Microstructure And Mechanical Properties Of Kaolin Under Different Sedimentary Environments

In recent years,the problems of environmental deterioration and soil pollution are becoming more and more serious.after the polluted soil has experienced soil-water chemistry,the particle microstructure changes,which leads to changes in the physical and mechanical properties of the soil.When the soil property of the building foundation changes,it will lead to the uneven settlement of the building and the cracking and collapse of the roadbed and other engineering disasters,resulting in huge economic losses.Therefore,it is of theoretical significance and practical value to study the influence of acid-alkali salt sedimentary environment on the microstructure and mechanical properties of kaolin at a certain stratum depth.In this paper,the deposition experiments of normal gravity(1g)and hypergravity(ng)were carried out on kaolin under different acid,alkali and salt environments.Through the XRD test of kaolin sediments,13 XRD maps were obtained based on the rotation of χ angle from 0°to 72°.Quantitative texture analysis of the diffraction spectra was carried out by using the Rietveld full-spectrum fitting method by MAUD software and the pole figure was introduced to illustrate the anisotropic characteristics of kaolin particle arrangement by using quantitative indicators such as orientation degree,pole density index,and grain shape,and explain the formation mechanism of structural anisotropy from a microscopic perspective.At the same time,the influence of different environments on the morphology and micromechanical indexes of kaolin was studied by means of atomic force microscopy.The main results of this paper are as follows:(1)The effect of salt concentration on the micro-anisotropy of kaolin is related to the surface charge intensity of soil particles,the repulsion of electric double layer and the arrangement of kaolin particles.The cations in salt solution will neutralize the anions on the surface of kaolin particles,the repulsion of the electric double layer will be reduced,and the van der Waals force will dominate,so that a surface-to-surface aggregation structure is formed between the particles.After the deposition,the central pole density of the(001)pole figure and the orientation increase,and the anisotropy along the horizontal direction increases.(2)The effect of pH value of the solution on the microstructure anisotropy of kaolin is controlled by the electrification at the edge of soil particles,the contact mode of particles,the corrosion degree of soil structure and so on.In the acid environment,the surface of kaolin particles has negative charges and the edges have positive charges.At this time,the edges and faces attract each other.Under the action of Coulomb electrostatic attraction,a flocculation structure with overlapping edges and faces is formed between the particles.After the deposition,the pole density and the orientation decrease,the particles are randomly arranged,and the soils exhibit isotropy.In alkaline environment,the anions on the surface of soil particles will attract the cations in the solution,resulting in the thinning of the electric double layer and a decrease in repulsion.The particles attract each other to form aggregated structures.After the deposition,the pole density and the orientation increase,and the anisotropy along the horizontal direction increases.But the strong alkali reacts with the cementitious materials in the kaolin,corroding the clay structure,expanding the soil pores and loosening the soil.After the deposition,the pole density and the orientation decrease,the particles are randomly arranged,and the soils exhibit isotropy.(3)With the increase of stratum depth,kaolin flake particles gradually show low-angle side-to-face contact or face-to-face contact,with a horizontal preferred orientation.With the increase of centrifugal force,the interparticle pores are squeezed,the porosity decreases,the pole density and orientation increase,the anisotropy along the horizontal direction increases gradually,and the structure is densified.(4)The microscopic Young’s modulus of kaolin is related to factors such as particle arrangement,porosity or compactness.In the strong acid environment,the kaolin aggregates in contact with each other edge-to-face are loose in structure and the Young’s modulus decreases;the reaction between the strong alkali and the kaolin cementing material increases the porosity of the kaolin and decreases the Young’s modulus;the kaolin particles in the salt environment are in face-to-face contact,the density increases after deposition,and the Young’s modulus increases;With the increase of centrifugal force,that is,with the increase of stratum depth,the structure of kaolin flake particles becomes densified and the stiffness increases.