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  Designing Climate-friendly Concrete—From the Nanoscale Up
  New understanding of concrete’s properties could increase lifetime of the widespread building material,decrease emissions June 1, 2016Nancy W. Stauffer, MITEIMITEI Office of Communications ( This article appears in the Spring 2016 issue of Energy Futures (/energyfuturesspring16) , the magazine of the MIT Energy Initiative. Subscribe today( Left to right: Roland Pellenq of civil and environmental engineering(CEE), Katerina Ioannidou of the MIT Energy Initiative, andFranz-Josef Ulm of CEE have developed a detailed understandingof the nanoscale forces at work as cement hardens—a critical steptoward developing stronger, longer-lasting, less carbon-intensiveconcrete for “greener” construction in the future. Photo: StuartDarsch. Overview  An MIT-led team has defined the nanoscale forces that control how particles pack together during the formation of cement “paste,” the material thatholds together concrete and causes that ubiquitous construction material to be a major source of greenhouse gas emissions. By controlling thoseforces, the researchers will now be able to modify the microstructure of the hardened cement paste, reducing pores and other sources of weakness tomake concrete stronger, stiffer, more fracture-resistant, and longer-lasting. Results from the researchers’ simulations explain experimentalmeasurements that have confused observers for decades, and they may guide the way to other improvements, such as adding polymers to fill thepores and recycling waste concrete into a binder material, reducing the need to make new cement.Each year, the world produces 2.3 cubic yards of concrete for every person on earth, in the process generating more than 10% of all industrial carbondioxide (CO 2 ) emissions. New construction and repairs to existing infrastructure currently require vast amounts of concrete, and consumption isexpected to escalate dramatically in the future. “To shelter all the people moving into cities in the next 30 years, we’ll have to build the equivalent of several hundred New York cities,” says Roland Pellenq (, senior research scientist in civil and environmental engineering(CEE) and research director at France’s National Center for Scientific Research (CNRS). “There’s no material up to that task but concrete.”Recognizing the critical need for concrete, Pellenq and his colleague Franz-Josef Ulm (/research/energy-faculty/franz-josef-ulm), professor of CEE anddirector of the MIT Concrete Sustainability Hub ( (CSHub), have been working to reduce its environmental footprint. Their goal: tofind ways to do more with less. “If we can make concrete stronger, we’ll need to use less of it in our structures,” says Ulm. “And if we can make it moredurable, it’ll last longer before it needs to be replaced.”Surprisingly, while concrete has been a critical building material for 2,000 years, improvements have largely come from trial and error rather thanrigorous research. As a result, the factors controlling how it forms and performs have remained poorly understood. “People always deemed what theysaw under a microscope as being coincidence or evidence of the special nature of concrete,” says Ulm, who with Pellenq co-directs the jointMIT-CNRS laboratory called MultiScale Material Science for Energy and Environment (, hosted at MIT by the MIT Energy Initiative(MITEI). “They didn’t go to the very small scale to see what holds it together—and without that knowledge, you can’t modify it.” ■ To shelter all the people moving into cities in the next 30 years, we’ll have to build the equivalent of several hundred New York cities.There’s no material up to that task but concrete. Roland Pellenq, senior research scientist Designing climate-friendly concrete—from the nanoscale up of 404-06-2016 PM 05:38  To validate their model of particle packing during cement hydrate formation, the researchers perform laboratory experiments on cement samples prepared with different ratios of water to cement powder to achieve carefully controlled packing fractions. Nanoindentation tests measure stiffness and hardness, and small-angle neutron scattering tests determinethe distribution of particles and pores within the sample. Photo: Stuart Darsch. Cement: the key to better concrete The problems with concrete—both environmental and structural—are linked to the substance that serves as its glue, namely, cement. Concrete ismade by mixing together gravel, sand, water, and cement. The last two ingredients combine to make cement hydrate, the binder in the hardenedconcrete. But making the dry cement powder requires cooking limestone (typically with clay) at temperatures of 1,500°C for long enough to drive off thecarbon in it. Between the high temperatures and the limestone decarbonization, the process of making cement powder for concrete is by itself responsible for almost 6% of all CO 2  emissions from industry worldwide. Structural problems can also be traced to the cement: When finished concretecracks and crumbles, the failure inevitably begins within the cement hydrate that’s supposed to hold it together—and replacing that crumbling concretewill require making new cement and putting more CO 2  into the atmosphere.To improve concrete, then, the researchers had to address the cement hydrate—and they had to start with the basics: defining its fundamentalstructure through atomic-level analysis. In 2009, Pellenq, Ulm, and an international group of researchers associated with CSHub published the firstdescription of cement hydrate’s 3-dimensional molecular structure. Subsequently, they determined a new formula that yields cement hydrate particlesin which the atoms occur in a specific configuration—a “sweet spot”—that increases particle strength by 50%.However, that nanoscale understanding doesn’t translate directly into macroscale characteristics. The strength and other key properties of cementhydrate actually depend on its structure at the “mesoscale”—specifically, on how nanoparticles have