The role of multi-walled carbon nanotubes for improving C-S-H creep property: An insight from atomic force microscope and nuclear magnetic resonance

Abstract

The effect of MWCNTs on the creep properties of C-S-H was investigated based on nanoindentation test, and the detailed mechanism analysis was carried out by atomic force microscope (AFM) and nuclear magnetic resonance (NMR)in this paper. The Portland cement and multi-walled carbon nanotubes (MWCNTs) grafted with COOH were used in the experiments. The water to cement ratio was 0.3 and the content of MWCNTs was 0.05 wt% of cement. The results of nanoindentation test showed that the creep modulus of low density C-S-H and high density C-S-H of the sample doped with MWCNTs (CC) was 39.2% and 21% higher than that of the control sample (PC), respectively, indicating the creep deformation of C-S-H was significantly reduced by MWCNTs. Foil shape in CC was observed, and the elastic modulus and Hamaker constant of C-S-H at submicron-scale were higher than those of PC measured by AFM. The results of mercury intrusion porosimetry test showed that the porosity and most probable pore size of CC decreased compared with PC. The mean chain length increased for CC measured by 29Si NMR. And the low-filed 1H NMR results showed that the proportion of interlayer water was almost not changed with the incorporation of MWCNTs, suggesting that the influence of interlayer water on C-S-H creep properties can be ignored. Based on the experimental results, the mechanism was proposed to explain the decrease of C-S-H creep deformation by MWCNTs: (1) foil shape C-S-H was not easy to slip; (2) less free volume available for globules to slip;(3) the stronger force existed between globules; (4) the network structure was formed in C-S-H due to the presence of MWCNTs and the elastic modulus of C-S-H increased, resulting in more resistance for deformation.