A recent study examined the inner structure of calcium silicate hydrate, a principal binding agent in concrete.
In a recent study published in Cement and Concrete Research, a team of researchers from the University of California, Berkeley, and Lawrence Berkeley National Laboratory have peered into the inner structure of calcium silicate hydrate (C-S-H), the principal binding agent in concrete, with an improved spatial resolution of just 5 nanometers (1). Led by Professors Paulo J. M. Monteiro and Shaofan Li, the research team combined advanced electron microscopy techniques to reveal the complex, heterogeneous structure of C-S-H at the nanoscale. This information is significant because it can help engineers model and improve concrete materials (1).
Portland cement is critical in concrete production. It helps form concrete when it creates a paste with water that binds with the rock and sand to harden (2). C-S-H is the most critical hydration product in Portland cement. It is primarily responsible for concrete’s mechanical strength and durability (1). Although researchers have studied its average properties for decades, much of the existing knowledge relies on broad characterizations that fail to capture the minute structural variations occurring at the nanoscale. This latest research shifts away from traditional ensemble analyses, offering a detailed look at the local atomic environment of individual C-S-H particles (1).
In this study, the research team utilized various high-resolution techniques, including electron energy loss spectroscopy (EELS) and electron nano-tomography, to examine over 10,000 local structural data points across C-S-H samples. EELS and electron nano-tomography allowed the researchers to discover contrasts in the behavior of silicon and calcium atoms of C-S-H (1). By examining C-S-H at the colloidal level with a better level of precision, they were able to learn more about C-S-H that could lead to more accurate models and stronger and more durable cementitious materials (1).
There were several unique findings in this study. The first is that the researchers observed pronounced heterogeneity in the silicate environment within C-S-H. Using Si L₂,₃ energy-loss near-edge structure (ELNES) analysis, the researchers identified variable degrees of polymerization in silicate chains and widespread distortion in the Si–O tetrahedral geometry (1). These irregularities indicate that silicate structures in C-S-H are not uniform. As a result, they exhibit significant local disorder, which can help scientists understand how to better simulate or manipulate the material at the atomic level (1).
Another key finding was that the calcium ions were mostly situated in environments with weak octahedral-like symmetry, featuring coordination numbers ranging from 7 to 9. The uniformity of calcium’s chemical surroundings, when compared to the variability in silicate structures, suggests differing roles for each element in influencing the material’s physical and chemical properties (1). Because calcium ions in C-S-H exhibit a quasi-octahedral coordination, their stability could play a key role in the overall behavior of the material, especially under mechanical or thermal stress (1).
Another unique finding in this study was regarding the average thickness of individual C-S-H colloids. Through 3D electron nano-tomography, the team determined that most C-S-H particles measure approximately 15 nanometers in thickness, corresponding to 13–14 structural layers (1). This measurement helps validate earlier theoretical models and provides a more grounded basis for future simulation work.
And finally, the researchers showed through EELS that the dielectric properties of C-S-H revealed minimal conductivity. The conclusion was that this can be largely attributed to the material’s layered structure, which was composed of calcium-water interlayers and silicate chains with inherent defects (1). Understanding this limited conductivity is essential for predicting how concrete structures perform in environments where electrical or moisture-related degradation might be a concern.
With global demand for more sustainable, durable, and high-performance concrete, a better understanding of C-S-H at the nanoscale could inform the development of novel cement formulations with reduced carbon footprints or enhanced longevity.
