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By virtue of its ultra-high strength, toughness and specific surface area, graphene oxide (GO) has been a research hotspot in nano-cement because of its excellent electrical, thermal and mechanical properties. The GO-cement composite owns greatly improved microstructure, and mechanical and permeability-related performances compared with plain cement. Clarifying the reinforcing mechanisms of GO on cementitious composites is a prerequisite for the large-scale application of GO in the cement industry. Hence, in this work, the effect of the GO nanosheets on the shear properties of the calcium silicate hydrate (C–S–H) composites and the corresponding reinforcing mechanisms were investigated using molecular dynamics (MD) simulations. The simulated results reveal that the orientation angle between GO nanosheets and C–S–H layers is a significant factor influencing the enhancing effects of GO on the shear behaviour of C–S–H/GO composites. As the orientation angle ranges from 30° to 60°, the shear strain would cause bending deformation of GO nanosheets in composites. The bent GO nanosheets then absorb the deformation energy in the system, effectively isolating the transfer of shear-induced force chains. The ultimate shear strength reaches about 861.2 MPa for the orientation angle equalling 30°, increasing 23.5% compared with that of plain C–S–H. The microstructural damage characteristics analysis further indicates that it is crucial to trade off the effective anchorage of GO nanosheets in C–S–H gel and the structural integrity of C–S–H matrix when using GO nanosheets to enhance the shear performance of C–S–H composites. We expect the findings from this work can enhance the understanding of GO-reinforced cementitious composites and assist GO-based cement design in future work. [ABSTRACT FROM AUTHOR]

In underground projects such as nuclear waste disposal facilities, the requirement of controlling the permeability is very important, and common grouting materials are not suitable. Therefore, creating a composite material to be used for the grouting in engineering projects is essential. The present study characterizes the efficiency of several kinds of carbon grouting composites based on nanomaterials, in terms of the hydraulic properties of single fractures in granites using a rock permeability test system. The pore structure and distribution of the carbon nanomaterials in the grouting pastes were investigated using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results reveal that the hydraulic properties of grouting samples can be well described by linear Darcy's law. These grouting composites can decrease the equivalent permeability (Kg) by 31.8–81.0% in different confining pressures. The enhancing effect of the graphene oxide (GO)/multi-walled carbon nanotube (MWCNT)–based ordinary Portland cement (OPC) grouted sample is the best, improving the permeability resistance by 240.3–315.0%. The MIP test proves that mixing carbon nanomaterials will improve the pore characteristics. The GO/MWCNT-OPC grouting presents the optimal decrease in all total intruded volume, average pore diameter and porosity, dropping by 33.8, 40.7 and 22.9%, respectively. Both the average pore diameter and porosity of grouting pastes have zero-intercept linear correlations with the equivalent permeability and can predicate permeability-related properties with considerable accuracy. The SEM images further confirmed that carbon nanomaterials can not only refine the pore structure of grouting materials through the superior properties of the MWCNTs to bridge micro-sized cracks and the interfacial adhesion of GO but also reinforce the flow resistance of the interface between the grouting material and the granite. The findings of this study can help guide the implementation of carbon nanomaterial-OPC grouting in engineering projects. [ABSTRACT FROM AUTHOR]

The superior mechanical properties of multi-walled carbon nanotubes (MWCNTs) and prominent dispersibility of graphene oxide (GO) make their hybrid materials have the potential capability for modifying the transport properties of cementitious composites. GO sheets will produce π–π stacking interactions between MWCNTs which not only assist the dispersion but also inhibit the re-agglomeration of the materials. However, as one type of tough two-dimensional materials, GO sheets would cut down the tube length of MWCNTs at the same time, particularly during the ultrasonication process. Thereby, this study characterizes the influence of GO on the dispersion and hydrodynamic size of the hybrid particles using an ultraviolet–visible spectrophotometer and laser particle-size analyzer, respectively. The transport properties of GO/MWCNTs-OPC pastes containing different ratios of hybrid materials were measured, and their microstructure was investigated by mercury intrusion porosimetry and scanning electron microscopy (SEM). The results of this study confirm that 0.02 wt% GO mixed into 0.04 wt% MWCNTs would balance the dispersion-assisted and nanoparticle cutdown effects well and obtain the optimal decrease in resisting water permeability of the materials, about 55.3%, while excessive GO mixing may deteriorate particle-size and influence reinforcing efficiency. Two analytical models and SEM images reveal the mechanism of transport properties reinforcement; GO/MWCNTs not only play a significant role as nucleating sites in assisting the hydration reaction but also form a net-like distribution in the microstructure of cementitious composites. The findings of this study can guide GO/MWCNTs hybrid composite implementation in the future. [ABSTRACT FROM AUTHOR]