Cementing the structure of concrete

By Jared SagoffAugust 1, 2010

Portland cement concrete is one of the most versatile and useful building materials ever created. But although we worry about carbon emissions from cars, airplanes, and power plants, this everyday material is also a hidden—and largely ignored—global warming culprit: it is responsible for about 5% to 7% of all the CO2 released by humans into the Earth’s atmosphere every year.

Those sobering facts have set the cement and concrete industry on an intensive quest to find ways to reduce CO2 emissions while improving concrete’s strength and versatility. These qualities are still not completely understood, even though Portland cement has been around for well over a century. A better understanding of the atomic-level structure of cement hydration products is the key to finding more environmentally-friendly ways to manufacture and use concrete.

Now, new insights into the nanostructure of concrete are coming to light thanks to studies carried out at the Advanced Photon Source (APS) at Argonne National Laboratory. The work, by researchers from the University of California, Berkeley, and Argonne and published in Physical Review Letters, is an important milestone in the push for stronger, more environmentally friendly concrete.

The researchers from the University of California and Argonne studied the nanostructure of calcium silicate hydrate (CSH), the main ingredient that gives concrete its great strength: “The glue that holds concrete together,” as Paulo Monteiro of UC-Berkeley put it. Pinning down the crystalline structure of CSH in cement-paste matrix has proven elusive because of its broad diffraction properties and difficulty in separating it from the other complex phases of cement formation.

In previous times, people would refer to these calcium silicates as almost like an amorphous gel, indicating there was very little structure in the very short range. Only recently have more precise characterization techniques become available to probe into the details of CSH structure. Using synchrotron x-ray diffraction at the X-ray Science Division 11-ID-C beamline of the APS, the group found that CSHs are more ordered than previously believed, and at a smaller scale. The unique contribution of the work is that the nanocrystals are in fact very ordered and that the planes need only be slightly bent to inhibit growth.

The next step will be to investigate the possible role of polymers and other materials—possibly including fly-ash residue from coal—that could be used to make hybrid CSHs with improved cementitious properties and a reduced carbon footprint. These new insights into the nanostructure of concrete are an important milestone in optimizing the engineering of improved CSHs for creating stronger, more environmentally benign concrete.

Related Article: L.B. Skinner et. al., “Nanostructure of Calcium Silicate Hydrates in Cements,” Phys. Rev. Lett. 104(19), 195502 (2010). DOI:10.1103/PhysRevLett.104.195502