Effect Of Iron Oxides On Expansion And Contraction Characteristics Of Kaolin

Iron oxide is the main cementing substance of the collapsed soil,which affects the stability of the collapsed soil.Kaolin is the main part of clay minerals in the soil of collapsed hills,and the cement formed by combining with iron oxides plays an important role in soil activity and soil structure.The cementation of iron oxides and kaolin may determine the stability of the collapse soil,so this study intends to explore the changes of the expansion and contraction characteristics of kaolin under different iron oxide concentrations through indoor load-free expansion test and linear shrinkage test.Through the electric double layer theory and DLVO theory,the influence of iron oxides on the surface of kaolin on the thickness of the sliding layer between particles and the internal forces of the soil（electrostatic repulsion and van der Waals force）was deeply analyzed,and how to further change the expansion and contraction characteristics of the soil.The main conclusions of the study are as follows:（1）Under the same conditions,the unloaded expansion rate and linear shrinkage rate of kaolin increased with the increase of iron oxide concentration.The expansion and contraction rate of KC specimen has the largest variation,and there may be a critical iron oxide concentration value of kaolin expansion and contraction between 35～50 g·kg^-1.The exogenous addition of iron oxides increases the water-holding capacity of kaolin.The phenomena of unsynchronized development of expansion and contraction,large amplitude differences,and uneven deformation indicate that expansion and contraction are irreversible.There was an obvious linear increasing relationship between iron oxide concentration and expansion and contraction rate,and the coefficient of determination of the fitted equation R² was greater than 0.935.（2）The surface charge density and surface potential（absolute value）of kaolin soil fluctuated with the increase of iron oxide concentration,and the absolute value of zeta potential decreased with the increase of iron oxide concentration.The absolute values of surface potential and zeta potential decreased with the increase of electrolyte concentration,and the amplitude of zeta potential was smaller than that of surface potential,and the amplitude variation of the two indexes in the 2:1 electrolyte system was about 1/2 of the amplitude in the 1:1 electrolyte system.The thickness of the sliding layer decreases with the increase of electrolyte concentration and increases with the increase of iron oxide concentration.The thickness of the sliding layer varies greatly at low electrolyte concentrations,and in1:1 electrolyte system.There was an obvious linear increasing relationship between the thickness of the sliding layer and the expansion and contraction rate,and the coefficient of determination of the fitting equation R² was greater than 0.695.The change of sliding layer thickness is macroscopic as expansion and contraction,and iron oxide affects soil expansion and contraction by changing the sliding layer thickness.（3）0.01 mol·L^-1 concentration as an example,electrostatic repulsion and van der Waals force（absolute value）decrease with the increase of particle spacing.The soil internal force is attractive when the particle spacing is less than 1 nm,which is converted into repulsion force at 1～1.5nm,gradually decreases after reaching a peak at 2 nm,and tends to stabilize after 10 nm.The changes of electrostatic repulsion and soil internal force affected by iron oxide concentration can be divided into KA～KC and KD～KE two parts,the former part has a larger amplitude than the latter part,and there may be a critical iron oxide concentration value between35～50 g·kg^-1 of kaolin electrostatic repulsion and internal force change.Water in and out of water causes changes in electrolyte concentration,changes the thickness of sliding layer and soil internal force,and then affects particle spacing,which will have an important impact on soil stability,while iron oxides change the surface charge properties and particle spacing of soil particles through cementation with clay minerals such as kaolin,thereby affecting the expansion and contraction process.Soil expansion and contraction may cause cracks to occur,which can destroy the soil structure and its stability,and then cause collapse erosion.