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Nature offers a new, free-access story by Ivan Amato that updates some of the efforts to lower CO2 emissions and the energy requirements for cement production going on at the Concrete Sustainability Hub (CSHub) at the Massachusetts Institute of Technology in Cambridge. The article particularly focuses on their work to make progress in modeling the calcium silicate hydrate (C-S-H) reactions, and understanding the difference characteristics of its alite and belite phases.
While the story, “Green Cement: Concrete Solution” covers some fairly familiar (if not somewhat misleading) grounds, it does suggest that CSHub researchers are getting a better handle on the pros and cons of increasing the belite content of clinker. Allow me to make a few verbal shortcuts here, but, in brief, clinker is eventually ground into a powder that is known as Portland cement. The alite phase, abbreviated in cement nomenclature as C3S, is more reactive with water and therefore hydrates (cures) in concrete mixtures faster than belite. The downside to alite is that it requires more energy and CO2 emissions than belite (alite forming about 1,500°C and belite around 1,200°C. Thus, the thinking goes that if there was a clever way to make better use of belite, there would be a payoff to the environment.
One group of researchers at the CSHub report in a recent newsletter (pdf) that they have been investigating the various polymorphs of belite, specifically its Beta β), Gamma (γ), and three Alpha (α) phases. In particular, they have been using “first-principles fully quantum mechanical analytical techniques” to model the different polymorphs. They have combined the modeling with statistical analyses to focus on and predict the most reactive and favorable structures of β and γ, and claim to have a deeper understand of how adatoms (atoms adsorbed into the crystal structure) and defects play a role in determining surface energies and crystal reactivity.
As Amato correctly notes, there is another downside to belite, besides lower reactivity: it is harder and therefore requires more mechanical energy to grind, a reality that decreases any potential net energy savings.
Some experts, such as Karen Scrivener who leads a European-based research initiative (and who I interviewed last year), have doubts that there will be a large payoff to tinkering with the clinker composition. Instead, Scrivener and others think larger gains can be made my replacing a significant amount of the clinker (”reducing the clinker factor”) by using supplementary cementitious materials (SCMs) that, conveniently, are often byproducts of other industries. These SCMs can include fine limestone, flash, blast furnace slag, silica flume, and natural pozzolans. In fact, the US may lag Europe and other regions in the use of SCMs, which Scrivener says European cement producers over the last two decades have successfully found substitutes for about 25 percent of the clinker.
One major difference, however, between Europe and the US is that the concrete industry, in the former, tends to use mixes containing SCMs that are pre-made at cement-making facilities, while US concrete makers who do use SCMs tend to add the SCMs while mixing the concrete. Waiting until the concrete-mixing stage introduces a significant change for variations from batch to batch.
Amato does reports on efforts by at least one US company, Ceratech, to produce and market cement mixes that contain flash. But Amato notes, “Ceratech is a bit player in the cement industry, and its approach to cutting carbon emissions amounts to a mere clink in a multibillion-tonne batch. The big carbon reductions will come only when next-generation cements are embraced by the construction industry’s thousands of independent producers, engineers, architects, city planners, and building inspectors.”
But above I mentioned that I thought there are some misleading points to Amato’s article, starting with the title (which, in fairness he may not have written). The admittedly nonintuitive truth is that concrete is already a relatively “green” construction material that is far less energy intensive than masonry, steel, aluminum, and even wood. Another inconvenient truth is that cement and concrete construction are absolutely essential to housing and infrastructure development, especially in the developing regions, and that it is a pipe dream to imagine some alternative in the remainder of the 21st Century. A final inconvenient truth is that the availability of some of the SCMs is actually very small—including waste slag and fly ash—compared to overall production quantities—and in some cases nonexistent because the regions lack the steelmills and powerplants that generate the byproduct. Therefore, research into the use of more common SCM sources, such as calcined clays and natural pozzolans is expected yield the most significant sustainability breakthroughs in the future. Scrivener says that a 10 percent reduction in the amount of CO2 currently associated with the production of cubic meter of concrete would be equivalent to removing all of the CO2 emissions associated with steel production.
All of this is not to say that CSHub’s work is not significant. In fact, it appears that they are doing a lot of valuable computation modeling work that will contribute to worldwide efforts to understand hydration and to build databases that can be used to create custom cement and concrete mixes based on regionally available sustainable resources. Even one or two percent efficiency gains in clinker production are of value given the enormous amount of cement and concrete produced world wide.
