Cement’s Magic Number

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.