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Cement’s Magic Number

Computational study reveals the optimum ratio of calcium to silicon for making cement, with the bonus of reducing CO2 emissions

by Stephen K. Ritter
October 6, 2014 | A version of this story appeared in Volume 92, Issue 40

Concrete is the world’s most-used construction material, but it comes with the drawback that its production accounts for about 10% of global industrial CO2 emissions. An international research team sponsored in part by the cement industry and led by MIT’s Roland J.-M. Pellenq has used a computational molecular model for evaluating cement hydrate—the CaO-SiO2-H2O mineral binding phase in concrete—to help overcome the problem. The researchers found that adjusting concrete’s calcium-silicon ratio downward leads to a stronger, more durable material that could reduce concrete demand and cut CO2 emissions by half (Nat. Commun. 2014, DOI: 10.1038/ncomms5960). Cement to make concrete is typically prepared by cooking limestone (CaCO3) with silica-rich clay at 1,500 °C. During the process, copious amounts of CO2 are liberated. The Ca/Si value of 1.7 has been adopted as an industry standard. However, the molecular structures of cement with different ratios had never been compared in detail, Pellenq explains. The researchers studied cement formulations with Ca/Si values from 1.1 to 2.1 and compared the results with experimental samples, finding that the optimum levels of stiffness and hardness with the best mechanical resistance to fracture occur at 1.5. With less limestone needed to make concrete, and with less concrete needed, CO2 emissions would be lower, the researchers note.

Molecular model of cement hydrate, which is the CaO-SiO2-H2O mineral binding phase in concrete, with varying Ca:Si ratios: 1.1, 1.5, and 1.8, left to right.
Credit: Nat. Commun.
Molecular models of cement hydrate, which is the CaO-SiO2-H2O mineral binding phase in concrete, with Ca/Si values of 1.1, 1.5, and 1.8 (from left).


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