Experimental Study On Pressure Sensitivity Properties Of Cement Slurry Specimens Mixed With GO/CNTs

Graphene oxide（GO）and carbon nanotubes（CNTs）are carbon nanomaterials with extremely high strength and toughness and very high specific surface area.Under the condition of uniform and stable dispersion state,both of them can significantly improve the mechanical properties,electrical conductivity,pressure sensitivity,corrosion resistance,freeze-thaw resistance and other properties of the traditional cement-based composites.It is a new idea and research direction of carbon nanomaterials to fabricate cement-based composites by mixing graphene oxide and carbon nanotubes.At present,researches on cement-based composites mixed with GO/CNTs mainly focus on the synergistic improvement of mechanical properties of these two kinds of nanomaterials.However,there is still a lack of research on the influence of these two kinds of nanomaterials on the conductive properties and pressure-sensitive response characteristics of cement-based composites.In this paper,the dispersion of mixed GO/CNTs under different dispersant conditions is studied.By observing the stratification phenomenon of dispersion solution and combining with the macroscopic performance of the compressive strength of cement-based specimens,the influence of dispersant types on the dispersion of GO/CNTs is analyzed.The compressive strength of cement-based composites mixed GO and CNTs with good dispersion at early（3 d）and 28 d age is studied,and the influence mechanism of mixed GO/CNTs on the compressive strength mechanical properties of cement-based composites is further discussed by means of scanning electron microscopy（SEM）and X-ray diffraction（XRD）.In addition,the conductive properties of cement-based composites mixed with GO/CNTs and the pressure-sensitive response characteristics under different maximum loading cycles are tested by the four-electrode method,and the influence of GO/CNTs on the conductive properties of cement-based composites is discussed.The pressure-sensitive properties of the cement-based samples are discussed based on the resistance rate pressure-sensitive response curves and stress curves of cement-based samples,and the sensitivity factors calculated.The feasibility of self-sensing cement-based sensors mixed with GO/CNTs and their application in structural health monitoring（SHM）are also prospected.Finally,orthogonal range analysis is carried out on the compressive strength and resistivity of cement-based specimens with different GO/CNTs content ratio,and the main factors affecting the compressive strength and electrical conductivity of cement-based composites mixed with GO/CNTs are quantitatively analyzed.The main achievements are as follows:（1）The mixed dispersion solution of GO/CNTs and cement-based specimens are prepared by using three dispersants:sodium dodecyl sulfate（SDS）,polyethylene-pyridoalkanone（PVP）and polycarboxylic acid water reducer（PCs）.The results of static observation and compressive strength test show that when PCs is used for dispersing,the mixed dispersion has the most stable dispersing state,and the cement-based specimens have the highest compressive strength,that is,PCs has the best dispersing effect on composite GO and CNTs in the solution and cement matrix.（2）Both GO and CNTs can improve the compressive strength of cement-based composites,and the early（3 d）and 28 d compressive strength of cement-based composites can be further improved when the two nanomaterials are mixed in an appropriate proportion.At the early age（3 d）,the compressive strength of0.08%GO-0.04%CNTs（8G4C）is the largest（43.1MPa）,which was 37.7%,8.0%and 29.0%higher than that of blank sample,8G0C and 0G4C,respectively.At 28days,the compressive strength of 0.02%GO-0.04%CNTs（2G4C）is the highest（58.2 MPa）,which is 18.1%,13.9%and 11.3%higher than that of blank sample,2G0C and 0G4C,respectively.It is observed by SEM that whether at 3 d or 28 d,the hydration products of cement specimens mixed with GO/CNTs are densified,and the hydration products are closely bonded.XRD analysis results show that hydration products of cement specimens mainly include calcium carbonate（Ca CO₃）,hydrated calcium silicate（C-S-H）,calcium hydroxide（CH）and ettringite（AFt）,etc.At the same time,the peak of C₂S and C₃S of cement raw material is significantly lower than that of blank cement sample,indicating that hydration reaction of cement mixed with GO/CNTs is more thorough.（3）The resistivity test results of the specimens show that the resistivity of cement-based specimens mixed with GO/CNTs is higher than that of cement-based specimens mixed with GO,but it is always lower than that of blank sample,indicating that the electrical conductivity of cement-based composites is improved by the mixed GO/CNTs.In addition,when CNTs are fixed at a large dosage（0.08%and 0.12%）,the resistivity of cement-based specimens show a downward trend after the addition of GO,indicating that GO can improve the dispersion of CNTs in the cement matrix,thus improving the electrical conductivity of GO/CNTs cement-based composites.（4）Under the same maximum loading cyclic load,when the content of GO is fixed,the amplitude of the FCR of the composite doped GO/CNTs samples first increases and then decreases with the increase of the content of CNTs.At the ratio of 0.02%GO-0.04%CNTs（2G4C）,the amplitude of the resistance change rate of the specimen is the largest.Under the maximum loading cycle of 10 MPa（4 k N）,the amplitude of the resistance change rate is about 1.4%,and the sensitivity factor reaches 36.0,which is about 4 times that of the specimens with 0.02%GO（2G0C）or 0.04%CNTs（0G4C）respectively.It indicates that the pressure sensitive response of cement-based composites can be significantly improved by the addition of GO/CNTs.（5）Under cyclic loads of different maximum loading forces,the amplitude of FCR of the composite GO/CNTs samples increases with the increase of loading amplitude,indicating that the pressure sensitivity of the composite GO/CNTs cement-based specimens is enhanced.However,sensitive factors of the specimens decrease with the increase of load amplitude,this is because the FCR variation amplitude and strain amplitude of the specimens along with the growth of the load amplitude is not a linear relationship,that is,the increase of FCR amplitude is not as good as the growth of strain value degree,therefore,the sensitivity factor calculated from the ratio of the absolute value of the FCR to the strain change decreases with the increase of the loading amplitude.The pressure-sensitive response characteristics of cement-based specimens mixed with GO/CNTs show that the cement-based composites mixed with GO/CNTs have the feasibility of preparing self-sensing cement-based sensors,which is expected to be used in the field of structural health monitoring.（6）The range analysis is carried out on the compressive strength and resistivity test results of cement-based specimens with different GO/CNTs content ratio.The results show that the content of GO is the main factor affecting the compressive strength,and the content of CNTs has a significant effect on the electrical conductivity of the cement-based specimens.According to the conclusion,GO/CNTs cement-based composites that meet the performance requirements can be prepared reasonably,and the cost can be reduced while the performance can be guaranteed.