Concrete Products

FEB 2017

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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70 • February 2017 From Rice University, Houston … Bringing order to disorder is key to making stronger and greener cement and concrete. In Nation- al Science Foundation and U.S. Department of Energy-backed research, Rice University scientists have decoded the kinetic proper- ties of cement and developed a way to "pro- gram" the microscopic, semicrystalline parti- cles within. The process turns particles from disordered clumps into regimented cubes, or spheres that combine to make the material less porous and more durable. Their study appears in the Royal Society of Chemistry's Journal of Materials Chemistry. The technique it describes may lead to stron- ger structures that require less concrete. Less is better, says lead author and Rice Univer- sity Materials Scientist Rouzbeh Shahsavari, who cites the 3 billion-plus tons of carbon dioxide released into the atmosphere from worldwide portland cement production. Through extensive experiments, he and colleagues decoded nanoscale reactions, or "morphogenesis," of the crystallization within calcium-silicate hydrate (C-S-H), the hydrat- ed-cement compound binding aggregates into concrete. For the first time, they synthesized C-S-H particles in a variety of shapes, includ- ing cubes, rectangular prisms, dendrites, core- shells and rhombohedra, then mapped them into a unified morphology diagram for pro- ducers and practitioners who wish to engineer concrete from the bottom up. "We call it programmable cement," explains Shahsavari. "The great advance of this work is that it's the first step in con- trolling 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-assem- ble into microstructures with far greater packing density compared with conventional amorphous C-S-H microstructures." 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 notes. "But when it comes to cement and concrete, it is extremely dif- ficult to control their bottom-up assembly. Our work provides the first recipe for such advanced synthesis. "The seed particles form first, automati- cally, in our reactions, and then they domi- nate the process as the rest of the material forms around them. That's the beauty of it. It's in situ, seed-mediated growth and does not require external addition of seed parti- cles, as commonly done in the industry to promote crystallization and growth." Previous techniques to create ordered crystals in C-S-H required high tempera- tures or pressures, prolonged reaction times and the use of organic precursors, but none were efficient or environmentally benign, Shahsavari observes. The Rice University lab created well-shaped cubes and rectangles by adding small amounts of positive or nega- tive ionic surfactants and calcium silicate to C-S-H, and exposing the mix to carbon diox- ide 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; increasing it formed clumped spheres and interlocking cubes. Continued on page 73 TECHNICAL TALK CEMENT RESEARCH Scientists link 'programmable' cement particles to improved concrete performance Work in the Rice University lab netted an isolated cement cube. Microscopic cubes and other shapes may serve as "seeds" in programmable cement for stronger, less porous and more environmentally friendly concrete. PHOTO: Rice University Multiscale Materials Laboratory

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