Mechanical Damage Behavior And Mechanism Of Negative Poisson’s Ratio Aluminate-sulphoaluminate Cement-based Materials

Cement-based materials are the most widely used building cementitious materials and are widely used in the construction of infrastructure such as people’s livelihood,industry and military.Due to the defects of its microstructure,cement-based materials have low tensile strength,high brittleness and poor impact resistance,so the concept of negative Poisson’s ratio is introduced to modify them.Based on the structural characteristics of negative Poisson ratio materials and the morphology of the main hydration products of aluminate cement（CAC）and sulphoaluminate cement（SAC）,11 kinds of composite cement formulations were designed in this paper.Three types of crystal regulators were selected to explore the influence on cement crystallization products.The basic performance test of compound cement shows that the standard consistency water requirement,setting time,fluidity,and hydration deformation of formula 4 composite cement have special changes,so it is speculated that the slurry of formula 4 composite cement has good properties,and the three crystal regulators have promoted the hydration of formula 4 composite cement.XRD,FT-IR,and EDS test results show that the main hydration products of composite cement are ettringite（AFt）,calcium aluminate hydrate（C₃AH₆）,and Ca CO₃.These three crystals interact with each other to form the structural strength of cement stone.SEM shows calcium carbonate crystals with massive morphology,ettringite crystals with rod-like morphology,calcium aluminate hydrate crystals with hexagonal flake and fibrous morphology,and some amorphous gels.TEM shows the influence of the crystal regulator on the morphology of the hydration crystallization product,and the calculation of the plane spacing of a concave crystal confirms that the crystallization product is composed of Ca CO₃ and ettringite.The digital speckle technique was used to measure the Poisson ratio of each formula cement net slurry under tensile and compressive loads.The results showed that the combination of the two kinds of cement significantly reduced the Poisson ratio of the net slurry,in which formula 4 composite cement net slurry decreased the most,and the Poisson ratio even decreased to-0.05 in some areas.According to the morphology of crystal products and the structure of concave negative Poisson’s ratio,the mechanism of negative Poisson’s ratio effect on cement slurry was analyzed,two crystal arrangements were predicted,and the ideal calculation model was given.The static compression,tensile and bending tests of 11 kinds of composite cement paste samples were carried out,and the following conclusions were drawn:the combination of CAC and SAC affected the mechanical properties of cement paste,and the cement paste obtained the best mechanical strength when the CAC content reached30%mass ratio.Combined with the morphology of the hydration products of composite cement,the mechanism of the change of strength properties of cement paste was explored,and the correlation law of hydration products on the change of properties of cement set was given.For experimental group and control group a total of six kinds of specimen in three point bending,falling ball impact,hopkinson impact test,the results show that the cement composite can significantly improve sample three-point flexural toughness and impact resistance,bending strength,bending deformation and impact energy consumption ability enhanced,introduce crystal control agent and can make the sample resistance to bending,and impact resistance to further improve,Compared with the control group,the bending toughness and impact energy consumption of adipic acid were increased by 81.3%and 89.1%,respectively.Using the digital speckle correlation method for experimental group and control group in six samples by characterizing the deformation behaviors of bending and impact load,the results show that the experimental group specimen deformation displacement value decreases,and the overall average shows that the cement composite and crystal control agents have the ability to increase the specimen deformation resistance load,the effect of adipic acid is best;The overall strain of the test group decreased,and the phenomenon of strain concentration in a wide range was also improved,indicating that the maximum bearing capacity and energy absorption effect of the test group were enhanced,and adipic acid had the best modification effect.