Aggregate knowledge: Scientists at MIT discover how concrete behaves on a molecular level, could spur material advances

Architecture International Technology
Mixing cement can be a laborious task with just a spade (Chris / Flickr)

Mixing cement can be a brutal task with just a spade (Chris / Flickr)

Suffice to say, we certainly know how concrete behaves at structural level—the material has been dominating cities and skylines since Joseph Monier invented a reinforced concrete in 1889. But until now, how the material works on a microscopic level has eluded scientists.

Now, researchers at the Massachusetts Institute of Technology (MIT) have unearthed concrete’s molecular properties, claiming their findings could lead to structural advances in the future.

Not so solid concrete (IanVisits / Flickr)

Not so solid concrete (IanVisits / Flickr)

Traditionally, concrete uses a mixture of gravel, sand, cement, and water. In this case, a compound known as calcium-silicate-hydrate (CSH or cement hydrate) forms when the cement powder mixes with water. It essentially causes all the ingredients to solidify and become one.

The phenomenon that has been baffling researchers many for years is whether concrete’s molecular structure is comprised of continual bonds as found in stone and metal, or rather, as a sea of aggregate particle clumps bonded by (in the case of concrete) CSH.

(Washington State Dep. of Transportation / Flickr)

(Washington State Dep. of Transportation / Flickr)

Researchers from MIT discovered in 2012 that during the first hour of the concrete mixing process, when CSH particles form, the size at which they form is apparently random and “not in homogenous spheres.” As a result, such “diversity in the size of the nanoscale units leads to a denser, disorderly packing of the particles, which corresponds to stronger cement paste.”

However, the question regarding whether concrete was “considered a continuous matrix or an assembly of discrete particles” still remained. Predictably, the answer was “a bit of both.”

Louis Khan used concrete extensively in his design for the SALK institute (Jacqueline Poggi / Flickr)

Louis Khan used concrete extensively in his design for the SALK institute (Jacqueline Poggi / Flickr)

In a press release, Roland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering explained that the particle distribution facilitated almost every gap in the molecular structure to be filled by even smaller grains. This seemingly iterative process continued to the extent that Pellenq and his peers could approximate the material as a continuous solid.

“Those grains are in a very strong interaction at the mesoscale,” said Pollenq. “You can always find a smaller grain to fit in between [the larger grains, hence] you can see it as a continuous material.”

Pollenq did however, conclude his findings by stating that concrete could never be considered a true continuous material. This is due to the fact that grains within the CSH, unlike those in metal or stone, cannot reach a resting state of minimum energy. In other words, larger molecules can cause solid concrete to “creep” which makes the material susceptible to cracking and degradation over time. “Both views are correct, in some sense,” Pellenq concluded.

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