Study On The Preparation And Properties Of Graphene Modified Cement-based Materials

This paper focuses on the study about the preparation and properties of graphene modified cement-based materials.Firstly,the dispersion of graphene is investigated and then the effects of graphene on the thermal properties,temperature difference,early shrinkage and cracking resistance are studied.At the same time,the mechanical strength,microstructure and steel corrosion of graphene modified cement composites are explored.The main results are as followed:(1)Dispersion of graphene oxide(GO)in cement matrix1.The agglomeration mechanism of GO:the equivalent radius of GO aggregations can reach 125?m.The thickness is about 12?m and the sphericity is between 0.2?0.7.The complexation between carboxyl group and divalent cations and the reduction under alkaline conditions are the main reasons forGO agglomeration.2.The dispersion method and spatial distribution of GO:Based on the agglomeration mechanism of GO,different methods are put forward.The spatial distribution of GO is studied by three-dimensional X-ray tomography and scanning electron microscope.The results show GO agglomeration can not be prevented by high-speed mixing.Polycarboxylate-ether dispersant(PCE)can significantly improve the dispersion of GO.Coating method and ball-milling can prevent GO aggregations by the blocking effect between cement particles.The distribution of strength data indirectly verifies the effects of different methods on dispersing GO.In conclusion,the high-speed mixing can not be used for the preparation of GO modified cement composites,PCE dispersion and ball-milling method are suitable for large-scale research,and the coating method is suitable for exploring subtle study.(2)The thermal conductivity and temperature gradient properties of reduced graphene oxide(rGO)modified cement-based materials1.Study on the dispersion of rGO:based on the dispersion method of GO,the dispersion of rGO is studied.When the mass ratio of PCE/rGO is 0.5,the dispersion of rGO is the best.The rGO uniformly disperses among the hydration products and no obvious agglomerations are observed.2.Influences of rGO on the thermal properties and temperature difference of mass mortar:after mastering the dispersion method of rGO,the influence of rGO on the thermal properties of cement composites is clarified.With the increase of rGO,the thermal properties of cement stone increase.The thermal conductivity and thermal diffusivity are respectively increased by 7.80%and 29.00%compared with the control group.The maximum temperature difference of surface layer,middle layer and bottom layer of rGO modified mortar is 1?,4?and 1.25?respectively,which is lower than that of plain group(5.5?,10.5?and 6?).At the same time,the strain value(69-76)is lower than that of control group(74-79).Combining with the simulation results of VG Studio MAX,the results show rGO improves the overall heat transfer capacity of mortar and reduces the temperature difference and temperature strain.3.Dispersion of rGO by ball-milling method and its effects on the thermal properties of cement:the feasibility and thermal properties of mortar with high content rGO prepared by ball-milling method are studied.The particle size distributions of four cement are consistent.The thermal conductivity and thermal diffusivity increase with the rising of rGO.The thermal conductivity and thermal diffusion coefficient increase by 31.48%and 40.83%,respectively.(3)The early shrinkage and cracking resistance of rGO modified cement-based materials1.Effects of rGO on the early shrinkage of cement:after controlling temperature difference by rGO,the early shrinkage of rGO modified mortar caused by the humidity are studied.The rGO increases the plastic shrinkage and inhibits the drying and autogenous shrinkage.Compared with the plain group,2.00 wt.%rGO can increase the peak value of plastic shrinkage by 11 times and reduce the dry shrinkage and autogenous shrinkage by38.25%.2.Effects of rGO on the crack resistance of cement composites:The amount,length and width of plastic shrinkage cracking are significantly reduced with the addition of rGO.There is no macroscopic crack on the surface of specimen with 2 wt.%rGO.The incorporation of 2wt.%rGO reduces the maximum crack width by 79.68%.3.Mechanism of rGO on improving shrinkage and crack resistance:the water capacity and water loss rate of rGO modified mortar are studied.The mechanisms of rGO on regulating shrinkage and improving crack resistance are clarified.The water retention effect of rGO on mortar leads to the larger meniscus radius in the internal hole,which reduces the surface tension,thus decreasing the autogenous and drying shrinkage.Additionally,rGO aggravates the imbalance relationship between the evaporation rate on the surface and the seepage rate of water,resulting in the increase of plastic shrinkage.The rGO could bridge hydration products,disperse capillary shrinkage stress and limit non-uniform shrinkage deformation.The positive effects of rGO can improve the cracking resistance of mortar,offset the negative effects caused by the increased plastic shrinkage.(4)The strength and microstructure of rGO modified cement-based materials1.Effects of rGO on mechanical strength:the mechanical strength of rGO modified mortar prepared by PCE method and ball-milling method is studied.With the increase of rGO,the compressive and flexural strength of mortar firstly increase and then decrease.When PCE method is used,3 days mortar strength with 0.6 wt.%rGO is increased by 7.8%and 6.5%respectively.When ball-milling method is used,the mortar strength with 1 wt.%rGO increases,the increasements of compressive and flexural strength at 3 days are 19.39%and14.59%respectively,and the increasements of strength at 28 day are 21.76%and 17.27%respectively.2.Effects of GO and rGO on cement hydration:based on the coating method,the effects of GO and rGO on the hydration properties of cement are studied.Firstly,the way of GO to rGO by annealing strategy is choosed:calcining at 300?for 60 min.The heat flow and total heat increase with the rising of GO or rGO.GO can better promote cement hydration than rGO.Additionally,GO and rGO do not change the type of hydrated crystals,but promote the formation of hydration products.The hydrated crystals are closely intertwined to form a more compact microstructure.3.Effects of GO on the hydration properties of ye'elimite(C₄A₃$):the hydration properties and mechanism of GO coated C₄A₃$are studied.GO is successfully coated on the surface of C₄A₃$and there is no chemical interaction between GO and C₄A₃$.With the increase of GO,the peak value of main exothermic peak of C₄A₃$increases by 32.3%,74.1%and 19.6%,respectively.Besides,0.12 wt.%and 0.23 wt.%GO can shorten the induction period of C₄A₃$,the hydration time of main exothermic peak can be prolonged by 0.4 wt.%GO.With the increase of GO,the content of AFt significantly decreases,while the amount of AFm increases.The complexation of Ca²⁺with-COOH group results in a decrease of Ca²⁺and delayes the formation of AFt.(5)The effects of rGO on steel corrosion in cement-based materials1.Analysis of potentiodynamic polarization curve:Before soaking in NaCl solution,the corrosion resistance of rGO modified mortar is improved.When the mortar is soaked in NaCl solution for 4 and 12 days,the corrosion resistance of 0.6 wt.%rGO modified mortar increases,1.2 wt.%rGO accelerates the corrosion of steel bars.After soaking in 28 days,the chloride ion erosion occurr.The corrosion resistance of rGO modified mortar decreases compared with the control group.2.Analysis of electrochemical impedance spectroscopy:at the same age,the charge transfer resistance R₃ firstly increases and then decreases before soaking in NaCl solution for28 days.The corrosion resistance of rGO modified mortar significantly decreases after chloride ion erosion.To sum up,0.6 wt.%rGO can improve the corrosion resistance of steel bars in a short time.When the content of rGO is too high or the soaking time is long,the corrosion of steel bars will be accelerated.