Mitigating drying shrinkage and enhancing mechanical strength of fly ash-based geopolymer paste with functionalized MWCNTs grafted with silane coupling agent

In this study, the functionalized MWCNTs grafted with silane coupling agent are obtained by means of successive chemical modification methods. The structure and dispersity of the functionalized MWCNTs are characterized using the FT-IR, XPS and HR-TEM, respectively. Then the reaction kinetics, mineralogical phases, morphology and elemental components of modified geopolymer incorporated with the functionalized MWCNTs are evaluated by means of the XRD, TAM, FT-IR and SEM-EDS analysis. In addition, the influences of functionalized MWCNTs on the mechanical strength and drying shrinkage behaviors are investigated. The MIP and 29Si NMR are employed to investigate the pore size distribution and microstructural rearrangement of gel products. The results show that the functionalized MWCNTs can be dispersed uniformly in aqueous solution without any dispersant. Importantly, the modified geopolymer incorporated with 0.2% functionalized MWCNTs-KH550 exhibits a lower drying shrinkage(0.098%) and higher mechanical strength compared with that of all geopolymer pastes. The hydrophobic modification of functionalized MWCNTs reduces significantly the mass loss of the geopolymer. The MIP results indicate that a critical pore size(ds)computed from both the pore size distributions and water loss is related with drying shrinkage, and the drying shrinkage of geopolymer pastes decreases with the increment of the ds value. A strong interaction between adjacent C(N)-A-S-H particles occurs in microstructure rearrangement period by means of the silicate polymerization and micropore closure. Moreover, the new dual interaction mechanism decreasing drying shrinkage and increasing mechanical strength of geopolymer pastes are proposed.