Rice University scientists working to develop a deeper understanding of concrete have detailed previously unexplored aspects that affect the energy used, and greenhouse gases produced, during its manufacture.

Using high-resolution electron microscopy, Rice materials scientist Rouzbeh Shahsavari has developed techniques not only to analyze but also to see dislocations in dicalcium silicates (aka belite), a component of Portland cement, and has detailed how each of five distinct polymorphic crystals contributes to concrete’s ease of manufacture and ultimate strength.

Close-up view of an edge dislocation defect in a cement crystal simulation. Image credit: Lei Tao/Multiscale Materials Laboratory. “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 says. But fine-tuning them for use in the cement that holds concrete together can help save energy, which in turn could lead to a reduction in carbon emissions, he adds.

“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,” Shahsavari says. “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 more dominant ingredient in cement—belite can be produced at a much lower temperature. However, it is harder to grind and reacts more slowly with water, which leads to delayed strength development in cement paste. Shahsavari says these issues have curbed the widespread use of belite-based cement in concrete.

Belite crystals of calcium, silicon and oxygen generally take one of two different forms, either monoclinic or orthorhombic, each of which behaves differently at the atomic level. Rice researchers subdivided those into five distinct polymorphic crystals. Through computer simulations and electron microscopy, Rice researchers determined one of the monoclinic forms, dubbed beta-C2S, is the most brittle and possibly the best suited for cements requiring low-energy manufacture.