Study On Sulfate Attack Process And Concrete Degradation Damage Under Multi-ion Environment

In the erosive environment,a wide variety of high-concentration salt ions migrate into the concrete,and corrode with the hydration products in the concrete,causing the initiation and growth of micro-cracks in the concrete,changing the microstructure in the concrete and forming damage,which eventually leads to the concrete.The deterioration and damage of the macrostructure can not even continue to serve.This is a complex physical-chemical-mechanical change process coupled with ion diffusion,chemical reaction and mechanical damage.In this study,a large number of experiments were conducted to study the ion migration and product distribution of concrete in the immersion of four high-concentration aggressive solutions of Na₂SO₄solution,Mg SO₄solution,Mg SO₄/Na Cl solution and Na₂SO₄/Na Cl/K₂CO₃solution,and the volume of concrete during erosion expansion and deterioration damage,and based on the meso level,revealed the volume expansion,crack propagation and deterioration damage laws of concrete under the action of different erosion solutions,which laid a theoretical foundation for clarifying the cracking and deterioration damage of concrete during the solution erosion process.1.The natural elastic modulus,mass loss of concrete of different ages in the immersion of four high-concentration aggressive solutions of Na₂SO₄solution,Mg SO₄solution,Mg SO₄/Na Cl solution and Na₂SO₄/Na Cl/K₂CO₃solution were studied by natural immersion damage and destruction;at the same time,the ion distribution law and the phase distribution law inside the concrete were studied.The following conclusions are drawn:(1)Under the action of Na₂SO₄solution,the ion concentration distribution decreases with the depth of erosion and increases with the age of erosion,and the change law is non-linear.The main corrosion products of gypsum and ettringite are the expansion stress generated by the formation and aggregation of the internal pores and aggregate-slurry interface transition area of the concrete,which resulting in the decrease in quality eventually leading to brittle fracture of the concrete body.The failure mode of concrete under the action of sodium sulfate solution is brittle fracture and spalling of concrete caused by penetrating cracks at the corners;(2)During the erosion process of Mg SO₄solution,Mg²⁺can reduce the surface ion concentration of SO₄^2-,but increase the relative content of SO₄^2-in the concrete.Mg SO₄solution raise pitting corrosion on the surface of the concrete,and flakes into powdery sand to form pits,and eventually a large amount of powder flakes off the slurry,forming a loose and porous honeycomb structure that extends from the concrete surface to the inside;(3)Cl^-pair can effectively delay the deterioration damage of concrete,but does not affect the type of concrete cracking and deterioration damage caused by Mg SO₄;(4)CO₃^2-can significantly hinder the transmission of ions into the concrete,which can effectively organize the sulfate corrosion and the formation of gypsum,and the formation of calcium carbonate leads to the densification of the concrete,which reduces the cracking and damage of the concrete.2.In view of the heterogeneity of concrete and the irregularity of migration of salt ions in concrete,three high concentrations of Na₂SO₄solution,Mg SO₄solution,Mg SO₄/Na Cl solution and Na₂SO₄/Na Cl/K₂CO₃solution were characterized by 3D-DIC system.Volumetric deformation under the action of aggressive solution:(1)3D-DIC technology reveals the non-uniform volume expansion of concrete during the erosion of different solutions.The volume expansion inside the concrete is not evenly distributed in all areas of the concrete,but as the erosion continues to deepen,it shows a non-uniform distribution.The volumetric deformation contour of concrete under the action of sodium sulfate solution presents a very obvious non-uniform distribution.On the same erosion horizontal line,the volume expansion in some areas is very obvious,but there is no obvious volume expansion in some areas,which fully explains the unevenness of the volume expansion distribution during the sulfate erosion caused by the unevenness of the concrete structure.The relationship between the expansion product content in concrete and the volume expansion of concrete under the attack of sodium sulfate solution is not a simple linear relationship;(2)When eroded for 60 days,the maximum volume expansion of concrete under Na₂SO₄solution and Mg SO₄solution are 0.159 mm/m and 0.129 mm/m respectively.The volume expansion is less than that of sodium sulfate solution system;(3)The presence of chloride ions does not change the migration and erosion laws of magnesium sulfate solution,but only delays to a certain extent;(4)The volume expansion of concrete in the multi-ion composite solution during each erosion age is relatively small.3.Based on the experimental research results,the Lattice model is used to simulate the volumetric deformation of concrete under the action of four high-concentration aggressive solutions of Na₂SO₄solution,Mg SO₄solution,Mg SO₄/Na Cl solution and Na₂SO₄/Na Cl/K₂CO₃solution at mesoscale.Crack development and deterioration damage,comparing the experimental results with the simulation results,are quite consistent.On this basis,the volumetric deformation,cracking and deterioration damage laws of concrete under long-term erosion conditions are simulated:(1)After the test results and fluorescent crack verification,the Lattice grid model can effectively simulate the volume expansion and cracking of concrete in erosion solution damage and predict the destruction of concrete under long-term erosion;(2)The displacement field of concrete under the action of Na₂SO₄solution is non-uniformly distributed,as the reaction proceeds,the volume expansion of concrete is mainly concentrated at the corners,while the middle area still remains intact;microcracks first germinate at the weak point of the interface transition area where the concrete aggregate is dense,and expand into the concrete along the direction of the aggregate and causing cracking and spalling of the outer concrete;(3)The displacement field of concrete under the action of Mg SO₄solution is non-uniformly distributed.The simulation results further illustrate the regional distribution of concrete surface cracks and the phenomenon of point spalling under the attack of Mg SO₄solution.Mg²⁺has a very obvious influence on the displacement field distribution and cracking damage of concrete during sulfate corrosion;(4)Under the action of Mg SO₄/Na Cl solution,the concrete displacement field distribution,crack distribution and final cracking damage law of concrete are similar to Mg SO₄solution.Cl^-has a significant delay and hinder effect on the displacement field distribution and crack damage of concrete during the erosion of Mg SO₄solution;(5)The displacement field distribution and crack growth pattern of concrete under the erosion of Na₂SO₄/Na Cl/K₂CO₃solution are similar to those of Na₂SO₄solution.The volume expansion and cracking of concrete in Na₂SO₄/Na Cl/K₂CO₃solution is lower than that of Na₂SO₄solution.In the early stage of corrosion,CO₃^2-hinders the volume expansion and deterioration of concrete,but in the long-term erosion process,CO₃^2-and SO₄^2-work together to cause deterioration and damage of concrete.