Phys.org Online Magazine reported: “Bringing order to disorder is key to making stronger and greener cement, the paste that binds concrete.” Scientists at Rice University have decoded the kinetic properties of cement and developed a way to “program” the microscopic, semicrystalline particles within. The process turns particles from disordered clumps into regimented cubes, spheres and other forms 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 A.
The technique may lead to stronger structures that require less concrete—and less is better, said Rouzbeh Shahsavari, Rice Materials Scientist and Lead Author. Through extensive experiments, Shahsavari and his 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. He said, “We call it programmable cement. 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.”
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. Shahsavari said 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. 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 (MIT) team that decoded cement’s molecular “DNA” in 2009. “There is currently no control over C-S-H shape,” Shahsavari said. “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 said. “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 research required the team to develop a method to test microscopic concrete particles for strength. The researchers used a diamond-tipped nanoindenter to crush single cement particles with a flat edge. They programmed the indenter to move from one nanoparticle to the next and crush it and gathered mechanical data on hundreds of particles of various shapes in one run.
For the original article from Rice University Current News, please go to: http://news.rice.edu/2016/12/07/decoding-cements-shape-promises-greener-concrete-2/.
For the ABSTRACT in “Journals of Materials Chemistry A”, please go to: http://pubs.rsc.org/en/content/articlelanding/2016/ta/c6ta09389b#!divAbstract.
For the full Phys.org article, please go to: http://phys.org/news/2016-12-decoding-cement-greener-concrete.html#jCp.
Photo (ISCP Home Page): Rice University scientists created microscopic cubes and other shapes of cement and crushed them to test their mechanical properties. The shapes may serve as ‘seeds’ for the bottom-up manufacture of stronger, more durable concrete.
Credit: Multiscale Materials Laboratory/Rice University