packed together over hundred-nanometer distances as the binder material forms.When dry cement powder dissolves in water, room-temperature chemical reactions occur, and nanoparticles of cement hydrate precipitate out. If theparticles don’t pack tightly, the hardened cement will contain voids that are tens of nanometers in diameter—big enough to allow aggressive materialssuch as road salt to seep in. In addition, the individual cement hydrate particles continue to move around over time—at a tiny scale—and thatmovement can cause aging, cracking, and other types of degradation and failure.To understand the packing process, the researchers needed to define the precise physics that drives the formation of the cement hydratemicrostructure—and that meant they had to understand the physical forces at work among the particles. Every particle in the system exerts forces onevery other particle, and depending on how close together they are, the forces either pull them together or push them apart. The particles seek anorganization that minimizes energy over length scales of many particles. But reaching that equilibrium state takes a long time. When the Romans madeconcrete 2,000 years ago, they used a binder that took many months to harden, so the particles in it had time to redistribute so as to relax the forcesbetween them. But construction time is money, so today’s binder has been optimized to harden in a few hours. As a result, the concrete is solid longbefore the cement hydrate particles have relaxed, and when they do, the concrete sometimes shrinks and cracks. So while the Roman Colosseum andPantheon are still standing, concrete that’s made today can fail in just a few years. Structure of hardened cement hydrate The left and center diagrams show the structure of cement hydrate as determined by the researchers’ model, which calculates the positions of particles based on particle-to-particleforces. Each simulation box is about 600 nanometers wide. The packing fraction (the fraction of the box occupied by particles) is assumed to be 0.35 in the left diagram and 0.52 inthe center one. Open pores, indicated by the white areas, are more prevalent at the lower packing fraction. The right-hand diagram is a sketch of cement hydrate published by T.C.Powers in 1958. The research challenge Laboratory investigation of a process that can take place over decades isn’t practical, so the researchers turned to computer simulations. “Thanks tostatistical physics and computational methods, we’re able to simulate this system moving toward the equilibrium state in a couple of hours,” says Ulm.Based on their understanding of interactions among atoms within a particle, the researchers—led by MITEI postdoc Katerina Ioannidou—defined theforces that control how particles space out relative to one another as cement hydrate forms. The result is an algorithm that mimics the precipitationprocess, particle by particle. By constantly tracking the forces among the particles already present, the algorithm calculates the most likely position for each new one—a position that will move the system toward equilibrium. It thus adds more and more particles of varying sizes until the space is filledand the precipitation process stops. Designing climate-friendly concrete—from the nanoscale up of 404-06-2016 PM 05:38  Results from sample analyses appear in the first two diagrams in the figure above. The width of each simulation box is just under 600 nanometers—about a tenth the diameter of a human hair. The two analyses assume different packing fractions, that is, the total fraction of the simulation boxoccupied by particles. The packing fraction is 0.35 in the left-hand diagram and 0.52 in the center diagram. At the lower fraction, far more of the volumeis made up of open pores, indicated by the white regions.The third diagram in the figure above is a sketch of the cement hydrate structure proposed in pioneering work by T.C. Powers in 1958. The similarity tothe center figure is striking. The MIT results thus support Powers’ idea that the formation of mesoscale pores can be attributed to the use of excessivewater during hydration—that is, more water than needed to dissolve and precipitate the cement hydrate. “Those pores are the fingerprint of the water you put into the mix in the first place,” says Pellenq. “Add too much water, and at the end you’ll have a cement paste that is too porous, and it willdegrade faster over time.” Measured and calculated cement hydrate stiffness and hardness These figures show stiffness and hardness of cement hydrate at various packing fractions as measured by nanoindentation in three laboratory samples (small symbols) andcalculated in a simulated sample generated by the researchers’ model (yellow squares). The simulation results match the experimental data well. The computed results extend downto packing fractions that produce material too soft to be tested experimentally. To validate their model, the researchers performed experimental tests and parallel theoretical analyses to determine the stiffness and hardness (or strength) of cement hydrate samples. The laboratory measurements were taken using a technique called nanoindentation, which involves pushing ahard tip into a sample to determine the relationship between the applied load and the volume of deformed material beneath the indenter.The figures above show results from small-scale nanoindentation tests on three laboratory samples (small symbols) and from computations of thoseproperties in a “sample” generated by the simulation (yellow squares). The figure on the left shows results for stiffness, the figure on the right results for hardness. In both cases, the X-axis indicates the packing fraction. The results from the simulations match the experimental results well. (Theresearchers note that at lower packing fractions, the material is too soggy to test experimentally—but the simulation can do the calculation anyway.)In another test, the team investigated experimental measurements of cement hydrate that have mystified researchers for