Visualize cement hydration at molecular level – Could enable 3D printing of concrete
The imaging technique could open up new avenues for reducing the heavy carbon footprint of concrete, as well as for 3D printing of concrete.
The concrete world around us owes its shape and durability to the chemical reactions that begin when ordinary Portland cement is mixed with water. Now, MIT scientists have demonstrated a way to observe these reactions under real conditions, a breakthrough that could help researchers find ways to make concrete more durable.
The study is a “Light Brothers moment for concrete science,” says co-author Franz-Josef Ulm, professor of civil and environmental engineering and faculty director of the MIT Concrete Sustainability Hub, referring to the two brothers who have inaugurated the era of the cinema project. Likewise, says Ulm, the MIT team provided insight into early-stage cement hydration that looks cinematic in Technicolor compared to black-and-white photos from previous research.
The cement in concrete contributes about 8 percent of total carbon dioxide emissions worldwide, rivaling the emissions produced by most individual countries. With a better understanding of the chemistry of cement, scientists could potentially “modify production or change ingredients so that concrete has less impact on emissions, or add ingredients capable of actively absorbing carbon dioxide”, explains Admir Masic, associate professor of civil sciences and environmental engineering.
Next-generation technologies such as 3D concrete printing could also benefit from the study’s new imaging technique, which shows how cement hydrates and hardens in place, says Student Hyun-Chae Chad Loh a graduate of Masic Lab, who also works as a materials scientist with the company. Black Buffalo 3D Corporation.
Loh is the first author of the study published in ACS Langmuir, joining Ulm, Masic and postdoctoral fellow Hee-Jeong Rachel Kim.
Cement from the start
Loh and his colleagues used a technique called Raman microspectroscopy to take a closer look at the specific and dynamic chemical reactions that occur when water and cement mix together. Raman spectroscopy creates images by projecting high intensity laser light onto the material and measuring the intensities and wavelengths of the light as it is scattered by the molecules that make up the material.
Different molecules and molecular bonds have their own diffusing “fingerprints”, so the technique can be used to create chemical images of molecular structures and dynamic chemical reactions inside a material. Raman spectroscopy is often used to characterize biological and archaeological materials, as Masic did in previous studies of mother-of-pearl and other biomineralized materials and ancient Roman concretes.
Using Raman microspectroscopy, scientists at MIT observed a sample of ordinary Portland cement placed underwater without disturbing it or artificially stopping the hydration process, mimicking the actual conditions of concrete use. Typically, one of the hydration products, called portlandite, starts off as a messy phase, seeps through the material, and then crystallizes, the research team concluded.
Before that, “scientists could only study cement hydration with medium bulk properties or with a snapshot of a given time,” Loh explains, “but it allowed us to observe all the changes almost in continuous and improve the resolution of our image in space and time. “
For example, hydrated calcium silicate, or CSH, is the main cement binding ingredient that holds concrete together, “but it’s very difficult to detect due to its amorphous nature,” says Loh. “Seeing its structure, distribution and how it developed during the curing process was something amazing to watch. “
Ulm says the work will guide researchers as they experiment with new additives and other methods to reduce greenhouse gas emissions from concrete: “Rather than ‘fish in the dark’, we are now in a position to to rationalize through this new approach how reactions occur or not. occur and intervene chemically.
The team will use Raman spectroscopy over the summer to test how well different cementitious materials capture carbon dioxide, Masic said. “Until now, it was almost impossible to track this, but now we have the ability to track the carbonation of cementitious materials, which helps us understand where carbon dioxide goes, what phases are formed and how to change them in order to potentially using concrete as a carbon sink.
Imaging is also essential for Loh’s work with 3D concrete printing, which depends on the extrusion of layers of concrete in a precisely measured and coordinated process, in which the liquid suspension turns into solid concrete.
“Knowing when the concrete is going to set is the most critical question everyone is trying to figure out” in the industry, he says. “We do a lot of trial and error to optimize a design. But monitoring the underlying chemistry in space and time is essential, and this scientific innovation will impact the concrete printing capabilities of the construction industry. “
Reference: “Time-Space-Resolved Chemical Deconvolution of Cementitious Colloidal Systems Using Raman Spectroscopy” by Hyun-Chae Loh, Hee-Jeong Kim, Franz-Josef Ulm and Admir Masic, June 7, 2021, Langmuir.
DOI: 10.1021 / acs.langmuir.1c00609
This work was partially supported by the scholarship program of the Kwanjeong Educational Foundation.