Study On The Characteristics And Time Duration Model Of Sulfate Attack-induced Expansion Of Cement-treated Subgrade Filler

In recent years,many ballastless track lines in China,such as Lanzhou-Wulumuqi High-speed Railway,have experienced expansion and deformation of cement-treated subgrade fillers under sulfate attack,which has caused continuous arch and deviation of track structure,and seriously affected the operation of the line.At present,the researches of cement-based materials attacked by sulfate are focused on concrete.But it is rarely researched on cement-treated subgrade fillers.However,there are obvious differences between concrete and cement-treated subgrade fillers in material composition,erosion mode,deformation law and other aspects.Based on this,the condition,laws and models of sulfate attack-induced expansion of cement-treated subgrade filler have been studied by macroscopic and microscopic tests,theoretical analysis and calculation in this paper.The main work and conclusions of this paper are as follows:(1)Based on the summary of filed cases caused by sulfate attack cement-treated subgrade fillers,it has been found that the deformation damage of subgrade caused by sulfate attack is a long-term process.It is because that under the certain temperature and chemical environment,the continuous formation of expansive minerals such as ettringite,gypsum and thaumasite caused by migration,recharge and local aggregation of salts in subgrade.(2)Under constant temperature,the conditions,main controlling factors and time-varying law of expansion deformation of sulfate attack in cement-treated aggregate have been studied.It has been found that:The sulfate content is the main controlling factor of expansion induced by sulfate attack,and the relationship between them is approximately logarithmic function.Within60 days,under 0.1～1% sulfate content,the overall deformation is very small,and the strain curve shows two stages: continuous increase slowly and stable stage.Under 1～3%sulfate content,the strain curve shows four stages: increase rapidly,increase slowly,increase rapidly and stable stage.When the sulfate content is 5% and above,the strain curve shows two stages of continuous increase rapidly and stable stage.The effect of cement content on expansion strain is not consistent,but the specific surface area of cement is positively correlated with expansion strain.(3)Under different freezing-thawing cycle temperatures and cycles,the expansion deformation conditions,the conditions,main controlling factors and time-varying law of expansion deformation of sulfate attack in cement-treated aggregate have been studied.It has been found that:Periodic frost heave and salt heave can cause recoverable deformation,chemical attack and water-salt migration can cause unrecoverable deformation,and unrecoverable deformation plays an important role.Under 1% sodium sulfate content and 3% cement content,unrecoverable deformation accounts for about 70% of the total deformation,and unrecoverable deformation caused by chemical attack for about 13-26% of the total deformation.(4)Through microscopic test and theoretical analysis,it is clear that the expansion induced by sulfate attack is caused by mirabilite,gypsum and ettringite crystals.A time duration model of expansion induced by sodium sulfate attack is proposed.The model calculation shows that:When the content of sodium sulfate is less than 1%,the expansion deformation is mainly caused by ettringite formation.When the content of sodium sulfate is more than1%,the expansion deformation caused by mirabilite formation is the main part.With the decrease of compaction degree,the total strain of subgrade filler and the strain caused by the formation of each crystal are decreased.Water content has little effect on the total strain.Under the lower water content,the expansion mainly caused by mirabilite crystal.And under the higher water content,the expansion mainly caused by ettringite crystal.