| The strength properties of the consolidations of the cement-based grouts are critical for the reinforcement effect in rock grouting.The further improvement on the mechanical properties of the cement-based grouts is quite significant for maintaining the stability of the fractured rock mass.By combining with the National Basic Research 973 Program of China(Grant No.2013CB36003)and National Natural Science Foundation of China(Grant No.51734009),this thesis studied the influences of carbon nanotubes on the mechanical properties of the cement-based grouts,and revealed the enhancing mechanism from the micro perspective by combinedly using various nano characterization techniques and molecular dynamics simulation method.The preliminary study on improving the dispersion stability of carbon nanotubes in cementitious environment was also conducted.The main conclusions are as the following:(1)Influences of carbon nanotubes on the mechanical properties of the hardened cement-based grouts with water to cement ratio of 0.6 at 28 days were studied through the conventional mechanical tests.Porosity and the microstructure characteristics of the hardened cement-based grouts were analyzed by using Mercury intrusion porosimetry method(MIP)and scanning electron microscope(SEM),separately.Results show that the addition of carbon nanotubes can enhance the properties of the compressive,tensile and shear strength.In the matrices,carbon nanotubes can work as nanofillers,thus decreasing the porosity and the amount of capillary pore,as well as bridge the nano cracks.As a result,the mechanical properties of the hardened grouts can be improved.The prepared grouts were used to reinforce rock specimens containing penetrated joints and it was found that the average energy release rate of the cemented interface increases with the increasing mass ratios of carbon nanotubes,indicating that carbon nanotubes enhanced cement-based grouts hold a great potential for being used in the rock grouting field.(2)Based on the atomic microscopy technique,the Peak Force QNM method was adopted to conduct a large number of testing on the nanoscale mechanical properties of the cement-based grouts.This research mainly focused on the influences of carbon nanotubes on the micro Young’s modulus and surface adhesion.The measurement results show that 0.018~0.036 wt% carbon nanotubes can improve the low-density and high-density hydration products by 21~36% and 5~13%,separately,and the micro surface adhesion by about 10~13%.The characterization results of on the nanoscale mechanical properties of cement-based grout revealed the enhancing mechanism of carbon nanotubes furtherly.(3)Based on the microstructure characteristic and the nanomechanical properties of the hardened cement-based grouts,molecular dynamics models simulating the micro-units of cement-based grout matrices containing carbon nanotubes were established,and their mechanical behavior under micro shearing was studied.Researches were done for explaining the enhancing mechanism of carbon nanotube from the nanoscale.The simulation results show that during the fracturing,the existence of carbon nanotubes could cause the improved fracture energy of the micro-unit by enhancing the nano friction,and the enhancement of the carbon nanotubes embedded in lower stiffness materials on the nanofriction was more pronounced.The increased incline angles of carbon nanotubes weakened their enhancing effect on the nanofriction.When the incline angle of carbon nanotubes increases to a critical value,the embedded carbon nanotubes even induced the propagation of the crack,resulting in the decreased strength characteristic.(4)For the phenomenon that the already dispersed carbon nanotubes may tend to re-agglomerate again in the cementitious environment,an amount of methylcellulose(MC)was introduced in the carbon nanotubes aqueous suspensions.In the cement pore solution,the MC molecular can not only prohibit the adsorption of various cations on the carbon nanotubes,but also increase the viscosity of the suspensions,which lower the probability of re-agglomeration of carbon nanotubes caused by the collision due to the Brownian movement.As a result,the dispersion stability of carbon nanotubes in cement environment was improved and the enhancing efficiency of carbon nanotubes on the strength performance of cement-based grouts was improved.The excess MC adversely affects the stable dispersion of carbon nanotubes and brings in a large amount of air bubbles in the matrices,thus the final strength performance of the consolidation can be reduced. |
