Study On Autogenous Shrinkage Properties Of Carbon Nanomaterials Cement-based Composites

With the large use of low water to cement ratio and high-strength concrete,the most prominent one is the cracking,which caused by autogenous shrinkage of concrete at the early time.Adding carbon nanomaterials to cement-based materials can not only effectively fill the cement pores,but also increase the hydration product content of the cement,thereby improving the compactness of the matrix,and improving autogenous shrinkage performance and reducing the early cracking of high-strength concrete,and then improving the service life of the building,so it has very important engineering significance and value.This paper mainly studied the aqueous dispersion process of three kinds of carbon nano-materials,including multi-walled carbon nanotubes,carbon nanofibers and graphene,and determined the best dispersion process for this experiment.Based on this,the preparation process of cement-based composites with multi-walled carbon nanotubes,carbon nanofibers and graphene was studied,and the self-shrinkage properties of the composites were systematically studied.Finally,using a variety of microscopic testing methods,the microscopic mechanism of carbon nano-materials in the cement matrix was thoroughly explored.The main results and conclusions are as follows:(1)The most suitable carbon nanomaterial dispersion process was determined in this experiment: gum arabic(GA)was used as a dispersant for multi-walled carbon nanotubes,methylcellulose(MC)as carbon nanofiber dispersant,nonylphenylether(CO890)was used as Graphene dispersant,the mass ratio of MWCNTs to GA was 1:6,the mass ratio of CNFs and MC was 1:2,GNPs and CO890 mass ratio of 1:5,using surface active agent and ultrasonic processor ultrasonic to prepare a uniform dispersed carbon nanomaterials suspension.(2)The variation of autogenous shrinkage of carbon nanomaterial cement-based composites was studied.The autogenous shrinkage values of the three carbon nanomaterial cement-based composites were tested by using autogenous shrinkage measurement instruments.When the water-cement ratio was 0.30 and 0.35,the autogenous shrinkage values of the three carbon nanomaterials cement paste samples first decrease with the increase of the age,and then increase again;while the water-cement ratio was 0.25,the autogenous shrinkage value of the specimen increases with the increase of the age,especially from the initial setting time of the test to the 1nd of the test age,the autogenous shrinkage value of the test specimen developed rapidly.When the water-cement ratio was 0.35 and the test age was 7 days,the autogenous shrinkage values of the 0.05 wt% multi-walled carbon nanotubes,0.12 wt% carbon nanofibers,and 0.10 wt% graphene cement paste samples were the lowest.Compared with the blank sample,the percentage reduction of autogenous shrinkage was 46.74%,98.73%,81.60%,respectively.(3)The autogenous shrinkage mechanism of carbon nanomaterial cement-based composites was studied.The addition of these three carbon nanomaterials to cement-based materials could be embedded in the cement matrix to fill the cement pores,reduce the porosity of the matrix,and refine the pore size distribution.In addition,when the multi-walled carbon nanotubes and carbon nanofibers embedded in the matrix were pulled out,the energy could be consumed and the autogenous shrinkage stress could relieved.The graphene could prolong the crack propagation path,effectively absorb and disperse the energy load,thereby improving the autogenous shrinkage performance of the cement matrix.These three kinds of carbon nanomaterials could also increase the content of cement hydration products,increase their structural regularity,and reduce the self-shrinkage value of cement-based materials.Thus,the addition of carbon nanomaterials can improve the autogenous shrinkage properties of cement-based materials.