Rice University study suggests changes to cement manufacturing will save energy
Making cement is a centuries-old art that has yet to be perfected, according to researchers at Rice University who believe it can be still more efficient.
Former Rice graduate student Lu Chen and materials scientist Rouzbeh Shahsavari calculated that fine-tuning the process by which round lumps of calcium silicate called clinkers are turned into cement can save a lot of energy. Their new findings are detailed in the American Chemical Society journal Applied Materials and Interfaces.
A cutaway illustration of a clinker, a pellet manufactured in a kiln and then ground to make cement, shows a defect called a screw dislocation. Rice University scientists studied the effect of such defects on the quality of cement used in concrete and how much energy could be saved by modifying the manufacturing process. Illustration by the Shahsavari Group
Manufacturers of Portland cement, the most common type in use around the world, make clinkers by heating raw elements in a rotary kiln and grinding them into the fine powder that becomes cement. Mixed with water, cement becomes the glue that holds concrete together. An earlier study by Shahsavari and his colleagues that viewed the molecular structure of cement noted that worldwide, concrete manufacturing is responsible for 5 to 10 percent of the carbon dioxide, a greenhouse gas, released into the atmosphere.
The researchers analyzed the crystal and atomic structures of five phases of clinkers representing stages of cooling after they leave the kiln. They focused on the internal stresses that make some more brittle (and easier to grind) than others. They also looked at the unavoidable defects called screw dislocations, shear offsets in the raw materials that, even when ground, influence how well the powders mix with water. That reactivity determines the cement’s ultimate strength.
They found that clinkers were not only most brittle when hottest, but also the most reactive. In ranking the five samples’ qualities, they suggested their research could lead manufacturers to consolidate processes and cut grinding energy that now absorbs around 10-12 percent of the energy required to make cement. Equally important, for each ton of produced cement, the grinding energy accounts for roughly 50 kilograms of carbon dioxide emissions into the atmosphere, they determined.
“Defects form naturally, and you cannot do anything about them,” Shahsavari said. “But the more brittle the clinkers are, the better they are for grinding. We found that the initial phase out of the kiln is the most brittle and that defects carry through to the powder. These are places where water molecules want to react.”
The National Science Foundation supported the research. Shahsavari is an assistant professor of civil and environmental engineering and of materials science and nanoengineering and a member of the Richard E. Smalley Institute for Nanoscale Science and Technology at Rice. Chen is now a structural engineer at Arup.
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Marc Melton
It is obvious that the researchers do not understand cement manufacturing. If you were to put clinker fresh from a kiln into a cement grinding mill there would be several problems with the finished product, regardless of the damage to material transport equipment in the mill system, motors, etc.
First and foremost would be the complete dehydration of the gypsum into its insoluble anhydrite form. Gypsum slows the reaction of the tricalcium aluminate phase. Secondly, the reactivity is only important once the cement is in contact water in the mixer. By this time the cement would have cooled considerably, thus losing any benefit of grinding it at the elevated temperature.
In practical experience, our cement mills typically run at 235-270F. This is to keep the mill hot enough to keep the mills humid to avoid dehydration of the gypsum phases. Then the finished cement is passed through a cement cooler to remove heat from the cement, again, to avoid dehydration of the gypsum.
The contractors who deal with the concrete would also not be happy with a hot cement. Setting time is decreased with an increase in temperature. They need time to work the concrete. Ask any worker that is pouring concrete and they will let you know in no uncertain terms that concrete that sets too early is not a good thing. Not only will the finishing be difficult, or impossible, the concrete itself will be of poor quality.
This just touches on a few points of why their assertion that not grinding the hotter clinker is wasteful of energy. Cement manufacturers are very aware of energy usage in all aspects of cement manufacturing and would not waste energy in places that it could be avoided.
— Comment copied from http://www.rdmag.com/news/2015/01/crush-those-clinkers-while-they%E2%80%99re-hot
Rouzbeh Shahsavari’s response:
@Marc, I am the author of the publication and believe the following paragraph will give you more information on what we researched and what we claimed. We are aware of the role of gypsum, stabilizers and other phase in cement clinkers. What we reported was a basic research for phase-pure clinker components devoid of any other phases or impurities. More precisely, we studied pristine belite polymorphs from a fundamental atomic standpoint. This was a first report on the role of very tiny (nanoscale) defects, so-called dislocations, on brittleness of belite polymorphs. We already highlighted in our published article (ACS Applied Materials and Interfaces, accepted, DOI: 10.1021/am509808) that this study is just the beginning of a more comprehensive solution that the industry requires. Our study only suggests that, unlike the common intuition, the phase pure clinker phase are more brittle at higher temperatures (the alpha polymorph of belite). Translating this finding to industrial clickers that have gypsum and other phases requires another study, or perhaps delaying the processes of adding gypsum as CT Yankee suggested in the comments. Nevertheless our study introduces a new phase space to search for potent sources of energy saving at higher temperature. For the reactivity, we did not suggest contractors to use a hot cement! of course working with a hot cement is not practical; again our report is the first sign of some potential higher reactivities at higher temperature for pure belite phases. Optimizing this to practical solutions at room temperature and/or incorporating the effect of other phases needs further study. I hope you agree with us that that creative solutions and innovation come step by step. If we had started with the full clinker phase (which typically has alite, belite, gypsum and other phases), we would have been lost with the complexity of the mixture, and thus never could have understood which effect comes from what phase! In dealing with complex materials such as clinker, it is logical to break the problem to several simpler phases to gain full understanding, and then work on the complete mixture. If you need further information, I suggest to study the article. Comments are always welcome.