Evaluation of the nanostructure of calcium silicate hydrate based on atomic force microscopy-infrared spectroscopy experiments

Calcium silicate hydrate (C–S–H) is the main product of cement hydration, which forms the microstructure of cement via the stacking of basic nanocrystals or gel units, and has a substantial influence on the mechanical performance of cement. Tetrahedron chains of silicon oxide form the main nanoscale structure of basic C–S–H units. Evaluation on the nanostructure of these tetrahedron chains facilitates to understand the source of cement strength. This article first introduced the atomic force microscopy-infrared spectroscopy (AFM-IR) technique into evaluating the nanostructure of C–S–H. The nano infrared spectroscopy of stacking C–S–H nanograins and tetrahedron spatial distribution mapping was obtained. The results demonstrate that the relative quantity of tobermorite-like and jennite-like units in C–S–H nanograins can be analyzed by AFM-IR. The stacking between C–S–H particles is facilitated to a large extent by silicate ( SiO 4 2 − {\text{SiO}}_{4}^{2-} ) tetrahedron chains formed of three tetrahedrons bridged by two oxygen atoms (i.e., Q2 chains), and there are Q2 chains acting as bridges between C–S–H particles. The proportions of different types of Q2 chains available for facilitating C–S–H particle stacking vary at the nanoscale. AFM-IR spatial mapping demonstrate that the orientations of these Q2 chains are not evenly distributed. These findings provide experimental information of the stacking C–S–H gaps.