I would be remiss if I didn’t note that 4th Advances in Cement-Based Materials meeting, coorganized by Cements Division of ACerS and the Center for Advanced Cement-based Materials, will be held July 8-10 at University of Illinois at Urbana-Champaign. (Abstract deadline is March 25).
Credit: Purdue University and INDOT Joint Transportation Research Program.
Many people in the field of high-performance cements and materials have been working on the goal of improving the performance of structures such as roadways and bridge decks, and recently there have been interesting developments in regard to the use of internal curing (IC) techniques and the creation of a new standard specification by ASTM International.
For researchers involved in cements and concrete, a fundamental task has been to prevent deterioration caused, to a large extent by ions from salts and other materials that can lead to crack formation and corrosion of steel reinforcements. A basic consideration is that cement systems must “cure” or hydrate sufficiently to become useful. A nemesis is early-age cracking that can lead to accelerated deterioration of concrete, which can lead to catastrophic outcome in the case of concrete bridge components.
A couple of factors come into play. First, curing is not instantaneous and requires access to water. Curing to a serviceable extent (e.g., to 75 percent of full curing) is typically measured in days and weeks, but the curing process can go on for years if conditions are right.
Another factor is the composition of the concrete constituents. Engineers are employing both “high-performance” concrete and cementitious materials that can substitute for some of the cements, such as fly ash. Unfortunately, both can also lead to curing problems. In the case of the former, although the high-performance materials have the positive property of limiting the ingress of briny fluids and destructive ions, according to John Ries, technical director of the Expanded Shale, Clay and Slate Institute, “these properties also limit the ability of externally applied curing water to reach the interior of the concrete.”
In the case of the latter, cement alternatives can lead to extended curing times. In a recent NIST Tech Beat story, NIST engineer Dale Bentz explains, “In these high-volume fly ash mixtures, internal curing is important because while the fly ash will react with the cement, it takes a lot longer. After 28 days, maybe 30 percent or less of the fly ash has reacted, so you really need to keep the concrete saturated for an extended period of time.”
In both cases, the solution is to achieve a way to encourage internal curing and, says Reis, “provide a source of additional water to maintain saturation of the cementitious paste and avoid its self-desiccation.”
As discussed in the above video, engineers from Purdue University and the Indiana Department of Transportation (INDOT) have been developing an IC approach that involves creating a longer-term internal water source instead of relying water in the mix or externally applied water. A Purdue news release reports that the IC approach is based on creating “water pockets” formed from small porous stones—or fine aggregate, as it is known in the industry—to replace some of the sand in the mixture. Purdue’s Jason Weiss says, “A key step in the process is to pre-wet the lightweight aggregate with water before mixing the concrete.”
Weiss, who is a professor of civil engineering and director of the Pankow Materials Laboratory, as well as a long-time collaborator on the annual meetings of ACerS’ Cements Division, reports that coming up with a suitable IC system did not happen overnight. “Nearly five years of research has been performed to fully understand how to proportion these mixtures and the level of performance that can be expected,” Weiss says.
The video and the Purdue release say a real-world IC study is underway. In 2010, INDOT (with the support of NIST, Lafarge North America and the Expanded Shale Clay and Slate Institute) built two adjacent bridges—one based on IC specifications and one based on traditional specs—and, so far, the results are looking good. In Purdue release, Weiss reports, “The control bridge has developed three cracks, but no cracks have developed in the internally cured bridge. Tests also show the internally cured concrete is approximately 30 percent more resistant to salt ingress.”
Another recent development is that NIST and Purdue successfully gained the approval of the ASTM’s Standard Specification for Lightweight Aggregate for Internal Curing of Concrete (ASTM C1761-12).
Finally, this is a good place to mention that the “4th Advances in Cement-based Materials: Characterization, Processing, Modeling and Sensing” meeting co-organized by ACerS’s Cements Division and the Center for Advanced Cement-based Materials will be he held July 8-10, 2013, at the University of Illinois at Urbana-Champaign.
Last week the ACerS Cements Division held its annual meeting at the University of Texas at Austin, Austin, Texas. The theme of the June 10-12 meeting was “3rd Advances in Cement-based Materials: Characterization, Processing, Modeling and Sensing.”
