An Accelerated Test Method For Low Temperature Sulfate Attack Of Cement Based Materials Using Electrical Field

Under the combined action of low temperature and sulfate,cement based materials will occur low temperature sulfate attack with various types of corrosion and complicated degradation behaviors.Typical degradation is represented by the thaumasite form of sulfate attack?TSA?which causes the materials to be non-adhesive.The external sulfate migrates into the material and reacts with the hydration products to form ettringite and gypsum,causing the material to crack or peel off.when the presence of carbonates,the formation of a more harmful muddy product,thaumasite,causes the cementitious material to lose gelation.The process of this series degradation poses a great threat to the cement concrete of constructions.Some studies have shown that the lower the temperature,the easier it is to form TSA,but there is no suitable accelerated test method for low temperature sulfate attack,which makes it difficult to quickly evaluate the material's resistance to low temperature sulfate attack.The feasibility of accelerating low temperature sulfate attack by electric fields has been proven,but the influence of the key factors must be clarified to determine a relatively uniform test standard if an electric field is used to establish an accelerated test method.Therefore,the aim of this paper is accelerating the low-temperature sulfate attack of cement-based materials by applying an electric field,and studying the degradation behavior and mechanism of low temperature sulfate attack under different electric fields,sulfates and electrode materials.The main research contents and results of this paper are as follows:?1?In this paper,the effects of voltage,period and current direction for electrical fields on the accelerated method were studied by the appearance damage observation,compressive strength test and microscopic composition analysis of the specimens.The results show that in the comparison of electric fields with different parameters,the efficiency of accelerated degradation and the severity of TSA increases with voltage increasing and period shortening.In the comparison of different current directions for electrical fields,the DC pulsed electrical field can accelerate degradation more effectively.Therefore,DC pulsed electrical field with high-voltage and short-period is more suitable for accelerated test methods.?2?In this paper,the effects of different sulfates on the appearance damage and strength loss of the material under electric field were tested.The phase composition changes were detected by FTIR and Raman spectroscopy.Finally,the reaction regularity under different sulfate conditions was analyzed by XRD.The results show that due to the combined impact of SO₄^2-and Mg²⁺,the magnesium sulfate solution is more favorable to accelerate the low temperature sulfate attack under the single sulfate solution condition.In the comparison of the accelerated efficiency of the mixed sulfate,the Na₂SO₄-CaSO₄-MgSO₄ mixed solution can more effectively accelerate the degradation of the specimens.To synthesize the degraded performance of specimens under different sulfates solutions,it is recommended to select Na₂SO₄-CaSO₄-MgSO₄mixed solution for the sulfate attack medium in the accelerated test method.?3?In this paper,the effects of electrode materials on the accelerated testing are discussed by measuring the corrosion resistance and stability of different metallic electrode materials and combining the performance changes of the degradation of specimens.The titanium electrode has the best corrosion resistance and stability under corrosion conditions and can accelerate the sulfate attack reaction more quickly,showing the advantage in the accelerated test method.?4?In this paper,the properties of materials such as appearance and strength during accelerating testing under different W/B ratio conditions were tested and the effects of W/B ratio on degradation of sulfate attack were investigated by microanalysis.The results show that the material is subjected to severe TSA under the conditions of higher W/B ratio for 0.40 and 0.30.When the W/B ratio decreases to 0.20,the risk of low-temperature sulfate attack is weakened,but when the W/B ratio is as low as 0.16,more obvious damage will occur.In this regard,when evaluating the low temperature sulfate attack of materials in the accelerated test method,the degree of TSA degradation is suitable for high W/B ratio materials,and the deterioration of ettringite and gypsum is more suitable for the low W/B ratio materials.According to the results of this paper,the feasibility of the electric field using as an accelerated test method for low-temperature sulfate attack of cement based materials can be further determined,and the influence of three key factors of electric field,sulfate and electrode materials in the accelerated test method was clarified.At the same time,the reference basis for the evaluation under different W/B ratio conditions is given.Through this research,it also laid a foundation for the establishment of an accelerated test method for low temperature sulfate attack within cement based materials.