From MIT News:

Concrete is the world’s most widely used construction material, so abundant that its production is one of the leading sources of greenhouse gas emissions. Yet answers to some fundamental questions about the microscopic structure and behavior of this ubiquitous material have remained elusive.  

Concrete forms through the solidification of a mixture of water, gravel, sand, and cement powder. Is the resulting glue material (known as cement hydrate, CSH) a continuous solid, like metal or stone, or is it an aggregate of small particles...

... Roland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering, director of the MIT-CNRS lab 2 hosted by the MIT Energy Initiative, and a co-author of the new paper, says the work builds on previous research he conducted with others at the Concrete Sustainability Hub (CSHub) through a collaboration between MIT and the CNRS. “We did the first atomic-scale model” of the structure of concrete, he says, but questions still remained about the larger, mesoscale structure, on scales of a few hundred nanometers. The new work addresses some of those remaining uncertainties, he says. (full article)