| Due to the characteristics of high natural water content,high compressibility,low strength,if it is encountered in the actual project,cement is generally used for in-situ solidification.However,the following problems will occur when the cement is used to solidify the marine silt:first,the cement production process will cause serious environmental pollution;second,the corrosive ions in the seavater,such as chloride ion(Cl-),sulfate ion(SO42-),magnesium ionon(Mg2+),will corrode the cement solidified soil,which will lead to the microstructure and growth of the solidified soil.The change of the formation will lead to the change of the physical and mechanical properties of the cement stabilized soil,which will reduce the durability of the soil and affect the service life of the project.In this paper,a new solidification agent(steel slag powder)is used to solidify marine silt,and the strength degradation performance of the silt under the action of aggressive ions is studied by replacing part of cement with steel slag powder.The change of physical properties and degradation mechanism of solidified soil are analyzed from macro and micro aspects.Through indoor physical and mechanical tests,under the conditions of distilled water,sulphate solution and sulphate chloride mixed solution erosion,the quality,unconfined compressive strength(UCS)and chloride ion concentration of SSP-CS and A-SSP-CS vary with the erosion time and concentration of erosion solution,and compared with cement stabilized soil(CS),to explore the feasibility of using steel slag powder to replace part of cement to solidify silt.Research findings,there was no loss in the quality of all kinds of solidified soil samples eroded by different concentrations of sulfate solution,In distilled water environment,only SSP-CS eroded 28 days before the mass loss of the sample.In the mixed solution,the presence of Cl-can reduce the corrosion rate of sulfate in the solidified soil,and the higher the concentration of Cl-,the more obvious the effect of delay.The quality of all kinds of solidified soil samples eroded in the mixed solution of sulfate and chloride has no loss.When Cl-and SO42-coexist in the erosion solution,Cl-can improve the compressive strength of the solidified soil to a certain extent,and reduce the erosion degradation degree of the solidified soil.However,when the Cl-concentration in the erosion solution exceeds a certain limit value,with the increase of Cl-concentration in the solution,this slowing down action will gradually lose.The results show that the concentration of Cl-in the solution is different when the strength of all kinds of solidified soil samples reaches the maximum value at different erosion time.The microstructure and phase change of CS,SSP-CS and A-SSP-CS in distilled water,sulfate solution and mixed sulfate chloride solution were analyzed by SEM and EDS,Furthermore,the strength degradation mechanism of cement stabilized soil with steel slag powder in sulphate solution and sulphate chloride mixed solution is obtained.In the early stage of erosion,Cl-will react with hydrated calcium aluminate to form calcium trichloroaluminate,or Friedel’s salt,because the penetration rate of Cl-is higher than SO42-.On the one hand,the expansion property of Friedel’s salt is lower than that of ettringite.On the other hand,Cl-will react with the hydrated products in the solidified soil to form hydrated calcium aluminate(3CaO*Al2O3*CaCl2*10H2O),which will reduce the production of ettringite,the product of sulfate corrosion,and thus weaken the expansion and destruction effect of sulfate on the solidified soil.In the later stage of erosion,on the one hand,the F’s salt generated by Cl-participating in hydration reaction has no gelling property,and the molten wax like substance covers the surface of particles,which makes the hydrate unable to exchange substance,and the hydration slows down or is forced to stop.On the other hand,SO42-is involved in the hydration reaction to produce ettringite.Ettringite has a small solubility and combines with a large amount of combined water.The volume of ettringite is 2.5 times that of the original calcium aluminate hydrate.Because of the huge volume change,the sample has a great internal stress due to the existence of ettringite,resulting in the expansion failure of the sample.Compared with calcium hydroxide,the volume of gypsum increased nearly 2 times.At the same time,calcium hydroxide is generated in the reaction,which increases the pH value in the pore solution,and in turn accelerates the formation of ettringite,resulting in more and more serious erosion,which makes the strength of the sample continuously reduce. |
