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Materials

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.

Technical summaries for each solution will be available May 1, 2017.

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