In modern cities, there's concrete at every turn. So it might surprise you to hear that, until now at least, we haven't really understood how it works at the microscopic level — despite the fact that we trust it to build huge structures. First, a recipe. Concrete's made up of water, gravel, sand and cement powder. As the mixtures dries out, a compound called cement hydrate (CSH) glues the whole lot together. But what's not been known until now is how it does that: Is the result a continuous material, like metal or stone, or more like an aggregate of particles that just fit together very snugly?
That's a question that researchers from MIT's department of civil and environmental engineering have been trying to figure out. They have discovered that the answer, perhaps predictably, is best summed up as "a bit of both".
While the material is made up of discrete particles, their size distribution turns out to be such that every single gap is filled up by smaller particles. "You can always find a smaller grain to fit in between," explains Roland Pellenq, one of the researchers, in a press release. In other words, no matter what size the gap you look at, there will be a cement hydrate particle filling the space. So, in effect, it can be considered as a continuous lump of material.
But there's a 'but'. A material made up like that can never be a true continuous material, because it can't settle into a permanent state of minimum energy: There's always a way that it can jiggle about to re-order itself into a potentially more perfect structure. That's unlike, say, a metal, whose crystal structure follows physical laws with precision. It's this phenomenon that can cause concrete to creep over time, according to the researchers. The results are published in the Proceedings of the National Academy of Sciences.
The team hopes that the finding will allow it to investigate new ways to make concrete better tuned to the design requirements for a given job: harder, more elastic, stronger or more durable. And for the rest of us, we can at least rest easy, knowing that experts now understand how concrete behaves at the micro — as well as the macro — level.