Cement is denser than it’s cracked up to be

When chemically modified with water in a process called hydration, cement morphs into the durable binder that holds gravel, sand, and other additives together to form concrete—the most used manmade material in the world. The main constituent of hydrated cement is CaO-SiO₂-H₂O (called C-S-H) in the form of nanoscale colloidal aggregates, the size, shape, and packing of which are crucial to the ultimate strength and stability of concrete. The solid C-S-H nanoparticles are generally thought to be analogous to the claylike minerals tobermorite and jennite, mixed with calcium hydroxide. But new neutron-scattering studies by Jeffrey Thomas and Hamlin Jennings of Northwestern University and Andrew Allen of NIST in Gaithersburg, Maryland, show that C-S-H has a higher-than-expected atomic packing density. The mass density of solid C-S-H is roughly 10% higher than that of a mixture of its widely used mineral analogues with the same composition. The result has important implications for the modeling of cement paste. (See, for example, PHYSICS TODAY, November 2009, page 23 , where that model’s starting point is dry tobermorite.) The researchers also investigated the composition and density of C-S-H cured at elevated temperatures and with various additives. In particular, they found that curing the cement at 80 °C led to a lower atomic packing density. Such atomic packing variations suggest the possibility to control chemical shrinkage and the associated cracking of concrete. (J. J. Thomas, H. M. Jennings, A. J. Allen, J. Phys. Chem. C , in press. --Stephen G. Benka