decades. A standard way todetermine the structure of a material is using small-angle neutron scattering (SANS). Send a beam of neutrons into a sample, and how they bounceback conveys information about the distribution of particles and pores and other features on length scales of a few hundred nanometers.SANS had been used on hardened cement paste for several decades, but the measurements always exhibited a regular pattern that experts in thefield couldn’t explain. Some talked about fractal structures, while others proposed that concrete is simply unique.To investigate, the researchers compared SANS analyses of laboratory samples with corresponding scattering data calculated using their model. Theexperimental and theoretical results showed excellent agreement, once again validating their technique. In addition, the simulation elucidated thesource of the past confusion: The unexplained patterns are caused by the rough edges at the boundary between the pores and the solid regions. “All of a sudden we could explain this signature, this mystery, but on a physics basis in a bottom-up fashion,” says Ulm. “That was a really big step.” In this novel CSHub experimental setup, a half-inch tire of rigid steel rolls along a polymer pavement, providing a detailed look at the interaction between the wheel and the pavementstructure. Tests show that the maximum pavement deflection is just behind the path of travel; in effect, the wheel is constantly rolling uphill. Making road surfaces stiffer—for example, by adding a concrete layer—would mitigate that effect, decreasing wasted fuel and associated emissions. Photo: Stuart Darsch. New capabilities, new studies ll of a sudden we could explain this signature, this mystery, but on a physics basis in a bottom-up fashion. That was a really bigstep. Franz-Josef Ulm (/research/energy-faculty/franz-josef-ulm), professor of CEE, director of the MIT Concrete Sustainability Hub Designing climate-friendly concrete—from the nanoscale up of 404-06-2016 PM 05:38  “We now know that the microtexture of cement paste isn’t a given but is a consequence of an interplay of physical forces,” says Ulm. “And since weknow those forces, we can modify them to control the microtexture and produce concrete with the characteristics we want.” The approach opens up anew field involving the design of cement-based materials from the bottom up to create a suite of products tailored to specific applications.The CSHub researchers are now exploring ways to apply their new techniques to all steps in the life cycle of concrete. For example, a promisingbeginning-of-life approach may be to add another ingredient—perhaps a polymer—to alter the particle-particle interactions and serve as filler for thepore spaces that now form in cement hydrate. The result would be a stronger, more durable concrete for construction and also a high-density,low-porosity cement that would perform well in a variety of applications. For instance, at today’s oil and natural gas wells, cement sheaths are generallyplaced around drilling pipes to keep gas from escaping. “A molecule of methane is 500 times smaller than the pores in today’s cement, so filling thosevoids would help seal the gas in,” says Pellenq.The ability to control the material’s microtexture could have other, less-expected impacts. For example, novel CSHub work has demonstrated that thefuel efficiency of vehicles is significantly affected by the interaction between tires and pavement. Simulations and experiments in the lab-scale setupshown in the image above suggest that making concrete surfaces stiffer could reduce vehicle fuel consumption by as much as 3% nationwide, savingenergy and reducing emissions.Perhaps most striking is a concept for recycling spent concrete. Today, methods of recycling concrete generally involve cutting it up and using it inplace of gravel in new concrete. But that approach doesn’t reduce the need to manufacture more cement. The researchers’ idea is to reproduce thecohesive forces they’ve identified in cement hydrate. “If the microtexture is just a consequence of the physical forces between nanometer-sizedparticles, then we should be able to grind old concrete into fine particles and compress them so that the same force field develops,” says Ulm. “We canmake new binder without needing any new cement—a true recycling concept for concrete!”This research was supported by Schlumberger; France’s National Center for Scientific Research (through its Laboratory of Excellence InterdisciplinaryCenter on MultiScale Materials for Energy and Environment); and the Concrete Sustainability Hub ( at MIT. Schlumberger is aSustaining Member  (/membership) of the MIT Energy Initiative. The research team also included other investigators at MIT; the University of Californiaat Los Angeles; Newcastle University in the United Kingdom; and Sorbonne University, Aix-Marseille University, and the National Center for ScientificResearch in France. Further information can be found in:K. Ioannidou, K.J. Krakowiak, M. Bauchy, C.G. Hoover, E. Masoero, S. Yip, F-J. Ulm, P. Levitz, R.J-M. Pellenq, and E. Del Gado. “Mesoscale texture of cement hydrates.” Proceedings of the National Academy of Sciences , vol. 113, no. 8, pp. 2029–2034, 2016.R.J-M. Pellenq, A. Kushima, R. Shahsavari, K.J. Van Vliet, M.J. Buehler, S. Yip, and F-J. Ulm. “A realistic molecular model of cement hydrates.” Proceedings of the National Academy of Sciences , vol. 106, no. 38, pp. 16102–16107, 2009.M.J.A. Qomi, K.J. Krakowiak, M. Bauchy, K.L. Stewart, R. Shahsavari, D. Jagannathan, D.B. Brommer, A. Baronnet, M.J. Buehler, S.Yip, F.-J. Ulm, K.J.Van Vliet, and R.J-M. Pellenq. “Combinatorial molecular optimization of cement hydrates.” Nature Communications , vol. 5, doi:10.1038/ncomms5960,2014. MIT Energy Initiative ( Massachusetts Institute of Technology77 Massachusetts Avenue, E19-307Cambridge, MA 02139-4307Phone: 617.258.8891Fax: 617.324.3665 Designing climate-friendly concrete—from the nanoscale up of 404-06-2016 PM 05:38
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