Degradation Mechanism Of Cement-based Materials Exposed To The Coupling Effects Of Stray Current And Sulfate

In our country,urban subway is developed rapidly.There is often aggressive media in the soil and groundwater around the subway structure.Especially,in saline soil or coastal areas,the concentration of aggressive ion is very high,which will seriously jeopardize the durability of concrete.Meanwhile,the DC traction power supply mode adopted by subway and the degradation of the insulation performance between rail and ground lead to the generation of“stray current".Therefore,this paper studies the durability of cement-based materials exposed to the stray current and aggressive ions.The degradation process and mechanism of cement-based materials under the coupling effects of stray current and aggressive media were studied and analyzed.Diffusion-migration-reaction equation of SO42transport process was established.Rubber powder was added into mortar to improve the performance of mortar against the coupling effects of stray current and sulfate.The main research contents and results are as follows:(1)The degradation mechanism of cement-based materials under the coupling effects of stray current and sulfate was studied.According to the changes of macroscopic properties,transport performance and micro structure of deteriorated specimens,the influence of stray current on the transport process of SO42-and the production process of ettringite were analyzed.The results showed that stray current accelerated the sulfate attack of cement-based materials.On one hand,stray current accelerated the transport of external SO42-into the material,on the other hand,stray current promoted the chemical reaction of sulfate attack to some degree,resulting in the formation of more degradation product,ettringite,in the material.In addition,the effects of external sulfate concentration,w/b ratio and cement type(C3A content)on sulfate attack in presence of stray current were studied.The results showed that the electric field force induced by stray current had more significant effect on the SO42transport process than the concentration gradient,and resulted in the change of the chemical reaction process of sulfate attack from "diffusion(migration)control" to "reaction control".The w/b ratio had a decisive influence on the rate of SO42-ingress into the specimen.With the increase of w/b ratio,the concentration of SO42-in the specimen also had an increase,and the specimen was subjected to sulfate attack more seriously.High sulfate resistant cement contains less C3A content than P·Ⅰ type Portland cement,therefore,it showed a good performance to resist the coupling effects of stray current and sulfate.(2)The degradation mechanism of cement-based materials under the coupling effects of stray current,chloride and sulfate was studied.The concentration ratio of Cl-/SO42-(0,0.5,1 and 2)was taken as variables,40V stray current was applied to specimens.According to the changes in the macroscopic performance,transport performance and microstructure of the deteriorated specimens,the degradation process and degradation mechanism of cement-based materials under the coupling effects of stray current,chloride and sulfate were analyzed.The results showed that Cl-can effectively inhibit SO42-ingress into the specimen,resulting in less SO42-concentrated in the specimen.Cl-could react with C-A-H to form Friedel’s salt,which reduced the content of Al-phase involved in the chemical reaction process of ettringite formation,and could significantly reduce the degradation of cement-based materials caused by sulfate attack.However,when the concentration ratio of Cl-/SO42-exceeded 1,the mitigation effect was limited.(3)Transport equation of SO42-drived by stray current in cement-based materials was established,and the SO42-transport process was simulated.At first,the transport driving force,the expansion caused by the chemical reaction process of sulfate attack,and the influence of cracks caused by the expansion effect were analyzed.On this basis,the diffusion-migration-reaction equation of SO42-transport in cement-based materials was established.COMSOL Multiphysics software was used to model SO42-transport process.Then,the effects of diffusion coefficient,chemical reaction constant,type of Al-phase,stray current and external sulfate concentration on SO42-transport process were analyzed.The effects of different levels of stray current and different types of cement(C3A content)on the transport process and distribution of SO42-in the material were simulated.The simulation results and experimental results were analyzed comparatively.The results showed that diffusion coefficient,chemical reaction constant,stray current and external sulfate concentration all had significant effects on transport process and distribution of SO42-.While different Al-phases have different effects on distribution of SO42-.The inhibiting effect of AFm(monosulfate)on SO42-transport was the largest,followed by C3AH6 and C4AH13 and the influence of unhydrated C3A was the least.A comparative analysis of simulation results and experiment results showed that the two results were consistent at the initial stage,but in the later stage,due to the significant deterioration of the materials,the influencing factors increased,which resulted in a bad consistency between the two results.However,the tendency of the two results was almost consistent.It proved that the transport equation was reasonable and effective.(4)Based on the electrochemical impedance spectroscopy(EIS)method,the properties of rubber powder mortar exposed to the coupling effects of stray current and sulfate were studied.According to the equivalent circuit(RS+Q1/(Rct1+W1)+Q2/(Rct2+W2)),the electrochemical parameters of rubber powder mortar were obtained by the method of equivalent circuit fitting,and the effects of rubber powder with different contents and different fineness on EIS parameters of cement mortar were analyzed.And the durable mechanism of rubber powder cement mortar was analyzed.The results showed that the strength of mortar decreased with the increase in the content of rubber powder,but the pore solution resistance Rs and electron transfer process resistance Rct1 had significant increase.Moreover,both Rs and Rct1 increased with the dosage of rubber powder,and 200 mesh rubber powder had a more significant improvement on the resistance of mortar than 80 mesh.The compressive strength of the mortar rubber powder showed that the rubber powder could improve the degradation resistance of the mortar,and the 200 mesh rubber powder mortar had better degradation resistance than the 80 mesh rubber powder mortar.Additionally,it was found that EIS parameters Rs and Rct1 had relationship with the compressive strength of rubber powder mortar during the degradation process,EIS parameters could be used to forecast the change rule of compressive strength of rubber powder mortar.In summary,the results of this paper showed that the presence of stray current could accelerate the external SO42-move into cement-based materials.Meanwhile,it could also promote the chemical reaction process of sulfate attack,resulting in an increase in the production of ettringite.Therfore,stray current could accelerate the deterioration of cement-based materials.Cl-could inhibit the transport of SO42-into the specimen,which could alleviate sulfate attack.Rubber powder could significantly improve the electrical resistance of cement-based materials,thereby improving the resistance of cement-based materials.This article provided a theoretical basis for the durability design of subway structure.