Rice University models detail how defects influence efficiency in concrete manufacture
Because concrete, the world’s most-used construction material, is such a major contributor to climate change, it’s worth knowing every detail about how it’s manufactured, according to Rice University scientists.
The Rice lab of theoretical physicist Rouzbeh Shahsavari is looking into those details down to the atomic level. The lab has published the results of computer modeling studies that detail how dislocations – screw-like defects – in raw crystals used for concrete influence how efficiently it can be made.
The research published this month in the Journal of the American Ceramic Society shows that tricalcium silicates (C3S) that consist of pure rhombohedral crystals are better than others for producing “clinkers.” Clinkers are round lumps of C3S that, when ground into a powder, mix with water to make cement, the glue that holds gravelly concrete together. The easier a clinker is to grind, the less energy a manufacturer needs to grind it.
Last year, the Shahsavari lab reported that hot clinkers were easier to grind. They also looked at the detrimental effects of screw dislocations on how well the resulting powder mixes with water.
This time, the lab built computer models of the molecular structures that make up several commonly used types of C3S to see which were prone to be more brittle, despite the inevitable dislocations that twist the crystals into unpredictable formations. (The more brittle, the better.)
They also sought to understand how defects in the microscopic crystals influence the powder’s ability to react with water and found that rhombohedrals are also more reactive to water than the two monoclinic clinkers they studied. Rhombohedral crystals have edges that are all the same length; monoclinic crystals do not.
“Understanding and quantifying the structure, energetics and the effect of defects on mechanics and reactivity of cement crystals is a fundamental and engineering challenge,” Shahsavari said. “This work is the first study that puts an atomistic lens on the key characteristics of screw dislocations, a common line defect in C3S, which is the main ingredient of Portland cement.”
Shahsavari insisted that maximizing energy use in the production of concrete is important, an attitude supported by attendees at last year’s climate summit in Paris. He noted at the time that the annual worldwide production of more than 20 billion tons of concrete contributes 5 to 10 percent of carbon dioxide to global emissions, surpassed only by transportation and energy generation as a producer of greenhouse gas.
Besides the findings of this work, this study paves the path to investigate such other defects as edge dislocation, brittle-to-ductile transitions and twinning deformations in cement, Shahsavari said. Together, these will provide new physical insights and strategies to tune the grinding energy and reactivity of cement, which would lower the energy consumption and carbon dioxide footprint associated with concrete.
Rice graduate student Lei Tao and alumna Lu Chen, now a structural engineer at Arup, are co-authors of the paper. Shahsavari is an assistant professor of civil and environmental engineering and of materials science and nanoengineering.
The National Science Foundation supported the research.
I have worked many years in the cement industry, mainly in cement and clinker microscopy, and find the Shahsavari report very puzzling. I have some specific questions. (1) In comparing brittleness, did you consider differences in chemical composition of C3S crystals? This might explain the differences in forces needed to break (grind, cleave) these crystals, and thus the surface area of C3S in the powder and its hydraulic response. (2) What criteria do you have to say that the crystals are rhombohedral (Hexagonal System)? Rhombohedral alite is not the prevailing crystal form in normal industrial clinker production. (3) Were the rhombohedral crystals lab-made? (4) Is this model based on pure compounds, that is pure C3S? (5) Did you calculate the differences in surface areas on a phase-by-phase basis in comparing hydraulic effects? Unfortunately, I was not able to download a copy of your paper because I am not a member of the ACS. Perhaps you could send me copies of your interesting work.
Sincerely, Donald H. Campbell, http://www.campbellpetro.com, retired
Posting answers from Professor Shahsavari:
(1) Our crystals were pure C3S, thus all had the same composition.
(2) Rhombohedral and other symmetries are known polymorphs of alite.
(3) Our study was computational, not lab-based.
(4) Yes, pure compounds
(5) No, we used a different approach. See the technical paper.