| The climate in Northwest China is arid,and there are extensive areas of sulphate saline soil.In the construction of roads,railways and other infrastructures in these areas,when lime and other improvers are used to improve the engineering properties of the soil,it will produce upward arch deformation for several years,which will affect the normal use of the facilities.The reason for the expansion deformation is that a large number of columnar ettringite crystals are generated by the reaction,which destroys the structure of the soil;the amount of deformation is mainly related to the sulfate content and the water replenishment environment.However,there are controversies about the conditions for the expansion deformation of the lime improved sulfate saline soil,and there is still a lack of related expansion deformation and expansion force calculation models.In this paper,through theoretical analysis,analytical chemistry test,closed system one-dimensional salt expansion test,open system one-dimensional salt expansion test,salt expansion force test and NMR test,the salt expansion deformation characteristics and mechanical properties of lime improving sulfate saline soil are studied.The main conclusions obtained are as follows:(1)The factors of the deformation expansion of the lime improving aluminum sulfate saline soil are sorted according to the degree of strength: sulfate content,temperature,moisture content,and degree of compaction.The salt expansion of the closed system and the open system increases with the increase of the sulfate content,and there is a strong linear relationship.However,the salt expansion of the open system is much greater than that of the closed system under the same conditions.(2)When the temperature is lower than 20℃,the corrosion reaction is not sufficient,not only ettringite crystal but also thaumasite will be produced,and the deformation behavior is complex and difficult to predict.When the temperature is higher than 20℃,the higher the reaction temperature,the faster the salt expansion rate,the shorter the salt expansion stability time,and the lower the salt expansion deformation.The reason is that:the formation reaction of ettringite is sufficient when the temperature is above 20℃,so the total amount of ettringite is the same;the longer the reaction time,the more significant the salt migration and aggregation,the larger the ettringite crystals produced,and the greater the expansion deformation.(3)The calculation model for the deformation expansion of the closed system is established,and the threshold value of the sulfate content for the deformation expansion is about 0.6% from the calculation model.(4)The lime improving sodium sulfate soil is divided into three stages: 1.This phase is generally completed within 10 hours.In this stage,gypsum is mainly produced;2.The stage of slow deformation.At this stage,gypsum transforms slowly to ettringite.3.The deformation continues to increase stage.At this stage,the clay particles continue to release active aluminum ions and the ettringite continues to increase,so the salt expansion will continue to increase,and the time will generally last for several months to several years.This is probably the reason why the deformation of sulfate saline soil can last for several years under natural conditions.The results show that the deformation law of the soil is consistent with that of the aluminum sulfate saline soil,and the expansion deformation of the sodium sulfate saline soil is linear with the sulfate content;and the deformation of open system is much higher than that of closed system.(5)The salt expansion force of aluminum sulfate sample is much greater than that of sodium sulfate sample,and its value is directly proportional to the content or deformation of crystal,up to 1200 k Pa.(6)Because the length of ettringite crystal is generally larger than the peak pore size of lime improving sulfate saline soil sample,the formation of ettringite will lead to the destruction of soil pore structure,resulting in larger expansion deformation and salt expansion force. |
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