Two-stage Corrosion Model And Experimental Study Of Cement Mortar Under The Action Of Dilute Sulfuric Acid

Atmospheric pollution,industrial production and microbial decomposition will cause sulfuric acid erosion to concrete structures.The degree and rate of erosion of cement-based materials such as concrete under the action of sulfuric acid are mainly affected by the pH value and mode of action of sulfuric acid.Therefore,research on cement-based materials the characteristics of sulfuric acid corrosion under different pH values and different modes of action are of great significance.In this paper,the long-term soaking test method is used to study the dissolution process of cement mortar under the action of sulfuric acid.Starting from the two-stage erosion process of sulfuric acid attacking cement-based materials,a long-term soaking test of two batches of cement mortar specimens in sulfuric acid solution is carried out to obtain the sulfuric acid corrosion characteristics of cement mortar under different pH values and different modes of action.Numerical simulation calculations based on experimental analysis are performed to verify the reliability of the numerical model.The specific research content is as follows:The first batch was an experimental study of cement mortars with different water-binder ratios under the action of sulfuric acid at pH=4.0.The test used simple immersion and replacement of the immersion solution and cleaning the surface of the test pieces as two experimental conditions.The curves of the acid consumption of cement mortar specimens with soaking time were obtained.And the surface morphology and mineral composition of the specimens after soaking were obtained by SEM and XRD.The results showed that all cement mortar samples did not show calcium sulfate precipitation during the soaking process,and the corrosion control mechanism was hydrogen ion dissolution.The composition of the surface layer,mainly composed by SiO₂,are the residual minerals after mortar leaching.Whether to remove the residue on the surface of the test piece has an important effect on the corrosion rate.The acid consumption of the test piece under surface cleaning conditions is about 16% to 25% higher than that of the test piece without surface cleaning.In order to study the effect of pH value and experimental conditions,the second batch of tests were carried out.The experimental research of cement mortar with different water-binder ratios under different pH sulfuric acid effects was set according to whether to clean the surface of the test piece and whether to replace the soaking solution.Three different experimental conditions were given.The law of the acid consumption of cement mortar with the power function of immersion time was obtained in the test,and the mineral composition and morphology of the surface layer of the test specimen after erosion were measured by SEM,XRD,and EDS.The results show that the dissolution process of cement mortar in the presence of sulfuric acid has two stages:before the precipitation of calcium sulfate and after the saturation precipitation of calcium sulfate.The first stage shows obvious hydrogen ion dissolution characteristics.The surface of the specimen is mainly cement stone which has been dissolved to varying degrees.When the remaining residue is sufficiently dissolved,it is mainly SiO₂.At this stage,when the pH value of the soaking solution is decreased by 1.0,the sulfuric acid consumption can be increased to about 6.5 to 7.5 times,which has a roughly stable similar scale.In the second stage,after the calcium sulfate reaches the saturation concentration,the surface of the specimens are mainly calcium sulfate.Calcium has a "blocking" effect on the erosion process to delay the erosion rate.There is no roughly stable proportional relationship between the increase in the cumulative sulfuric acid consumption of the specimen and the decrease in the pH value of the soaking solution.Based on the experimental analysis,further numerical simulation calculations were performed.Based on the calcium dissolution partial differential equation,an unconditionally stable implicit difference method was constructed for numerical solution.A concentration profile correction method was introduced to simulate the cleaning process in the experiment.The dissolution process of cement mortar specimens was analyzed,and the variation law of the simulated acid consumption of the specimen with time and the distribution of the solid phase calcium ion concentration were obtained.The change of the simulated acid consumption of the specimen with the experimental measured value over time was compared and analyzed to verify the reliability of calcium dissolution model.