Abstract
One-step combined modification using graphene and polyvinyl alcohol(PVA)to improve mechanical properties of cementbased materials and overcome limitations of other modification methods by polymer and nanomaterials is presented in this paper.The complementary and synergistic ef fect of graphene and PVA can solve key problems such as high cost,low production of graphene and poor dispersion of graphene in cement-based material.
An important advantage of PVA is that it functions as a stabilizer,preventing graphene aggregation by its adsorption on graphene surfaces.Figure 1 shows pictures of a dispersion that is freshly prepared and allowed to sit for 6 h,12 h,7 d,14 d and 28 d,respectively.As desired,no difference can be found indicating no agglomeration even after long durations.The concentration of graphene in the resulting PVA dispersion was measured using absorbance,vacuum filtration and TGA measurements.Figure 1(a)presents the measured absorbance relative to the concentration of graphene/PVA dispersions.The linear relationship also suggests the stability of graphene dispersion in PVA.The calculated graphene concentration is consistent with the results obtained from vacuum filtration measurement and TGA test.The proposed method of graphene/PVA aqueous dispersions by shear exfoliation has high stability and can directly substitute water in cement casting.The TEM image of the prepared graphene free from PVA is shown in Figure 1(b).
Figure 1 Absorbance relative to the concentration and TEM image of the prepared graphene
A series of MD simulations were performed to provide complimentary information at a molecular level on the role that PVA plays in stabilizing graphene dispersion.Firstly,adsorption of a single chain of PVA(N=20)from the aqueous phase onto graphene sheet was simulated.The graphene sheet was modeled as a square in the x-y plane with a lateral length of 2.8 nm2.Geometry optimization was first performed to relax the system to a local energy minimum.Afterwards,the position of the graphene sheet was fixed and dynamic simulation was performed until reaching equilibrium.Figure 2(a)displays the time evolution of the distance between the center-of-mass of the PVA chain and the graphene sheet.This distance is slightly larger than the interlayer distance of graphite(0.34 nm),which means graphene can be stabilized by PVA in water without aggregation.Figure 2(b),(c)show snapshots of PVA absorption onto graphene surface,which reflects the time evolution of PVA motion towards the graphene surface.Initially,the PVA chain and the graphene sheet are separately solvated in water.PVA chain gradually approaches the graphene surface and attaches itself to the graphene surface.At 2 ns,the PVA chain is completely absorbed on the graphene surface.These simulation results show that even though graphene is hydrophobic,water does not hinder PVA absorbance onto graphene.In the second step of MD simulation,the possibility of graphene aggregation after shear mixing was considered.As shown in Figure 2(e),(f),the two graphene sheets that were initially separated would aggregate spontaneously in water.This result is consistent with observations of poor graphene dispersion in water[1,2].From the time evolution of center-of-mass distance between the top and bottom graphene sheets(Figure 2(d)),the distance between the two graphene sheets varies little with time after 0.8 ns,indicating an equilibrium state.The equilibrium distance between the two graphene sheets was 0.34 nm,which is the interlayer distance of graphite.A model with PVA chain(N=20)placed between two graphene sheets was built.The time evolution of center-ofmass distance of the PVA chain from the top graphene sheet and between the top graphene sheet to the bottom graphene sheet is shown in Figure 2(g),with Figure 2(h),(i)showing snapshots at 0 ns and 2 ns.The two distances remain stable(Figure 2(g)),indicating a geometric equilibrium of the system.The equilibrium distance between the two graphene sheets is 0.75 nm,while the distance from PVA to the bottom graphene sheet is 0.37 nm.It can be observed that due to presence of the PVA chain,the top graphene sheet cannot attach to the bottom graphene sheet(Figure 2(h),(i)).This result confirms that the adsorption of PVA onto the graphene surface can prevent aggregation of graphene in water[3].
Figure 2 MD simulations of PVA stabilized graphene dispersion
The properties of hydrated cement depend on the characteristics of its microstructure.On the fracture surface of plain cement(Figure 3(a)),needleshaped crystals and flocculation structures can be seen,representing cement hydration products ettringite and calcium silicate hydrate(C—S—H)gel,respectively[4,5].Distinctly different morphology(Figure 3(b))can be observed when graphene was introduced into the cement.The size and shape of the pores of well-dispersed cement and the crystalline hydration products were completely dif ferent.With graphene in cement,clusters of rod-like ettringite crystals and granular C—S—H can be observed.The shape of ettringite changed from a needle to a rod,indicating that the incorporation of graphene into cement promoted the hydration process.With the assistance of PVA,graphene dispersed uniformly in cement paste and functioned as a nucleation-inducing agent[6].In the cement hydration stage,as the amount of water decreased,PVA was gradually restricted in capillary pores,forming a gel membrane on the surface of the hydrated cement[7].The membrane wrapped the hydrated cement gel,the unhydrated cement particles and graphene together to form a three-dimensional interpenetrating network structure,thereby enhancing the mechanical properties of the cement,especially its flexural strength.
Figure 3(c)compare compressive and flexural strengths of cement pastes with different graphene concentrations.Compared to cement with 1.8% PVA by weight,the modified cement with graphene and PVA has increases of 71.1% and 106.5%in compressive and flexural strength,respectively.The corresponding values are-4% and 36.2% respectively when the cement is modified by 1.8%PVA only.This strongly demonstrates the synergistic effects of graphene/PVA in improving cement properties.
Figure 3 SEM images and mechanical properties of graphene/PVA synergistically-modified cement-based materials
The graphene processing approach of this paper provides a new method of synergistic modification of cement-based materials for practical application of graphene in mass produced concrete in large scale projects.