Multi Scale Study On Physical And Mechanical Properties Of Kaolin In Salt Environment

With the deployment of "The Belt and Road" initiative,East,West and South Asia,where the saline soils are frequently encountered,have become the hotspot for the quick growth of geotechnical engineering.However,the geotechnical engineering projects in these areas often face problems like high cost,long construction period,or even the engineering failures due to the special physical and mechanical properties of saline soils.Thus,there is an urgent need to study the physical and mechanical properties of saline soils in a more detailed and in-depth way to ensure the project safety and rational investment.In this paper,high-purity nano kaolin was taken as the test material,two kinds of salts(sodium chloride and calcium chloride)were used to simulate different salt environments(different salt types/valence states and different salt concentrations)in engineering practice.The physical and mechanical properties of kaolin under different salt environments were thoroughly studied in multi-scale,the main contents include the following aspects:(1)The essential source and influencing factors of particle interactions are clarified from the atomic/molecular scale,the interaction forces between kaolin particles and AFM probe in different salt environments are measured by atomic force microscope,then the stability of kaolin system is analyzed by DLVO theory.Test results show that the DLVO force between kaolin particles is repulsive force under low salt condition(Zeta potential<-30 mV).In this case,kaolin particles disperse each other and the system is relatively stable.With the increase of ionic strength,the zeta potential increases(-30～-15 mV)and the DLVO potential reaches the second well,the kaolin system begins to destabilize.When the zeta potential is greater than-15 mV,the DLVO potential will break through the energy barrier to the first minimum energy value,and the kaolin particles attract each other to form stable and independent aggregates.(2)The microstructures of kaolin aggregates in different salt environments are studied by standard phase formation and high-resolution atomic lattice imaging under AFM tapping mode,the underlying formation principles of different microstructures are analyzed based on the measured interaction forces in(1).The results show that under our experimental condition(pH=5.6),the silica face of kaolin is negatively charged and the alumina face is positively charged,so the silicon surface and aluminum surface of different kaolin particles will attract each other first,making the FF structure always exist in any salt environment.In addition,the edge face of kaolin is positively charged at this pH,so the positively charged edge will attract the negatively charged silicon surface to form EF structure.In low salt condition,the kaolin particles are repulsive on the surface but attractive on the edge,making the EF structure unstable.In high salt condition,the repulsion force between particles decreases,the van der Waals force plays a leading role,so the particles attract each other to form a stable FF structure,while EE is the intermediate state from EF to FF.(3)Based on the results of(1)and(2)and the measurement of the physical and chemical properties of kaolin suspension(such as the size of aggregates,turbidity,etc.),the micro mechanism of settlement behavior of kaolin in different salt environments is thoroughly analyzed through batch sedimentation tests.The results show that under low salt condition,the gravity only has very limited effect on small kaolin aggregates,causing the slow settling of kaolin suspension,which is characterized by dispersed free settling.However,the settled EF and EE aggregates form a card house at the cylinder bottom,forming a three-dimensional network structure with continuous space,which shows high final sediment height and strong positive thixotropy.Under high salt condition,aggregate size increases,the gravity effect becomes prominent,and the speed of kaolin settling accelerates,which is manifested as flocculated zone settling.The settled EE and FF aggregates form relatively independent and dense structure.In this case,the connection between kaolin aggregates is weakened,reducing the thixotropy of kaolin system.(4)The macroscopic physical and mechanical properties of kaolin with different salt content are tested,including compactness,shear strength,compressibility,shear wave velocity(small strain shear modulus)and thermal conductivity,the micro-mechanism of these macroscopic behaviors are thoroughly analyzed by using the results of(1),(2)and(3).The results show that salts can shrink the soil pore,the crystallized salt can act as "filler" in the soil pores to make the soil denser.Therefore,the microscopical performance of saline soil is that the compactness is larger,and the shear wave propagation is faster.However,due to the salt can also promote the soil aggregation and destroy the soil structure,the compressibility and thermal conductivity of saline decrease.In addition,a small strain shear modulus(G0)model and a thermal conductivity(K)model are formulated for saline soil for the first time,and the effectiveness and feasibility of the proposed models are validated by comparisons of the model predicted values and experimental data.