| As a kind of two-dimensional nano materials,multi-layer graphenes(MLGs)possess excellent mechanical properties such as high strength,high modulus and strong toughness.MLGs also have outstanding functional performances such as excellent thermal conductivity,electrical conductivity and electromagnetic shielding/absorbing properties.Moreover,MLGs have low density,good chemical stability and thermal stability.Therefore,MLGs are incorperated into cementitious composites to make composites possess both excellent mechnical properties and functional/smart properties.Therefore,this dissertation mainly studied the preparation process,rheological property of the cementitious composites filled with MLGs before hardening,hydration characteristics and mechanical performance,electrical conductivity,thermal conductivity,sensing property and electromagnetic shielding/absorbing property after hardening.The main contents and conclusions are as follows:(1)Physical and mechanical methods were used to disperse MLGs in cementitious composites.Observing methods of microstructure and discreteness of electrical resistance,electrical capacitance and alternating current(AC)impedance were employed to evaluate the dispersion status of MLGs in cement matrix.Rheological characteristics of the composites were analysed.Rheological modle was chosen to calculate rheological characteristic parameters.The influence laws of MLGs on rheological behaviour of the cement slurry were studied.Hydration products of the composites were analysed.Research results showed that ultrasonication treatment with low-speed and high-speed stir and polycarboxylate superplasticizer dispersion method can disperse MLGs uniformly in cement matrix.Combining qualitive composites microstructure observation method and quantitative macro properties discreteness calculation method can effectively evaluate the dispersion status of MLGs in cement matrix.Owning to its high specific surface area,MLGs lowered the fluidity of cement slurry when they were added into cement slurry.The viscosity of MLGs filled cement slurry increases slowly first and then fast with increase of MLGs content.The rheological behaviour of MLGs filled cement slurry is close to dilatant fluid.Addition of MLGs can lower the orientation index of calcium hydroxide crystals inside hydration products and promote 3d and 28 d hydration of the composites without influencing its 90 d hydration degree.(2)Influence laws of different MLGs content on compressive strength,flexural strength,elastic modulus,abrasion resistance,hardness,and damping performance of the cementiitious composites were studied.Influence mechanisms of MLGs to mechanical performance of the cementitious composites were analysed through calclating the distribution of MLGs in cement matrix,combining micro hardness,thermal conductivity and micro structure of MLGs filled cementitious composites.Research results presented that MLGs could notably enhance the mechanical properties of the cementitious composites.The compressive strength,flexural strength,elastic modulus,abrasion resistance,hardness,and damping performance of the cementiitious composites increase by 47.7%,20.45%,24.5%,70.88%,150% and 45.1%,respectively.Improvement mechanism of MLGs on the cementitious composites are mainly owning to their nano size and two dimensional shape which make MLGs easily distribute to form network structure,nucleation effect lowering calcium hydroxide crystal orientation index in hydration products of cementitious composites,enhancing thermal conductivity of the cementitious composites to reduce original crack,absorbing water in early hydration stage to lowering water/cement ratio and releasing water to form self-curing inside in later hrdration stage and bridging effect to stop/delay micro cracks from developing,etc.(3)Electrical property and mechanism of the cementitious composites filled with different content of MLGs were studied.Electrical conductive path model was established through AC impedance spectroscopy method.Relationship equation between MLGs content and electrical property of the cementitious composites was deduced based on effective medium theory.Thermal conductive property(including coefficient of thermal conductivity and specific heat capacity)of the cementitious composites filled with different content of MLGs was studied.Influence mechanism of MLGs on thermal conductivity of the cementitious composites was studied combining the characteristics of MLGs.Results showed that the addition of MLGs enhanced the electrical conductivity by 120 times.The percolation threshold of MLGs filled cementitious composites is about 2%.There presentes second percolation phenomen when the content of MLGs reaches 9%.Electrical conductive path model can be simplified into series-parallel circuit model.Pore solution resistivity of the cementitious composites decreased with the increase of MLGs content.Relationship equation between MLGs content and electrical resistivity of the cementitious composites was established through Fixman,electrical resistivity and effective medium equation.Thermal conductivity of the cementitious composites filled with MLGs increases with the increase of MLGs content,while the sepecific heat capacity decreases.Coefficent of thermal conductivity and specific heat capacity increases and decreases most by 76.85% and 17.7%,respectively.This is due to the high thermal conductivity of MLGs.(4)The change characteristics of direct current(DC)and AC electrical resistance(i.e.,piezoresistivity)of the cementitious composites filled with different content of MLGs under compressive loading were studied.The influence factors such as MLGs content,water/cement ratio,loading rate,loading amplitude,constant loading,loading times,matrix type and sample size to the piezoresistivity of the cementitious composites filled with MLGs were also studied.The change of DC and AC electrical resistance and capacitance(i.e.,temperature-sensitive property)of the cementitious composites filled with MLGs was investigated.Occurrence mechanism of piezoresistivity and temperature-sensitive proerties was explored.Results illustrated that the stress and strain sensitivity reach 0.8%/MPa and 156 seperately.The strain sensitivity is 50-80 times of that of usual metal strain gauge.Dispersion status of MLGs is closely related to the piezoresistivity of the cementitious composites filled with MLGs.Low dispersion of small content of MLGs is beneficial for the formation of conductive network while high dispersion of MLGs is opposite,which affects the electrical conductivity and piezoresistivity of the composites.The size of cementitious samples has no effce on the piezoresitivity of the composites.Results of piezoresistivity tested by DC and AC two-electrode method are the same.Piezoresistivity of MLGs filled cementitious composites under static and high speed loading is stable and repeatitive.Temperature sensitivity of MLGs filled cementitious composites decreases with the increase of MLGs content,while the linerity increases.(5)Influence laws of MLGs content to the electromagnetic parameters of the cementitious composites were studied.Electromagnetic wave shielding effectiveness and absorption reflection of the composites were calculated based on the study above.Influence mechanisms of MLGs to the electromagnetic wave shielding and absorbing property were also analysed.Differences of electromagnetic wave shielding and absorbing property of MLGs filled cement paste and mortar were compared.MLGs filled cementitious plates were prepared to test the electromagnetic wave absorbing property and compared with theoretical calculation value to verify the reliability of the theoretical calculation method.Results show that the electromagnetic wave shielding and absorbing properties of MLGs filled cementitious composites increase most by 95% and 9 times,respectively.The absorption depletion to electromagnetic wave by MLGs filled cementitious composites is mainly dielectric depletion without magnetic depletion.Employment of indirect testing method of testing electromagnetic wave parameters can effectively illustrate and predict the electromagnetic wave shielding and absorbing properties of MLGs filled cementitious composites. |
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