Electron energy loss spectroscopy of nanoscale local structures in calcium silicate hydrate

Calcium silicate hydrate (C-S-H) is a crucial cement hydration product for the strength and durability of concrete. While previous studies have extensively investigated the structural and compositional characteristics of C-S-H, they mostly focused on average properties within ensemble systems. In this work, we use electron energy loss spectroscopy (EELS), electron nano-tomography, and other spectroscopies to study the local structure of C-S-H at an unprecedented spatial resolution of 5 nm. The chemical environments of silicon (Si) and calcium (Ca) elements, thickness, and dielectric properties are scrutinized. Statistical analysis of over 10,000 data points reveals significant heterogeneity in the silicate chemical environment, including different polymerization degrees and tetrahedral distortions. In contrast, the local Ca environment exhibits more homogeneity with a coordination number ranging from 7 to 9, indicating a weak octahedral-like symmetry for C-S-H. Additionally, our findings show that the local thickness of C-S-H predominantly hovers around ∼15 nm consisting of 13–14 layers, validated through electron tomography. This work provides insights into the local structural features of C-S-H from the single colloid perspective and thus facilitates the future development of more realistic C-S-H models.