Edward Garboczi, NIST Fellow, Materials and Construction Research Division, gave the Della Roy award lecture, “The Computational Materials Science of Concrete: Past—Present—Future,” to an audience of about 100 attendees. Overall, there were 35 presentations, including three invited tutorial workshop presentations and 35 posters (32 presented by students). The workshop theme was “Novel Experimental and Computational Tools,” and the presenters were Rolf Arvidson and Rouzbeh Shahsavari, both from Rice University and Jeffrey Chen from Lafarge Centre de Recherche.
The Division also awarded its 2011 Brunauer Award for the best paper published on cements in 2010 to Jeff Bullard of NIST and Robert Flatt of Sika Technology, AG, Switzerland.
The meeting was organized by Zach Grasley, Maria Juenger and Jeff Chen in coordination with the Center for Advanced Cement-based Materials
Here are a few pictures from the event in Texas, provided by meeting organizer and UTA associate professor, Maria Juenger, and ACerS staffer, Marcia Stout.
Energy gurus often talk about reducing CO2 emissions. Why not be more aggressive and talk about eliminating CO2 emissions? And, why not start with a heavy CO2-producing industry, such as cement?
Some estimate that cement production generates five to six percent of all anthropogenic (human-generated) CO2 emission. There is an almost one-to-one correspondence of CO2 generated to cement made — 10 kg of cement generates 9 kg of CO2. The global annual consumption of cement is more than 3 x 1012 kg , and 90 percent of that is a lot of CO2. That translates to about 3,300 million tons of cement and just under 3,000 million tons of CO2.
Indeed, cement researchers often ponder how to to significantly reduce the emissions problem, and many strides have been made in partnership with large cement makers.
The cement-making process generates CO2 from the decomposition reaction of calcium carbonate to calcium oxide (lime) and from the combustion of fossil fuels to fire kiln reactors (to about 900˚C). Ninety percent or so of the total energy needed to make concrete is used just to make the cement.
Decomposition is a brute force approach to making lime — heat the stuff until it gives up its bonds and falls apart. Professor Stuart Licht at George Washington University is a STEP ahead, though, and has demonstrated the feasibility of making lime by electrolysis with a process he calls Solar Thermal Electrochemical Production. In a paper published this month in Chemical Communications, (DOI: 10.1039/C2CC31341C), he describes a solar-driven process that exploits “a new thermal chemistry, based on anomalies in oxide solubilities, to generate CaO, without CO2 emission.”
In the process, molten carbonates heated by solar energy are electrolyzed and form oxides, which in the presence of calcium carbonate precipitate as lime. The solubility of calcium carbonate is high in molten carbonates at high temperatures (in the 750-950˚C range). However, the solubility of calcium oxide in molten carbonates is low, up to 100 times lower than calcium carbonate.
The team experimented with two kinds of electrolyte, a eutectic mix of carbonates and pure lithium carbonate. The paper explains how lime forms in the electrolytic cell, “when molten carbonates undergo electrolysis to form oxides, added calcium carbonate will precipitate the desired CaO product for extraction, and the added carbonate replenishes the electrolyte for continued, ongoing CaO production.”
At temperatures below 800˚C, the calcium carbonate electrolyzes to CaO, C and O2. Above 800˚C, the reaction products are CaO, CO and O2. (CO is a commercially valuable compound.) No CO2 is produced in either temperature regime.
Electrolysis of carbonates is endothermic, which means much of the thermal energy required to drive the process can be provided by solar energy. And, if all of the heat is provided by solar energy, no fossil fuels are burned and no CO2 is generated by the process itself.
The resulting calcium oxide is high density and appears to be easy to harvest as it “forms a slurry at the bottom of the vessel where it may be removed by tap in the same manner in which molten iron is removed from conventional iron production kilns.”
The authors realize that scaling-up to industrial production levels and incorporation into production systems will be challenging. But, this is familiar territory for industries and engineers. There is precedent, too, for industrial-scale electrolysis processes. Electrolysis is the basis of the Hall-Heroult process (pdf) for extracting aluminum from bauxite. Similar to Licht’s experiment, the key step is to dissolve alumina in a molten salt, in this case, sodium aluminum fluoride.
Cementiers will be interested to know that the American Ceramic Society’s Cements Division is holding its Third Advances in Cement-based Materials meeting in June. The theme is “Characterization, Processing, Modeling and Sensing.” The plenary speaker will be Edward Garboczi from NIST whose talk is titled, “Computational Materials Science of Concrete: Past-Present-Future.”
Also watch for the June issue of The Bulletin, which will cover new cement technologies.
