone and aluminosilicate clay is roasted in a kiln. At high heat, limestone’s calcium carbonate splits into calcium oxide (the desired lime content) and carbon dioxide (the waste). Decarbonizing limestone causes roughly 60 percent of cement’s emissions. The rest result from energy use.

To reduce emissions from the decarbonization process, the crucial strategy is to change the composition of cement. Conventional clinker can be partially substituted for alternative materials that include volcanic ash, certain clays, finely ground limestone, ground bottle glass, and industrial waste products—namely blast furnace slag (from manufacturing iron) and fly ash (from burning coal). These materials leapfrog the most carbon-emitting, energy-intensive step in the cement production process.  

The average global rate of clinker substitution could realistically reach 40 percent and avoid up to 440 million tons of carbon dioxide emissions annually. Standards and product scales will be key for realizing the opportunity of alternative cements.


Pantheon temple in Rome: Moore, David. The Roman Pantheon: The Triumph of Concrete. Mangilao, Guam: MARC/CCEOP, University of Guam Station, 1995.

cement one of the most used substances: Scrivener, Karen L., Vanderley M. John, and Ellis M. Gartner. Eco-Efficient Cements: Potential, Economically Viable Solutions for a Low-CO2, Cement-Based Materials Industry. Nairobi: United Nations Environment Programme, 2016.

Decarbonizing limestone…emissions: WBCSD and IEA. Cement Technology Roadmap 2009. World Business Council for Sustainable Development & International Energy Agency, 2009; Amato, Ivan. “Concrete Solutions.” Nature 494, no. 7437 (2013): 300-301. 

4.6 billion tons of cement: USGS. Mineral Commodity Summaries 2015. Reston, VA: U.S. Geological Survey, 2015.

5 to 6 percent of society’s…emissions: Amato, “Green Cement”; Scrivener et al, Eco-Efficient Cements.

blast furnace slag [and] fly ash: Scrivener et al, Eco-Efficient Cements.

clinker substitution…avoid…emissions: Scrivener et al, Eco-Efficient Cements.

strength can…be higher: Amato, “Green Cement”; Crow, James Mitchell. “The Concrete Conundrum.” Chemistry World, March 2008: 62-66; WBCSD and IEA, Cement.

European Union reuses…fly ash: Moon, Steven T. “Regulatory and Legal Application: Fly Ash Use in Cement and Cementatious Products.” World of Coal Ash (WOCA) Conference, Lexington, KY, April 22-25, 2013.

New York City…ground bottle glass: Ellen Macarthur Foundation. The Circular Economy and the Promise of Glass in Concrete. Isle of Wight, UK: Ellen Macarthur Foundation, 2016.

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