New insights into the improvement of volume stability: Plant polyphenol modified calcium silicate hydrate (C-S-H)

Abstract

The internal water migration within calcium silicate hydrate (C-S-H) in dry environments is considered to be the primary factor affecting the volume stability of cementitious materials. In this research, the in-situ polymerization product of plant polyphenol tannic acid (TA) was applied to modify C-S-H based on an organic-inorganic composite modification method. The chemical structure, microstructure, composition, dimensional changes and water migration characteristics of modified C-S-H were analyzed. Experimental results showed that TA improved the polymerization degree of siloxane chains in the C-S-H nanostructure, with a maximum improvement of 80.70%, and increased the interlayer spacing in the C-S-H structure, confirming the modification of C-S-H at the nanoscale, exhibited by ²⁹Si Nuclear Magnetic Resonance (NMR), X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). This modification by TA optimized the C-S-H nanostructure and microscopic pores, and increased the interlayer and gel pore water content, especially gel pore water increased by 56.53% compared to that of the unmodified C-S-H, revealed by nitrogen adsorption and ¹H LF-NMR. In the dry environment, the blocking and cladding effects of TA on C-S-H effectively inhibited the gel water loss and reduced the drying shrinkage, especially at low TA concentration. This research aims to improve the volume stability of C-S-H by in-situ polymerization product of plant polyphenol, which provides new insights into improving the volume stability of cementitious materials.