Test And Analysis Of Sulfate Erosion Process Of Cement-based Materials

Concrete structure is one of the most common structural forms in construction engineering currently.The porosity of the concrete material and the complexity of the structure service environment make it more susceptible to erosion of harmful media in the environment,leading to severe durability problems.Sulfate could induce expansion and cracking of the concrete materials,which facilitates the other harmful ions into the concrete structure and accelerates sulfate attack process;in order to clarify the mechanism of sulfate attack,it is necessary to trace the transport of sulfate ions inside the concrete,which is an valuable research for improving the durability of concrete;therefore,sulfate attack is an important part of the study of unsolved issues in durability.To characterize the deterioration of concrete caused by sulfate attack,this experiment uses non-destructive electrochemical impedance spectroscopy to measure the degree of cement paste damage induced by sulfate attack,aiming to provide in-situ non-destructive monitoring method for cement concrete sulfate erosion test.In this paper,the electrochemical impedance spectroscopy?EIS?model was established to explain the sulfate erosion process of cement paste.Impedance spectroscopy behavior of cement paste under two erosion modes of long-term immersion and electric pulse,different water-cement ratio and different concentration of corrosive solution was systematically and thoroughly studied.Combining the results of X-ray diffraction?XRD?,scanning electron microscopy?SEM?,X-ray fluorescence spectroscopy?XRF?,X-ray micro-tomography?X-ray microtomography?and impedance spectroscopy fitting,the sulfate erosion process of cement paste is analyzed.The following conclusions are drawn:?1?An electrochemical impedance spectroscopy?EIS?method was used to nondestructively track the impedance spectra of sulfate corrosion process of cement paste samples.by taking into account the solid/liquid double-phase interaction,the impedance spectra of cement paste samples were fitted by equivalent circuit model R_S(Q₁(R_ct1W₁))(Q₂(R_ct2W₂)).The impedance arc radius and the equivalent impedance curve increased firstly and then decreased.?2?In the long-term immersion test,the impedance curve of R_ct1 varies with the water cement ratio and the concentration of the corrosive solution,but the impedance curve of R_ct1 increases first and then decreases(regardless of the beginning of the decline of R_ct1t1 value).The smaller the water cement ratio is,the longer the rising section of R_ct1 impedance curve is,and the more compact the R_ct1 impedance curve is,to a certain extent,hindering the sulfate ion from entering the sample and delaying the sulfate corrosion process.?3?Under long-term immersion and accelerated electric pulse test,the impedance curve of Rct1 shows a trend of rising first and then decreasing,which reveals two stages of pore evolution in sulfate erosion process?without considering the Rct1 value beginning to decline?.The first stage is the pore filling stage of erosion products,that is,the rising stage of Rct1;the second stage is that the expansion stress caused by erosion products is greater than the tensile strength,resulting in cracks,and the damage evolution is serious,that is,the decline stage of Rct1.At the same time,it reveals that the sulfate erosion process has undergone a dense-cracking process.?4?Under the accelerated test of electric pulse,the strength of S element on the erosion surface increases with the erosion time.XRD composition of the slice powder is analyzed.The formation of ettringite and gypsum is found.The corrosion products and cracks are found by SEM,which proves that electric pulse can accelerate the rapid entry of sulfate ions into the sample.Continuous accelerated erosion,SEM and X-ray Ultra-CT results showed that the corrosion products accumulated in the cracks and pore again,resulting in micro-cracks again,leading to further deterioration of the sample.