Reduced / Sequestered
(To Implement Solution)
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.