By Debbie Sniderman
If concrete is known for anything, it’s toughness. Concrete has been used for centuries in applications where strength and durability are imperative, so what can the concrete industry learn from glass? Researchers at MIT’s Concrete Sustainability Hub found that the cement industry may be able to benefit from improvements made in the glass industry. The new Gorilla Cements are enhanced by the same atomic theory that Corning used to develop Gorilla Glass, offering improved strength and lower carbon dioxide emissions.
Change motivated by industry
Concrete is the most consumed synthetic material on Earth. It’s the backbone material our societies need for housing, shelter, hospitals, infrastructure and more. But Franz-Joseph Ulm, professor at MIT and faculty director of the CSHub, said concrete’s fundamental unit, its DNA, has not been explored to the same extent as other high-tech materials. “This provided enough motivation for us to dive into the mature field of concrete sciences and engineering with a new approach based on molecular sciences and glass physics,” he said. “We wanted to see whether it was possible to nano-engineer the material to a point that one could reduce the environmental footprint without compromising concrete’s performance.”
Fellow researcher Roland Pellenq, a senior research scientist at MIT and CSHub, adds the project was originally driven by industry demand to do the same or more with less cement. Achieving a better-performing cement can change the industry. A more resistant and durable cement will last longer and require less maintenance, providing lower cost options.
Environmental implications
Clinker is by far the largest contributor of CO2 in cement paste. When properly calibrated, a more technical ready-mix or “smart” cement could be produced that uses less clinker. Less clinker reduces CO2 emissions.
CSHub researchers screened the chemistries of available dry cement clinker powders, measuring how much calcium and silicon were present in the calcium-silicate-hydrates – the binding phase of cement. They developed an atomistic numerical model for the Ca/Si ratio for each. When mixed with water, cement hydrate has a Ca/Si ratio ranging from 1.0 to 2.0, with typical ordinary Portland cement at 1.7.
At first they studied the mechanical properties of cement with a Ca/Si ratio of 1.7 but expanded their measurements to probe the entire range of possible cement paste chemistries. By looking at 150 molecular structures of cement hydrates, they found the fracture toughness of samples at Ca/Si 1.5 was twice that of normal cement paste at 1.7, and found a family of compositions with improved toughness.
Gorilla Glass designers at Corning used the same methods, screening silica glasses according to composition and using rigidity theory to understand whether residual stresses were present in the network. In cement, calcium and silicon content was measured in a ratio. Rigidity theory says that at 1.5, the cement is isostatic, which means it has no residual stress. Glass or cement that is not under stress will age well, won’t degrade with time, and in the end, be more durable.
Picoliter-volume droplets allow visualization.
(Dr. Romain Grossier, CSHub@MIT)
Cement’s evolution
CSHub researchers looked at today’s cement and asked, “Why not use the same rigidity theory that was developed for smartphones?”
“Cement is a colloidal system, so the physics of glasses and soft matter has important ideas that we can use to help understand cement from the nanoscale and provide a basis for better engineering opportunities for cement,” Pellenq said. “As far as we know, this is the first time that atomistic physics principles have been applied to cement. That’s how we found this improved cement composition – by looking in from the outside.”
Clinker powder is mixed with water at room temperature to create a paste that will solidify into rock. Instead of changing the clinker powder, CSHub discovered industry additives containing silica could be added to the paste that will produce a secondary reaction and increase the amount of silica, shifting the Ca/Si ratio to 1.5. Silica additives are frequently used in the oil industry to produce cement wells 2 km tall that need high stability. “In those cases, silica is used to decrease the Ca/Si ratio to 1.0 for better rheology and flow,” Pellenq said. “No one has ever tried to produce cement with a Ca/Si ratio of 1.5. Different types of silica, grain sizes and crystalline cores may react differently. We need to understand the degrees of freedom that exist to achieve this.”
Prime for the precast industry
The research looked at the specific strength, or strength per ton, of material. Using fine-tailored calcium-silicate-hydrate binders would allow more structural performance with less material. Ulm said the precast industry, with its highly controlled material quality assurance systems, is best situated to use and further develop these next-generation cement-binder materials.
“The precast industry has a long tradition of moving forward with material innovations, such as the rapid adaptation of self-compacting concrete or the employment of high-strength and ultra-high strength materials by the industry at large,” Ulm said. “Thanks to this innovation culture and the quality control systems well in place, the precast industry is well placed to make this transfer of research results from the lab to the plant. The transfer barriers are certainly lower than in classical on-site concrete applications.”
According to Ulm, these stronger, more durable materials could be used in smaller niche applications such as city furniture, and in larger applications such as concrete pipe, roof beams, or in precast shelter structures that could be implemented in homes in tornado and hurricane areas, enhancing their resiliency. Precast solutions could also be developed for enhancing sustainability and resilience of structures.
This cement also offers another advantage to the precast industry. “In current ductile precast forms, the strength can be improved by a factor of 2, controlling the temperature of the initial reaction between the clinker and the water,” Pallenq said. “It may be possible to produce objects, systems and end products that are more resistant using this cement by directly addressing the paste. Improving the paste’s strength by changing its composition is a new degree of freedom.”
Crystals of portlandite seen in the scanning electron microscope, secondary electrons mode. Research subject – improving properties of cement-based materials exposed to extreme conditions.
(Dr. Konrad J. Krakowiak, CSHub@MIT)
What’s ahead for the technology
At this point, the cement research is in a pre-competitive stage. The technology is shared with the entire industry, from cement producers to concrete manufacturers, architects, engineers and contractors.
“We want to reach all sectors of the industry,” Ulm said. He thinks that at this stage of technology development and transfer, a specific task group on precast applications could be assembled that represents a sample of the industry, from large to small.
“Together, this task group could develop showcase examples, identify the low-hanging fruits and the more advanced applications before the R&D goes in-house, which certainly will need IP-production (intellectual property production),” he said.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company. She can be reached at [email protected].
Resources
CSHub@MIT MIT Concrete Sustainability Hub
http://cshub.mit.edu/
MIT Research Brief: Gorilla Cement, Tougher Yet Greener
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