A new method transforms particles in cement from disordered clumps into regimented cubes, spheres, and other forms, which makes the material less porous and more durable.
Cement is the paste that binds concrete. The new technique may lead to stronger structures that require less concrete—and less is better, says materials scientist and lead author Rouzbeh Shahsavari of Rice University. Worldwide production of more than 3 billion tons of concrete a year now emits as much as 10 percent of the carbon dioxide, a greenhouse gas, released to the atmosphere.
Shahsavari and colleagues decoded the nanoscale reactions—or “morphogenesis”—of the crystallization within calcium-silicate hydrate (C-S-H) cement that holds concrete together.
For the first time, they synthesized C-S-H particles in a variety of shapes, including cubes, rectangular prisms, dendrites, core-shells, and rhombohedra and mapped them into a unified morphology diagram for manufacturers and builders who wish to engineer concrete from the bottom up.
“We call it programmable cement,” he says. “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.”
He says the idea is akin to the self-assembly of metallic crystals and polymers. “It’s a hot area, and researchers are taking advantage of it,” Shahsavari says. “But when it comes to cement and concrete, it is extremely difficult to control their bottom-up assembly. Our work provides the first recipe for such advanced synthesis.
“The seed particles form first, automatically, in our reactions, and then they dominate the process as the rest of the material forms around them,” he says. “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.”
Previous techniques to create ordered crystals in C-S-H required high temperatures or pressures, prolonged reaction times, and the use of organic precursors, but none was efficient or environmentally benign, Shahsavari says.
The Rice lab created well shaped cubes and rectangles by adding small amounts of positive or negative ionic surfactants and calcium silicate to C-S-H and exposing the mix to carbon dioxide and ultrasonic sound. The crystal seeds took shape around surfactant micelles within 25 minutes. Decreasing the calcium silicate yielded more spherical particles and smaller cubes, while increasing it formed clumped spheres and interlocking cubes.
Once the calcite “seeds” form, they trigger the molecules around them to self-assemble into cubes, spheres, and other shapes that are orders of magnitude larger. These can pack more tightly together in concrete than amorphous particles, Shahsavari says. Carefully modulating the precursor concentration, temperature, and duration of the reaction varies the yield, size, and morphology of the final particles.
The discovery is an important step in concrete research, he said. It builds upon his work as part of the Massachusetts Institute of Technology team that decoded cement’s molecular “DNA” in 2009. “There is currently no control over C-S-H shape,” Shahsavari says. “The concrete used today is an amorphous colloid with significant porosity that entails reduced strength and durability.”
The new technique has several environmental benefits, Shahsavari says. “One is that you need less of it (the concrete) because it is stronger. 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 study appears in the Journal of Materials Chemistry A. Coauthors are from Rice University, C-Crete Technologies in Houston, and the University of Houston.
The National Science Foundation and the Department of Energy supported the research.
Source: Rice University