Effect Of Electric Field On Sulfate Attack Of Cement-based Materials Under Different Conditions And Its Mechanism

Sulfate attack is one of the important factors which affects the durability of cement-based materials,and it is also an erosion that is more complex and has a greater degree of damage.At present,the similar effects of sulfate and other factors such as Cl-,carbonization,loading and alkali aggregate reaction have been studied and discussed.With the continuous development of subway rail construction,the service condition of cement-based materials is becoming more and more serious.In the urban subway DC power supply system,the use of rail as a return line.As the rail can not be completely insulated,there is always a part of the current through the track to the track bed,station,subway structure and the surrounding soil medium leakage,the formation of stray current.Stray current not only will itself corrosion of reinforced concrete structure,but also with other factors together to attack cement-based materials.In this paper,the specimen was fully immersed,semi-soaked and immersed in the soil condition by salt solution with electric field to simulate the conditiont of the cement-based materials in the subway condition,and studied the performance and mechanism of the cement-based materials under the interaction of sulfate attack and electric field.The pulsed instrument was used to simulate the stray current with the voltage intensity of 30 v and the pulse period of 20 s.At the same time,sodium sulfate was used to study the performance change and mechanism of the cement paste in the different conditions under the action of sulfate and electric field.By micro-analysis and macro-analysis,the main ways of test were used as follows:(1)The appearance change of the specimen in different conditions was recorded,and characterized the destruction of cement-based materials effected by the combination of sulfate and electric field;(2)The macroscopic examination of the cement-based materials was compressive strength and erosion resistant coefficient;(3)The distribution of sulfate ions in the specimen was tested by the barium sulfate gravimetric method;(4)Microscopic application of scanning electron microscopy(SEM)/ energy dispersive spectrometer(EDS)was used to study the effect of the content of hydrosulfide on the ion transport properties of sulfuric acid.Qualitative analysis was carried out and the corrosion products were quantitatively analyzed by X-ray diffraction(XRD).In this paper,the performance and mechanism of cement-based materials in different conditions under sulfate and electric field were studied.The results showed that:(1)The no sulfate erosion is relatively slow without electric field,the general test age ranging from several years to several decades.Cement-based material sulfate erosion significantly accelerated with electric field,and the performance was significantly changed after a few months,which was reflected in the appearance of the destruction rate,erosion resistance,sulfate ion transport and the kinds and quantity of erosion products.(2)The sulfate corrosion products are mainly ettringite and gypsum under the test conditions,which absorb water and damage the cement-based materials.The main factors influencing the types of eroded products are the concentration of sulfate ions.The erosive products was mainly ettringite when the concentration of sulfate ions was low,and the gypsum when the concentration of sulfate ions was high.(3)The sulfate erosion of cement-based materials were similar in different conditions,but the reaction product and the degree of erosion is different under the same ratio of the degree of erosion: sulfate solution semi-soaking> sulfate full immersion> sulfate full immersion in soil condition.(4)Considering the water-cement ratio,the sulfate concentration,the mineral admixture type and the content and the strength of the electric field applied,the prediction of sulfate resistance is built,which choose the compressive strength as the evaluation index.When the admixture is fly ash:a=3.24,?=-0.0069When the admixture is limestone powder:a=3.27,?=0.007According to the prediction model,the durability life of cement-based materials under sulfate and electric field conditions is less than 100 years under the experimental conditions.The addition of fly ash and limestone powder can extend and shorten the durability life.This experiment used the sulfate and electric field in different conditions to simulate the metro rail condition,and studied the change and mechanism of cement-based material properties in the condition,and established the model to predict the durability life of cement-based materials.However,the durability of cement-based materials Life is the result of a variety of factors,in addition to sulfate erosion and other factors,such as Cl-,carbonation,load,etc.,so we need further study to predict the durability life of subway rail condition cement-based materials.