At the recently concluded Advances in Cement-based Materials meeting organized by ACerS’ Cements Division and ACBM, Karen Scrivener of the Ecole Polytechnique Fédérale de Lausanne (Switzerland) was selected by the organizers to deliver the Della Roy Lecture. Scrivener is a highly respected expert in the field of cements and she was an appropriate pick, having followed in Della Roy’s footsteps as the editor of the Cement and Concrete Research journal.
Scrivener is also the founder of Nanocem, a Europe-based initiative working on collaborative approaches (not just among institutions, but also between institutions and industry) to cements questions. One of the aims of Nanocem is to spur a constant effort to reduce CO2 emissions:
“Evolving CO2 emission caps in Europe mean that cement manufacturers will have to find solutions, or pay more to produce cement, which will reduce their competitivity. Nanocem is sponsoring fundamental research that will support technological solutions, not only to help secure the long-term health of the European cement industry, but also to address global CO2 reduction by ensuring that cement manufacturing is not just shifted to regions of the world that have less stringent emissions regulations.”
The title of Scrivener’s Della Roy Lecture was “Modeling Hydration Kinetics of Cementitious Systems,” which was quite a good discussion about the what has and what’s yet to be done in the world of modeling cement microstructures.
But, as a non-expert in cements, I found Scrivener’s opening remarks, which provided the context for her technical presentation, a balanced and compelling discussion about the importance of cements and concrete to, well … the world!
A few of her salient points:
• Concrete is most used material in the world. It is the only material that can satisfy the demand for low-cost decent housing and infrastructure. There is no way to satisfy the demand for low-cost housing and infrastructure without concrete.
• The demand for concrete is growing and will continue to soar, especially in the developing nations. The demand may double or triple by 2050.
• Although cement production is energy intensive, the energy and CO2 emissions of concrete (per ton) is among the lowest of all building materials, even lower than wood.
• The CO2 problem associated with cements (5-8% of CO2 production world wide) is primarily because of the volume of demand.
• There has been talk of achieving 5-10% reduction in emissions per cubic meter of concrete through the use of substitutes for Portland cement. It may be more like 1-2% given the amounts and choices of supplements (see below), but even a 1% saving would be equal to removing all the CO2 emissions associated with steel production. “So that shows how much impact we can have by research to increase the sustainability of cement,” she said.
• The available elements in the earth’s crust imposes a fundamental limit on the options for substitute cementitious materials. Eight elements—oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium—make up 98 percent of earths crust. So, forget about making cement out of any other elements.
• The way this has been pursued over the last 20-some years is first of all process optimization. Cement kiln and other production operations have gotten much better and state-of-the-art plants are achieving 80% of their theoretical efficiency.
• So, recently the goal has shifted more to reducing the “clinker factor,” i.e., instead of grinding clinker and gypsum, add more and more supplementary cementitious materials. SCMs my be byproducts or waste products from other industries, such as limestone, fly ash, blast furnace slag, silica flume, natural pozzolans, etc.
• This has been a good and successful strategy (over the last 20 years 25% of the previous amount of clinker in cements is now substituted by SCMs).
• But … and this is a big “but” … this is going to be a difficult strategy going forward because under the best of circumstances, the amount of SCM available will be dwarfed by amount of cement produced.
• For example, people talk about using fly ash as an SCM and it is probably the most widely available. But, there simply isn’t enough fly ash worldwide to replace cement in any big amount. The availability of SCMs in underdeveloped countries, where the demand is going to be coming from, is small because by definition they don’t have the scale of industries to provide large volumes of slag and fly ash byproducts.
• So where are SCMs going to come from? More limestone, probably, but more calcined clays and natural pozzolans. Cuba may be a good source of calcined clays.
• There is no one single answer. All sustainability possibilities must be pursued in parallel. Eventually we are going to see a very diverse range of cements, which are adapted to locally available materials. But diversity means performance questions will be more and more complicated.
• For researchers to support sustainability, they must provide end users of concrete information that will make them confident about the use of the SCMs. This means having relevant performance tests. In particular, researchers need to know how to start with the variables—the composition of the cements, the SCMs, mixing techniques, curing times, relative humidities, etc.— and from this predict performance.
• Short term performance can be relatively easy to predict in the laboratory, but long term performance—where we expect structures to last 40-50 years without hardly any maintenance—is much harder to measure in the lab.
• Therefore, researchers have to pursue mechanisms to predict microstructure via advanced modeling techniques to pull all of this complex information together and have it make sense.