Cement’s Basic Molecular Structure Revealed: Robustness Comes From Messiness, Not a Clean Geometric Arrangement

By Denise Brehm
Civil & Environmental Engineering

In the 2,000 or so years since the Roman Empire employed a naturally occurring form of cement to build a vast system of concrete aqueducts and other large edifices, researchers have analyzed the molecular structure of many natural materials and created entirely new building materials such as steel, which has a well-documented crystalline structure at the atomic scale.

Oddly enough, the three-dimensional crystalline structure of cement hydrate — the paste that forms and quickly hardens when cement powder is mixed with water — eluded scientific attempts at decoding, despite the fact that concrete is the most prevalent man-made material on earth and the focus of a multibillion-dollar industry that is under pressure to clean up its act. The manufacture of cement is responsible for about 5 percent of all carbon dioxide emissions worldwide.

But an interdisciplinary MIT research team finally decoded the three-dimensional structure of the basic unit of cement hydrate in 2009, publishing their work in the Proceedings of the National Academy of Sciences.

Scientists had long believed that at the atomic level, cement hydrate (or calcium-silica-hydrate) closely resembles the rare mineral tobermorite, which has an ordered geometry consisting of layers of infinitely long chains of three-armed silica molecules (called silica tetrahedra) interspersed with neat layers of calcium oxide.

But the MIT team found that the calcium-silica-hydrate (C-S-H) in cement isn’t really a crystal. It’s a hybrid that shares some characteristics with crystalline structures and some with the amorphous structure of frozen liquids, such as glass or ice.

At the atomic scale, tobermorite and other minerals resemble the regular, layered geometric patterns of kilim rugs, with horizontal layers of triangles interspersed with layers of colored stripes. But a two-dimensional look at a unit of cement hydrate would show layers of triangles (the silica tetrahedra) with every third, sixth or ninth triangle turned up or down along the horizontal axis, reaching into the layer of calcium oxide above or below.

And it is in these messy areas — where breaks in the silica tetrahedra create small voids in the corresponding layers of calcium oxide — that water molecules attach, giving cement its robust quality. Those erstwhile “flaws” in the otherwise regular geometric structure provide some give to the building material at the atomic scale that transfers up to the macro scale. When under stress, the cement hydrate has the flexibility to stretch or compress just a little, rather than snapping.

“Now, we’ve finally been able to look inside to find cement’s fundamental signature. I call it the DNA of concrete,” said CEE Macomber Professor Franz-Josef Ulm. “Whereas water weakens a material like tobermorite or jennite, it strengthens the cement hydrate.”

Senior Research Scientist Roland Pellenq pinned down the exact chemical shape and structure of C-S-H using atomistic modeling and a Monte Carlo simulation. He first removed all water molecules from the basic unit of tobermorite, watched the geometry collapse, then returned the water molecules singly, then doubly and so on, removing them each time to allow the geometry to reshape as it would naturally. After he added the 104th water molecule, the correct atomic weight of C-S-H was reached, and Pellenq knew he had an accurate model for the geometric structure of the basic unit of cement hydrate.

The team, which also included Associate Professor Markus Buehler and graduate student Rouzbeh Shahsavari, then used that atomistic model to perform six tests that validated its accuracy.

“Now that we have a validated molecular model, we are working to manipulate the chemical structure to design concrete for strength and environmental qualities,” said Ulm.