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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]

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]

By virtue of its low cost, high workability, wide availability and suitability for large-volume fabrication, cement-based grouting is the most applied material in underground projects. However, due to the weak tensile strength and high-frequency impact dynamic load disturbance in underground engineering, cement-based grouting is vulnerable to tensile damage, seriously influencing the stability and safety of underground projects. Hence, in this work, the graphene oxide (GO) coated fly ash was adopted to reinforce the dynamic tensile behaviour of the cement-based matrixes to fabricate high-performance, cost-effective and environmentally friendly cementitious composites. The test results show that the GO-coated fly ash could significantly enhance the dynamic tensile performance of the cementitious composites. With only 0.08 wt% GO mixing, the dynamic tensile properties can be strengthened by 19.5–88.5%. After GO nanosheets accelerate the cement hydration process and improve the chemical composition of hydration products, GO nanosheets exhibit macroscopic nucleation and "crack-bridging" effects on the cement matrixes, and the cracks propagation of the hardened specimens would be suppressed under dynamic loading, thus enhancing the tensile resistance of the specimen. The fractal dimension calculation results prove that the GO-coated fly ash-modified cementitious composites have higher integrity after tensile impact, about 2.8–11.9% higher than plain cement-based slurry. Finally, the theoretical analysis reveals that GO nanosheets can enhance the tensile behaviours of the cement matrix by optimizing the dynamic load coefficient. Nevertheless, with the crack propagation, the reinforcing efficiency of GO would be gradually weakened. The findings of this study can promote an understanding of the enhancing mechanisms of GO on the dynamic tensile behaviour of cementitious composites and inspire the potential application of GO-coated fly ash to reduce cement usage in grouting engineering. ● GO-coated fly ash can significantly enhance the dynamic tensile performance of cementitious composites. ● GO nanosheets can suppress the cracks propagation of the cement under dynamic loading. ● The GO-coated fly ash-modified cementitious composites have high integrity after tensile impact. ● One theoretical model of GO in reinforcing cementitious composites under dynamic tensile loading was proposed. [ABSTRACT FROM AUTHOR]

• GO/MWCNTs enhance impermeability and refine pore structures of cement pastes. • High W/C ratios hinder the reinforcing effects of GO/MWCNTs in cement pastes. • There is a linear correlation between fractal dimensions and permeability. • Analytical models and SEM unveil the mechanism of impermeability enhancement. The prominent dispersibility of graphene oxide (GO) and superior mechanical properties of multi-walled carbon nanotubes (MWCNTs) make them potential materials for reinforcing cementitious composites. The aims of this study were to investigate the effectiveness and mechanisms of GO/MWCNT admixtures on the impermeability of cementitious composites. Comparative tests concerning the transport properties between plain cement pastes and GO/MWCNT-OPC pastes at different water-to-cement (W/C) ratios were conducted in this study. Mercury intrusion porosimetry (MIP) tests were carried out to characterize the pore structures of the cement pastes, and the fractal dimensions were utilized to exploit the relationship between the pore structures and permeability-related properties. Power's model, general effective media (GEM) theory, and scanning electron microscopy (SEM) images were used to illustrate the mechanism of pore structure enhancement. The results imply that mixing GO/MWCNTs into cementitious composites enhances the impermeability of hardened pastes, and the reinforcing efficiency rate in impermeability decreases linearly with the increment of W/C ratio. The fractal dimensions based on the MIP data characterized the pore structures and predicted the transport properties with considerable accuracy. Furthermore, the role of GO/MWCNTs as nucleating sites for hydration reaction and net-like distribution in the composites may explain the impermeability enhancement mechanism induced by the additional GO/MWCNTs. However, GO/MWCNTs tend to re-agglomerate due to the longer setting times at high W/C ratios and lead to a nonuniform distribution of hydration products, which is responsible for the incompetent reinforcing effects. [ABSTRACT FROM AUTHOR]

