Experimental Study On The Dissolution Characteristics Of Cement-based Materials Under The Action Of Low-velocity Water Flow

In the long-term service in the flowing water environment,the concrete structure such as the bridge pier,the dam is subjected to the damage caused by calcium leaching and hydraulic attack.On the one side,for the concrete,the hydration product such as calcium hydroxide and C-S-H gel is dissolved due to the calcium leaching,which making the macrostructure and microstructure of the concrete degraded.As a result,the mechanical properties of the surface concrete such as shear strength is reduced.One the other side,the flowing water can provide the hydraulic attack on the surface of concrete.With the reduction in concrete strength caused by calcium leaching,the surface of the concrete will pell off since the surface concrete can not resist the hydraulic attack any more.Then,a layer-by-layer failure of concrete on surface of the structure will occur.As a result,for the concrete structure and membe,the service performance is reduced and the service life is shortened.Therefore,in order to reveal the degradation mechanism in physical and chemical performance and evaluate the leaching resistance and service life of the concrete,it is of great significance to study the leaching behavior of the concrete in the flowing water with low velocities.In this paper,a circular water conveyance experiment setup was designed.By using the setup,the cylindrical cement paste/mortar instead of the concrete was taken as the research object to carry out the accelerated corrosion experiment in the static 1M ammonium chloride solution and the flowing 1M ammonium chloride with the velocitie of 0.5m/s,1.0m/s,2.5m/s and 5.0m/s.The leaching behavior evolution of the cement mortar and cement paste/mortar specimen changing with the flowing velocity of the solution and the corrosion time was studied and the following results and conclusions was obtained:(1)A setup that can provide the circulating flowing solution with velocities of 0.5m/s,1.0m/s,2.5m/s and 5.0m/s was designed according to the experiment and it was used to carry out the accelerated corrosion experiment on the cylindrical cement paste/mortar specimen in the 1M ammonium chloride solution.(2)The leaching depth and porosity of the cylindrical cement paste/mortar specimen under different corrosion conditions were tested by the phenolphthalein titration and water-saturated drying.It is found that,the development of the pore structure and the corrosion process is accelerated by flowing solution.When the corrosion time is same,the higher flowing velocity results in greater leaching depth and porosity.In the static solution,the leaching depth has basically a linear increase with the square root of corrosion time.But,for the specimen in the flowing solution,the relationship between the leaching depth and the square root of corrosion time is nonlinear,and the higher the flowing velocity of the solution,the higer non-linearity.(3)The phase composition,the microstructure morphology and the content of hydration products in the surface of the leached cement paste/mortar specimens under different corrosion conditions was respectively tested by the X-ray diffraction method,the scanning electron microscope method and the thermogravimetric analysis method.And it is found that,at the early stage of corrosion,there exists few unhydrated phases such as belite and alite and the flowing solution accelerates the hydration process of the cement paste/mortar.What is more,in the flowing solution,when the the flowing velocity of the solution increases,for the surface of the specimen,the content of calcium hydroxide is decreased,and its microstructure morphology is gradually loosen.But,at the later stage of corrosion,calcium hydroxide in the surface of the cement paste/mortar specimen is basically leached and the conctent(mass percentage)of C-SH gel is reduced with the increase in flowing velocity.Additionally,a denser microstructure morphology can be observed on the specimen surface as the flowing velocity increases.(4)Under the corrosion action by the static solution,the content of the hydration product in the cement paste/mortar specimen surface reduces with the increase in corrosion time.But,in the flowing solution,with the increase in corrosion time,the content of calcium in the specimen surface decreases first and then gradually stabilizes to a constant value,which is defined as the critical calcium/solid calcium content when the surface damage on the cement paste/mortar specimen occurs.The critical calcium/solid calcium content is decided by the flowing velocity of the solution,and the higher content is corresponded to the higher flowing velocity.Finally,based on the fitting of the experimental data,for the cement paste/mortar specimen in the 1 M ammonia chloride solution,the function relationship between the critical calcium/solid calcium content of cement paste/critical solid calcium of cement mortar and the flowing velocity of solution is obtained.(5)Based on experience formulas,the energy conservation law and the ANSYS simulation,the shear force generated by the flowing solution with different velocities on the surface of the cement paste specimen is obtained by theoretical calculation and ANSYS simulation.And it is found that,in the flowing solution with low velocities,values of the shear force calculated by experience formula is basically in agreement with the results obtained by ANSYS.But when the flowing velocity increases,the difference between the two becomes greater.