Keywords

1 Introduction

By virtue of superior mechanical properties, ultra-high specific area and abundant oxygen-containing functional groups, graphene oxide (GO) exhibits outstanding performance in optimizing the microstructure [1], improving mechanical properties [2], enhancing permeability resistance [3], increasing durability [4], and improving the thermal and electrical conductivity [5] of cement-based materials through nucleation effects [6] and pore-infilling effects [7]. Furthermore, some latest studies indicate that graphene-based enhancement of cementitious composites could be a cost-effective strategy for decreasing CO2 emissions and energy consumption in practical engineering [8].

The concentration of GO sheets significantly influences the mechanical performance of cement-based grouting materials when they are incorporated into the cementitious material. In the past study, researchers have primarily determined the proportion of GO with the help of experiments and experience [2]. There are few studies on the mechanism of composite enhancement by different ratios of GO. Hence, to better understand the regulatory effect of GO on calcium–silicate–hydrate (C–S–H) gel, it is necessary to understand the effect of different mixing concentrations of GO sheets on the mechanical properties of C–S–H gels. Therefore, in this study, we built a molecular dynamics (MD) model with different ratios of GO nanolayers incorporated into the C–S–H nanocomposite, and performed tensile tests to investigate the effects of the GO content on the reinforcing efficiency of C–S–H composites and the enhancing mechanism.

2 Methods

Following a previous study [9] and using the same model parameters of GO and C–S–H gel, GO/C–S–H nanocomposites were built in which one or two parallel GO nanolayers were embedded in the C–S–H matrix and the spatial orientation of the sheets was aligned with the \(x\) direction (Fig. 1). The whole system was firstly relaxed for 2 ns in the NPT ensemble at 300 K and 0 atmosphere to achieve equilibrium. Next, tensile deformation was applied to the nanocomposite with a stretching rate of 1 m/s. The NPT ensemble was applied in the \(y\) and \(z\) directions during the tensile deformation at 300 K and 0 atmosphere. The tensile process was completed when the tensile strain in the \(x\) direction reached 110%. The C–S–H gel was described by the CLAYFF force field [10] and GO was described by the consistent-valence forcefield [11], with a cutoff radius of 1.5 nm for both Lennard–Jones 12–6 potential and Coulomb electrostatic potential. The intermolecular force between the C–S–H gel and GO nanolayer was described by the Lennard–Jones 12–6 potential and Coulomb electrostatic potential, and the corresponding parameters were derived by the Lorentz–Berthelot combining rules. The time step was set as 0.5 fs and the long-range electrostatic interactions were resolved by the particle–particle–particle-mesh method.

Fig. 1
An illustration of an atomic model that represents a G O slash C S H nanocomposite, composed of two layers of graphene oxide nanosheets. It highlights the presence of two G O nanolayers.

Atomic model of GO/C–S–H nanocomposite with two GO nanolayers. The model with only one GO nanolayer can be found in Fig. 3a and is not shown here

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3 Results and Discussion

3.1 Tensile Mechanical Properties of the GO/C–S–H Model

The ultimate tensile strength of the MD system did not change significantly after embedding different numbers of GO nanosheets, and the tensile stress–strain curves are presented in Fig. 2. Similar to the visible test results [2], when the MD model reached its peak strength, some damage was generated inside the C–S–H model, but the C–S–H model still had a certain amount of residual strength until its complete failure after further stretching. The monolayer GO/C–S–H model underwent damage at a tensile strain of 0.5. By contrast, the tensile ductility of the bilayer GO/C–S–H model was significantly enhanced to 0.9, with an enhancement ratio of 80%.

Fig. 2
A graph plots stress in the x-direction versus strain. It gives values for 1 and 2 layers. The data points vary for both layers which increase first and then decrease in different directions.

Mechanical responses of GO/C–S–H models with different GO content under stretching

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3.2 Failure Process of GO/C–S–H Models Under Stretching

In order to reveal the enhancement mechanism of GO incorporated into C–S–H, typical atomic model images (Fig. 3) during stretching were selected to display the microdamage evolution of the C–S–H models with different GO layers embedded. Figure 3 shows that the deformation of the GO/C–S–H model can be divided into the following stages: structural loosening stage, pore development stage, and failure stage.

Fig. 3
Two sets of images labeled as a and b. a and b have four images each. a depicts a C S H model with visible microdamage, represented by cracks or fractures, which evolve over time. b showcases two stacked C S H layers with visible microdamage, represented by cracks or fractures.

Microdamage evolution of C–S–H models with different GO content: a single layer and b two layers

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In the monolayer GO/C–S–H model, the C–S–H structure loosened as the tensile strain reached 0.2. As the tensile strain increased to 0.5, noticeable pores began to appear in the structure, but there was still a certain amount of bridging C–S–H, so the model was still a monolithic structure. As the tensile strain continually increased to 0.6, the C–S–H completely separated and fractured, consistent with Fig. 2. At the same time, the molecular structure of the bilayer GO/C–S–H model slowly started to be loosen. Pores gradually appeared and expanded when the strain increased from 0.6 to 1.1. At this time, the C–S–H gel still had residual strength and ability to resist tensile deformation. In addition, it was not hard to find that with increasing deformation, the molecular structure around the GO nanosheets became more closely arranged, especially the C–S–H molecules sandwiched between the two GO layers. It can be concluded that the ductility of the C–S–H was greatly improved with the addition of two GO nanolayers compared with the intercalation of a single layer of GO. Our results also proved that an appropriate amount of GO can effectively enhance the stretching mechanical performance of cement-based materials.

4 Conclusions

The effects of incorporating different numbers of layers of GO nanosheets on the tensile performance of C–S–H gels were studied by MD simulations, the microdamage evolution of GO/S–H models in the tensile process were investigated, and the enhancement mechanism of GO on the tensile properties of hydrated calcium silicate composites was analyzed at the atomic scale. Our main conclusions are listed below.

  1. (1)

    Incorporating an appropriate amount of GO was able to effectively improve the ductility of hydrated calcium silicate composites. MD simulations showed that the bilayer GO/C–S–H model increased the post-peak plastic strain by 80% compared with a monolayer GO/C–S–H model.

  2. (2)

    The deformation of the GO/C–S–H model can be divided into structural loosening stage, pore development stage, and failure stage. Compared with the monolayer GO/S–H model, the strain values corresponding to the bilayer GO model entering the third stage were delayed.

  3. (3)

    At the atomic scale, the structure of C–S–H around the GO nanosheets was more compact, showing a higher ability to resist tensile deformation, and also reflected the enhancement effect of an appropriate amount of GO on the mechanical properties of C–S–H composites.