• CWRB is an essential composite in supporting technology for safe and green production in mining. • GO is promising for CWRB composite reinforcement and solid waste recycling. • The reinforcing effects of GO on CWRB is more pronounced under the low Talbot index. • GO-FA hybrid assists CWRB in creating a high-performance material for backfill. Cemented waste rock backfill (CWRB) is an important composite in supporting technology for safe and green production in mining. However, the mechanical performance of CWRB is highly correlated to cement consumption. This study utilized industrial graphene oxide (GO) and fly ash (FA) to create a high-performance, low-cost, and environment-friendly material for backfill. The results show that the GO-FA hybrid improves the microstructural matrix of hydrated cement by generating nucleation effects that refine the microstructure to a denser state and promote pore-filling effects that form barriers at pore throat junctions. This significantly reinforced the compressive strength (up to 53.6%) of the CWRB specimens. The reinforcing efficiency is more pronounced when using fine particle size distributions due to the lack of skeleton effect of gangue aggregates. By monitoring the acoustic emission and analyzing fractal properties, it can be further revealed that GO improves the integrity of CWRB material during loading. Compared to traditional backfill materials, the study found that 0.008 wt% industrial GO can assist FA to lower the consumption of cement by over 20% and improve the engineering properties by 2.6–53.6% at the same time. This research further proves that GO is promising for CWRB composites reinforcement and solid waste recycling. [ABSTRACT FROM AUTHOR]

• IGO can significantly reinforce the tensile behavior of CWRB. • The low W/C ratio benefits the enhancing effects of IGO on CWRB's tensile properties. • GO nanosheets improve the ductility of cementitious nanocomposites. • IGO strengthens the ITZ of CWRB through nucleation and pore-filling effects. Cemented waste rock backfill (CWRB) is an important supporting technology for underground engineering safety and waste recycling. However, the brittle nature of cementitious materials, particularly weak tensile behavior, always compromises the performance of the CWRB. In this study, the industrial graphene oxide (IGO) was adopted to reinforce the tensile behavior of the CWRB specimens and be mixed with fly ash to fabricate environmentally friendly, cost-effective, high-performance backfilling materials. The results showed that the tensile strength of the CWRB specimens was significantly improved up to approximately 230%, by only mixing 0.007 wt% IGO. The low water-to-cement (W/C) ratio benefited from the enhancing effects of the IGO on CWRB's tensile performance. Molecular dynamics simulations further revealed the underlying nanoscale reinforcing mechanisms of IGO, indicating that the tensile strength enhancement is mainly due to GO nanosheets effectively improving the ductility of the nanocomposites and delaying the failure of cementitious materials. The microstructure characterization proved that IGO could generate nucleation effects and pore infilling effects to optimize the microstructure in the interfacial transition zone (ITZ) of CWRB. Acoustic emission monitoring and digital photography characterization technologies implied that incorporating IGO could effectively reduce the micro-damage degree of the CWRB specimens during the failure process. This phenomenon ensures the integrity of the specimen during the destabilization failure process and reinforces resistance to the compressive load. This research not only broadens our understanding of GO in enhancing cementitious composites but also inspires the potential application of the IGO for strengthening the performance of cemented rockfill and the sustainability of waste rock recycling. [ABSTRACT FROM AUTHOR]

By virtue of its great dispersibility, ultra-high specific surface area and superior mechanical properties, graphene oxide (GO) has been a potential candidate for nanoengineered concrete. A recent experimental study has provided nanoscale evidence for the improved nanomechanical performance of calcium silicate hydrate (C–S–H) with the addition of GO. The further numerical studies are needed to validated the exiting findings and revealed the underlying nanoscale reinforcing mechanisms. In this study, we investigate the mechanical characteristics of C–S–H/GO nanocomposites under tensile loading at the nanoscale via molecular dynamics simulation and reveal the reinforcing mechanisms of GO in C–S–H gel. The results show that only mixing 9 wt% of GO nanosheets to the C–S–H gel can effectively reinforce the ductility of the C–S–H by 83% and delay the failure of cement-based materials. Ensuring a sufficient proportion of GO is essential for its enhancement effect. Insufficient GO content would cause stress concentration in the C–S–H gel, failing to produce the reinforcement and weakening the performance of the cement-based materials. GO acts as two main roles in C–S–H/GO nanocomposites as bridging roles and embedded roles. The bridging GO can reinforce the ductility of the C–S–H gel under stretching and increase the energy required for the C–S–H/GO nanocomposites to break, while embedded GO may destroy the integrity of the C–S–H and cause adverse effects such as stress concentration. The findings of this work can promote understanding of the reinforcing mechanisms of GO in C–S–H composites and assist in balancing the reinforcing effects and the economic cost of GO application in practical engineering. • The tensile behaviour of C–S–H/GO nanocomposites was studied via MD simulation. • Mixing GO in C–S–H gel can effectively reinforce the ductility of composites. • GO would act as bridging roles or embedded roles during C–S–H breaking. • Ensuring appropriate GO content is significant for its enhancement effect. • The preferred orientation of C–S–H induced by the deviatoric stress is confirmed. [ABSTRACT FROM AUTHOR]