After stone, concrete is the world’s most widely used construction material. Yet answers to some fundamental questions about the microscopic structure and behavior of this ubiquitous material have remained elusive.
As you know, 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?
As basic as that question is, it had never been definitively answered. In a paper published in the Proceedings of the National Academy of Sciences, a team of researchers at MIT, Georgetown University, and France’s CNRS (together with other universities in the U.S., France and U.K.) say they have solved that riddle and identified key factors in the structure of CSH that could help researchers work out better formulations for producing more durable concrete.
Roland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering, director of the MIT-CNRS lab hosted by the MIT Energy Initiative, and a co-author of the new paper, said 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 said, 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 noted.
One key question was whether the solidified CSH material, which is composed of particles of many different sizes, should be considered a continuous matrix or an assembly of discrete particles. The answer turned out to be that it is a bit of both — the particle distribution is such that almost every space between grains is filled by yet smaller grains, to the point that it does approximate a continuous solid. “Those grains are in a very strong interaction at the mesoscale,” Pellenq said.
“You can always find a smaller grain to fit in between” the larger grains, Pellenq said, and thus “you can see it as a continuous material.” But the grains within the CSH “are not able to get to equilibrium,” or a state of minimum energy, over length scales involving many grains, and this makes the material vulnerable to changes over time, he said. That can lead to “creep” of the solid concrete, and eventually cracking and degradation. “So both views are correct, in some sense,” he explained.
The analysis of the structure of hardened concrete found that pores of different sizes play important roles in determining the material’s characteristics. While smaller, nanoscale pores had been previously studied, mesoscale pores, ranging from 15 to 20 nanometers on up, had been more difficult to study and not well-characterized, Pellenq said. These pore spaces can play a major role in determining how susceptible the material is to water that can enter the material and cause cracking, eventually leading to structural failure. (This cracking, perhaps surprisingly, has nothing to do with the expansion of the water when it freezes, however).
According to Pellenq, the new mesoscale simulations are the first that can adequately match the sometimes conflicting and confusing results seen in experiments measuring the CSH texture.
The new simulations make it possible to match the values of key characteristics such as stiffness, elasticity and hardness, which are seen in real concrete samples. That shows that the modeling is useful, Pellenq explained, and might help guide research on developing improved formulas, for example ones that reduce the required amount of water in the initial mix with cement powder.