The Study On Engineering Characteristics And Improvement Mechanism Of Sulfate Salty Soil Modified By Lime-silica Fume

A large number of saline soil are distributed in the northwest of China,whose undesirable engineering characteristics,such as salt-frost heave,dissolution collapse and corrosion,have seriously hindered the construction of China’s projects,caused huge economic losses and threatened the health and safety of the people.It is very important to carry out the prevention and control of saline soil and improve its engineering characteristics.Therefore,lime-silica fume was selected to improve the saline soil.In this experiment,sulfate salty soil prepared in laboratory was used as the ameliorating object and lime-silica fume was used as the ameliorating agent.The content of the test is to monitor the salt-frost heave value of the soil before and after improvement,as well as the shear strength,unconfined compressive strength and splitting tensile strength of the soil under freeze-thaw action.The independent variables are the salt content,the content of the improver,and the curing age.The mechanism of soil improvement was analyzed and studied by electron microscopy.The main research results are as follows:1.In terms of the compaction characteristics.The optimal water content of sulfate salty soil increases with the increase of salt content.The maximum dry density decreased with the increase of salt content.The optimum water content of the soil with 5% fixed lime and silica fume increased with the increase of the silica fume content,and the maximum dry density decreased with the increase of the silica fume content.2.In salt-frost heave of sulfate salty soil.Salt-frost heave values of sulfate salty soil with different salt contents are cumulative with the increase of freeze-thaw times.Salt-frost heave of saline sulfate soil samples with salt content of 1% or more is relatively obvious.The freezing peak salt-frost heave and thawing residual salt-frost heave increase linearly with the number of freeze-thaw cycles.3.In terms of salt-frost heave properties of modified sulfate salty soil.The effect of adding lime and silica fume together is much better than that of adding lime and silica fume alone.In order to achieve a better salt-frost heave inhibition effect,for sulfate salty soil with salt content of 3% or less,the lime content should be 5% as a fixed value,and the silica fume content should be 5%,and the curing age should be up to 7 days;For the super sulfate salty soil with 6% salt content,the curing age should reach 28 d when the content of 5% lime and 5% silica fume is fixed.4.In terms of mechanical strength of modified sulfate salty soil.A representative3% sulfate salty soil was selected for the test.The test results show that the shear strength,unconfined compressive strength and splitting tensile strength of the soil before and after freezing-thawing were improved with the addition of the improver.Among them,the combined addition of lime and silica fume has the most significant effect,and its strength is far higher than that of the single addition of lime and silica fume.Among them,the single addition of silica fume has the worst effect,and its strength will not significantly improve with the growth of time.Based on three mechanical tests of shear,compression and tensile strength,it is concluded that when lime and silica fume are added together,if the fixed lime content is 5%,the reasonable starting point of silica fume is 5%,and the reasonable starting point of curing is 7days.5.In terms of microstructure.The most obvious feature of the joint improvement of lime and silica fume compared with the single improvement is that the cementitious substances such as hydrated calcium silicate are generated,most of the pores and cracks are filled with micro-aggregates and hydration products,and the soil particles develop into aggregates,which are embedded with each other and have a stronger integrity.The internal structure of the improved soil with lime added alone has more pores and pores,showing granular and overhead structure,and poor integrity.Although the pores of the improved soil with silica fume added alone are filled with silica fume particles,no hydration products are generated and the strength is low.For the soil improved by lime and silica fume,when the lime content is constant and the silica fume content increases,the most obvious feature is the decrease of pores and large pores,while the number of cemented objects such as micro-aggregates increases.With the increase of age,the flocculent gel material began to develop continuously,and the soil particles were further closely linked,and the structural linkage type was further transformed to cementation type.For the improvement mechanism,it mainly includes physical filling and replacement,ion exchange,volcanic ash and carbonation.Four reactions occur simultaneously,among which pozzolanic reaction is the main reason for the increase of soil strength.