Study On Salt Expansion Mechanism And Performance Control Of Cement Solidified Sulfate Salty Soil

With the in-depth implementation of the western development strategy and the proposal of the “Belt and Road” construction,there is an urgent need to upgrade and improve the construction of western highways and railways.However,the saline soil in the western region is widely distributed.The soil body is uplifted and cracked due to salt loss,dissolution,salt expansion,and frost heaving,which greatly reduces the stability.Among them,the damage of sulfated saline soil is particularly serious,which threatens the normal use of roads and railways.Cement solidified soil is widely used in base materials because of its wide distribution of materials,ease of mining,good strength,stiffness,and cement stability.However,cement-cured sulfated saline soil will undergo chemical reaction to form expansive products such as ettringite,which will cause physical and chemical expansion of the soil at the same time.It is urgent to continue to develop and study on the basis of cement as a solidified material to increase the stability and durability of the saline soil subgrade.In this paper,the effects of mineral composition,salt content and salt type,compaction degree on the moisture transmission of cement-solidified soil and the influence of mineral composition and compaction degree on sulfate ion transmission of cement-solidified soil are studied.Secondly,the finite volume method and the finite difference method are used to numerically simulate the water transport in the dried cement-solidified soil and the sulfate transport in the unsaturated cement-solidified soil material.In addition,the volume strain model of the improved sulfated saline soil material during the cooling process was established.Finally,on the basis of cement solidified sulfate saline soil,lime and fly ash are added for improvement,and unconfined compressive strength test and salt expansion test are carried out.The main conclusions are as follows:(1)A variety of microscopic test methods are used to analyze the one-dimensional water transmission law of cement-solidified soil.The results show that kaolin is a flocculent structure with little combined water content.Due to its obvious layered structure,montmorillonite has more combined water content and less free water content;as the density of cement solidified soil increases,the capillary water absorption rate gradually increases,which is Because the greater the degree of compaction,the more capillary pores;the results of capillary water absorption with different kinds of sulfates and different sulfate contents are mainly affected by the surface tension,the surface tension of sodium sulfate-containing samples is large,and the water absorption rate is faster.In addition,the sulfate transmission law of cement-solidified soil was studied.The sulfate ion transmission rate is closely related to the fractal dimension of the soil.For sulfate ion transmission at different degrees of compaction,the greater the degree of compaction,the higher the sulfate concentration at the same depth.The smaller the degree of compaction,the greater the fractal dimension of the soil sample,the greater the resistance to ion penetration,and the slower the rate of sulfate ion transmission;for the sulfate transmission under the influence of different mineral components,the sulfate ion concentration is the highest and the damage is the most serious in the montmorillonite sample.The sample with montmorillonite has the lowest fractal dimension value.This is because montmorillonite has good dispersibility,small overall porosity,small particle agglomeration degree,and low resistance to ion transmission.The viscosity of kaolin is too high,the degree of agglomeration is larger,and it has a larger specific surface area,so the fractal dimension is the largest.(2)Establish a water transport model in cement-cured soil materials under dry conditions,and use Cs Cl enhanced X-CT method to successfully realize real-time observation of water transport in cement-cured soil,therefore,the water transport formula is verified,and the experimental results are in good agreement with the theoretical calculation results;the finite difference method is used to simulate the transmission of sulfate ions in unsaturated cement-solidified soil materials,and the fractal dimension is introduced into the ion transmission model to achieve the quantification of pore tortuosity.And compared with the test results,the two have a good consistency.(3)Based on the mechanics theory of porous media,the constitutive equation of linear elastic material under the crystallization of sodium sulfate was obtained,and the volume strain model of the improved sulfated saline soil material during the cooling process was established and compared with the macro test.A good agreement verified the reliability of the model.Criteria for critical saturation are proposed to quantify the relationship between the salt expansion rate and supersaturation,and to achieve the sulfate erosion control index from empirical determination to quantitative calculation.(4)The best ratio of anti-salt swell curing agent was obtained through orthogonal test.The test results show that the addition of lime and fly ash can help reduce the problem of salt swelling and cracking caused by cement reinforced sulfated saline soil.And the mechanism of action of the three materials on the sulfated salted soil was obtained: Physical filling and chemical gelation,calcium hydroxide carbonization,reducing the production of ettringite,etc.are combined to improve the density of sulfated saline soil and reduce the salt expansion rate.The test results showed that the optimal content of improved sulfated saline soil was 8% cement content,15% fly ash content,and 6% lime content.