The modern world is likely home to more concrete jungles than natural ones, and although we've been using the material for hundreds of years, the recipe can always use some improvement. Researchers at Rice University have found a way to "program" cement particles into specific shapes in order to make concrete that's stronger, less porous, and more environmentally friendly.

It may not seem like the most exciting stuff in the world, but concrete has had some pretty intriguing advances over the last few years, making the material more fire-resistant, bendable, and even self-healing.

To make their improvements, the Rice researchers looked to the nano scale, studying how calcium-silicate hydrate (C-S-H) cement crystallized, and used that to synthesize C-S-H particles of specific shapes. Rather than the amorphous blobs that these particles normally form, the team turned them into cubes, rectangular prisms, dendrites, core-shells and rhombohedra, which are able to pack together more densely. The end result is concrete that's better at keeping water out and preventing it from destroying the material from the inside.

"We call it programmable cement," says Rouzbeh Shahsavari, lead author of the study. "The great advance of this work is that it's the first step in controlling the kinetics of cement to get desired shapes. We show how one can control the morphology and size of the basic building blocks of C-S-H so that they can self-assemble into microstructures with far greater packing density compared with conventional amorphous C-S-H microstructures."

To guide the particles to form these shapes, the team added surfactant compounds and calcium silicate with a positive or negative charge, before exposing the C-S-H mix to carbon dioxide and ultrasound. Changing the amount of calcium silicate affected the shapes that the particles would take on: less of it made for more spherical shapes and smaller cubes, while adding more calcium silicate led to clumps of spheres and interlocking cubes.

After about 25 minutes, these crystal "seeds" formed around the surfactants, and instructed other nearby molecules to self-assemble into larger versions of those shapes. The team was able to control the amount, size and shape of these final particles by adjusting the original seeds' concentration, temperature and the duration of the creation process. This information was then mapped into a unified morphology diagram that can be shared with manufacturers and builders, to allow them to engineer concrete that has specific desired properties.

"The seed particles form first, automatically, in our reactions, and then they dominate the process as the rest of the material forms around them," says Shahsavari. "That's the beauty of it. It's in situ, seed-mediated growth and does not require external addition of seed particles, as commonly done in the industry to promote crystallization and growth."

To test the strength of the different-shaped particles, the team used a nanoindenter with a diamond tip to crush hundreds of individual particles one by one, producing detailed mechanical data.

"Other research groups have tested bulk cement and concrete, but no group had ever probed the mechanics of single C-S-H particles and the effect of shape on mechanics of individual particles," says Shahsavari.

Concrete production is one of the biggest culprits in greenhouse gas emissions, and although recent research suggests that the material may be a significant carbon sink that offsets much of its own environmental cost, finding ways to reduce the amount of cement manufactured is still a priority. To that end, the Rice team's new technique has several advantages.

"One is that you need less of it because it is stronger," explains Shahsavari. "This stems from better packing of the cubic particles, which leads to stronger microstructures. The other is that it will be more durable. Less porosity makes it harder for unwanted chemicals to find a path through the concrete, so it does a better job of protecting steel reinforcement inside."

The research was published in the Journal of Materials Chemistry A.

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