Mechanical Properties And Microstructure Of Nmcs Under Sulfuric Acid Corrosion Environment

There are many soft soil distribution in the coastal area of our country,and there are some defects in the soil,such as large compression capacity and poor bearing capacity.In practical engineering,reinforcement measures are needed,so researchers are constantly looking for new materials to improve the bearing capacity of soft soil layer.As a new multi-functional material,nano Magnesium Oxide(Nm)has quantum size effect,small size effect,and superficial area effect and so on.It has broad application value for the improvement of cement soil.With the acceleration of industrialization,the factory discharges acid industrial sewage to erode the structure of underground cement soil.Therefore,it is of great engineering value and reference significance to study the mechanical properties and microstructure analysis of nano Magnesium Oxide modified cement soil(Nmcs)under sulfuric acid erosion environment.The main research work and achievements are as follows:(1)Direct shear and UCS tests for cement soil were carried out on different cement,Nm ratio and different Nm ratios under sulfuric acid corrosion environment.The results show that the shear strength and UCS increase linearly with the cement mixing ratio.For shear strength,the optimum mixing ratio of Nm is 10‰,and the optimum Nm incorporation ratio is 5‰in sulfuric acid corrosion environment.For UCS,the optimum mixing ratio of Nm is about 15‰-20‰,and the optimum mixing ratio of Nm is 15‰in sulfuric acid environment.By direct shear and UCS tests of Nmcs with different erosion concentrations and time,it is found that the shear strength and UCS decrease with the increase of erosion concentration.With the increase of erosion time,the strength began to increase,and then slowly increased or even decreased.(2)Based on the test data of direct shear and UCS draw the shear stress displacement(?-?)and stress strain curve.The?-?curve b ehaves as strain hardening under the condition of no cement mixing and low cement mixing ratio.When the ratio of cement is increased,the curve is changed from hardening to softening.The?-?curve behaves as strain droping under high cement mixing ratio or Nm's incorporation.The Nmcs's?-?curve behaves as“S”type before peak value under sulfuric acid corrosion environment,after the peak also appeared drop section.For stress-strain constitutive relationship,the curve characteristics are similar in the test,all of which are hump type.(3)According to the mathematical characteristics of?-?curve,summarizes the defects of hyperbola and exponential function and power function of traditional constitutive relation model,and a new model(REP model)is built on the basis of the traditional model.For the hardened and modified droping curves,the REP model has a high fitting accuracy,and the traditional model fitting effect is relatively poor,and the double“X”error occurs in the modified droping curve.For the“S”curve,the CEL model is obtained when the parameter?is minus one in the original REP model.Compared with the traditional model,it is found that the traditional model can not meet the curve fitting point,and the double“X”type error is more significant,while the CEL model agrees well with the measured curve.(4)According to the stress strain hump curve characteristics,the parameter range of REP model is released,and the simulation accuracy is higher than the traditional power function and the second-degree parabola model.In order to study the influence of erosion concentration on the parameters of REP model,the sulfuric acid erosion factor?(?_s)and damage model is introduced,and the parameter values of the new model can be determined according to the actual engineering sulfuric acid erosion conditions,soil cohesion and internal friction angle.(5)The microstructure of Nmcs was analyzed by Search Engine Marketing(SEM)and Energy Dispersive X-Ray Spectroscopy(EDX)tests.The results of SEM test show that the adsorption capacity of soil particles increases with the addition of Nm,which makes the whole structure more compact.Excessive Nm reduced the formation of C-S-H gel and lost the cementation between the particles.In corrosive environment,more acicular and rod-shaped ettringite filling in between the particles,the whole structure becomes loose overhead.The results of EDX test show that there are obvious diffraction peak of elements such as O,Si,Al,Ca,K,Na,Fe and Mg in the energy spectrum.The weight ratio of Si increases with the increase of Nm mixing ratio,which is similar to the mechanical properties.In sulfuric acid etching environment,the weight ratio of Mg decreases with the addition of excessive Nm.The weight ratio of other elements has no obvious change with the increase of Nm incorporation.