This shows a close-up view of an edge dislocation defect in a cement crystal simulation conducted by scientists at Rice University. Image: Lei Tao/Multiscale Materials Laboratory.As part of efforts to develop a deeper understanding of the world's most widely used man-made material, concrete, scientists at Rice University have investigated previously unexplored aspects that could help to improve its environmental credentials.
The Rice laboratory of materials scientist Rouzbeh Shahsavari has developed techniques to not only analyze but also to see dislocations in dicalcium silicates (aka belite), a component of Portland cement. Using these techniques, the scientists have been able to determine how each of five distinct types of belite crystal contribute to concrete's ease of manufacture and ultimate strength. They report their findings in a paper in Cement and Concrete Research.
"Though belite is crystalline in nature, the crystals are so small and the material so amorphous that nobody has looked at them with the kind of analytical eye they deserve," Shahsavari said. Fine-tuning the belite crystals for use in the cement that holds concrete together can help save energy, which in turn leads to a reduction in carbon emissions, he explained.
"Putting an atomistic lens on the role of defects on the mechanics and water reactivity of belite crystals can provide new insights on how to modulate the grinding energy of cement clinkers and strength development of concrete," he said. "Both of these factors can significantly contribute to energy saving and reduced environmental footprints due to the use and manufacture of concrete."
Calcium silicates are a key ingredient in industrial clinkers, which are ground and mixed with water to make cement. Compared with tricalcium silicate, the most dominant ingredient in cement, belite can be produced at a temperature that is at least 100°C lower. But belite is harder to grind and reacts more slowly with water, which leads to delayed strength development in cement paste. Shahsavari said these issues have curbed the widespread use of belite-based cement in concrete, but this latest research could help to change that.
Belite crystals of calcium, silicon and oxygen mainly take one of two different forms, either monoclinic or orthorhombic, each of which behaves differently at the atomic level. Shahsavari and his team subdivided these two forms into five distinct polymorphic crystals. Through computer simulations and high-resolution electron microscopy, they determined that one of the monoclinic forms, dubbed beta-C2S, is the most brittle and possibly the best-suited for cements requiring low-energy manufacture.
Shahsavari said their research provides new insight into the bottom-up engineering of materials with the properties of cement. "The physical understanding gained by our high-resolution electron microscopy images, the first of their kind for cement, combined with our atomistic-level computations, can put cement-based materials on equal footing with metallic systems and semiconductors in the emerging application of 'defect-engineering' to boost performance in manufacturability and functionality," he said. "We expect this will lead to energy savings and environmental benefits."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.