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AbstractThe present study compares the mechanical behavior, failure mode and microstructure of sandstone with a single infilling fracture grouted by common cement and multi-walled carbon nanotubes (MWCNTs) based cement. The mechanical properties of the samples were tested under the MTS816 testing system with the assistance of the acoustic emission system and a digital video camera. The microstructure of the grouting materials was observed using scanning electron microscopy (SEM). The results indicate that the incorporation of MWCNTs into grouting reinforces the mechanical behavior of samples at an inclination angle (α) ranging from 15° to 75°. The highest reinforcing effect occurs at α = 30° for improving the compressive strength (147.8%) and at α = 15° for enhancing the elastic modulus (99.4%). The samples show three main failure modes at different inclination angles. The addition of MWCNTs reinforces the bonding strength and shearing resistance of the grouting surface, compared with the reference group, the infilled specimen presents more fracture occurring in the final rupture mode particularly at α = 15° and 30°. SEM images further illustrate that the reinforcing effects mainly result from the crack bridging and pulled-out distribution of MWCNTs in the grouting.

Nano cement additive using a hybrid of graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) combines the excellent affinity of GO and the superior mechanical properties of MWCNTs. Ultrasonication is the key process to disperse the GO/MWCNTs and further optimizes the pore structures of cement-based pastes. Fractal dimension can effectively and quantitatively characterize the pore structures of cementitious composites. The present study investigates the fractal dimensions of pore structures of GO/MWCNT-OPC pastes under power- and time-controlled ultrasonication based on the mercury intrusion porosimetry (MIP) tests data. The finding of this study shows that comparing to calculating the fractal dimension of the overall pore size range, assessing the variations of fractal dimension of individual pore size range is more effective in evaluating the pore characteristic. The fractal dimension of larger capillary pores D>104nm$$\left( {{D}_{>{{10}^{4}}nm}} \right)$$can be use to describe the change of pore structure of GO/MWCNT-OPC pastes under ultrasonication treatment with sufficient accuracy as higher value of D>104nm$${{D}_{>{{10}^{4}}nm}}$$indicates better pore characteristics. The fractal dimension change trend of mesopores is always opposite to that of bigger capillary pores. Modest increment in both power- and time-controlled ultrasonication seems to result in the increase of the fractal dimension of capillary pores and lead to better reinforcement effects. Prolongation of ultrasonication time slightly influences the pore structure of the specimens, while nano cement additives exposed to excess ultrasonication power fail to afford adequate reinforcing effect and finally cause the deterioration of the pore structures. The findings of this study can provide helpful information of GO/MWCNT-OPC pastes and ultrasonication treatment in the future.