Effect Investigation of Nanofillers on C-S-H Gel Structure with Si NMR

This paper aims to use ${}^{29}\mathrm{Si}$ nuclear magnetic resonance (NMR) for investigating the effect of nanofillers on the C-S-H gel structure of hardened cement paste, analyze the modification mechanism of nanofillers from a microscopic perspective, and then provide fundamentals for controlling the macroscopic properties of cement-based materials. Different curing temperatures (25°C and 90°C) and nanofiller types [including $\mathrm{nano}\text{-}{\mathrm{SiO}}_2$, $\mathrm{nano}\text{-}{\mathrm{ZrO}}_2$, $\mathrm{nano}\text{-}{\mathrm{TiO}}_2$, carbon nanotubes (CNTs), nano-boron nitride (nano-BN), and multilayer graphenes (MLGs)] are considered as the influencing factors. Three characterization parameters [polymerization degree, mean molecular chain length (MCL), and hydration degree] are calculated and used as the evaluation indexes. Experimental results show that the incorporation of most types of nanofillers can cause increases in all of these three parameters. With the increase of curing temperature, the polymerization degree of C-S-H gel is further enhanced, because the layered or even spatial network structures of silicate tetrahedron occur in the C-S-H gel. The composites with MLGs present the maximum increase values of polymerization degree, MCL, and hydration degree, by 786.2%, 166.5%, and 27.4% compared with control cement paste, respectively, which is mainly attributed to the nanofiller morphology (unique two-dimensional stacked flake structure) and the functional group (carboxyl-functionalized) on the surface of nanofillers. Fluctuations of polymerization degree and MCL appear in composites with $\mathrm{nano}\text{-}{\mathrm{TiO}}_2$ and with CNTs, relative to the particle sizes, crystal phases, and surface modification of nanofillers. The effect mechanisms of nanofillers on C-S-H gel can be attributed to two main aspects: (1) the nucleation effect and pozzolanic effect (just for $\mathrm{nano}\text{-}{\mathrm{SiO}}_2$) of nanofillers facilitate the cement hydration; (2) the high water absorption capability of nanofillers reduces the proton water inside C-S-H gel, and shortens the distance between the structural groups of Ca, O, and Si atoms. The chemical bonds (ionic bonds and covalent bonds) between these groups are enhanced and the values of polymerization degree and MCL are therefore increased.