Experimental Study On The Boundary Layer Effect In The Process Of Cement Mortar And Concrete Corroded By The Dilute Sulfuric Acid

The building will be corroded and shortened the life of the service by Sulfuric acid.Structures had been corroded by Chemical processes such as atmospheric pollution,industrial production,weathering of sulfur-containing minerals,and the metabolic processes of sewage treatment plants,sewage pipelines,and microbials in farms.Because of its own particularity,sulfuric acid will cause adhesion layer to produce boundary layer effect during erosion of cement mortar or concrete,resulting in the decrease of sulphuric acid erosion rate and the increase of acid resistance of cement mortar or concrete.Therefore,studying the effect of sulfate layer on cement mortar or concrete is very important for building durability.This paper focuses on the role of boundary layer effect in the process of dilute sulfuric acid corrosion in cement mortar and concrete.The main contents and methods of the study are as follows::(1)due to the boundary layer effect of sulfate erosion of concrete and cement mortar in the process will affect the sulfate erosion rate,so as to describe the effect of boundary layer effect in sulfate erosion of concrete and cement mortar in the process,combined with the reaction boundary layer model and the diffusion reaction equation,the theoretical formula of acid consumption,analysis calculation method of apparent diffusion coefficient conversion coefficient,and to express the material changes of the apparent diffusion coefficient in different time.(2)the influence of surface layer effect in the process of dilute sulfuric acid corrosion of cement mortar is studied.Test method for division 2 immersion ways for 2300 h long term immersion test of different water cement ratio of cement mortar,in dilute sulfuric acid solution of PH=4 as the solution,to eliminate the boundary layer accumulation,one batch of specimens taken long-term soaking soak the regular replacement of calcium sulfate cannot be accumulated precipitation formation the boundary layer,another batch of specimens as the control group was taken without replacing the soak of long-term conventional immersion mode.The consumption of sulfuric acid in different periods of each specimen was obtained,and the apparent diffusion coefficient conversion coefficient of mortar material at different time was calculated.The relationship between the change of water cement ratio and the boundary layer effect during the soaking process was obtained by nonparametric regression analysis.The surface morphologyand phase composition of the sulfuric acid corroded cement mortar were further studied by using the X diffraction instrument and the electron microscope scanner.It was found that the apparent diffusion coefficient of the specimen decreased according to the power function of time,and whether the replacement of soaking solution would produce the boundary layer effect and the apparent diffusion coefficient to decrease,but the effect of the boundary layer effect would be reduced by changing the way of soaking liquid.The change of water cement ratio of cement mortar will hardly affect the effect of surface layer effect on the specimen.The early stage of the reaction does not precipitate calcium sulfate,it is silica alone as boundary layer to reduce the erosion rate of sulfate cement mortar,the reaction will begin after the precipitation of calcium sulfate for a period of time,this time by the silica and calcium sulfate as the boundary layer to reduce the erosion rate of sulfate sand slurry who.(3)the influence of surface layer effect in the process of dilute sulfuric acid corroded concrete is studied.The test method for 3 kinds of soaking methods for the same water cement ratio of concrete specimens for 1200 h long-term immersion test.Taking dilute sulfuric acid with different pH(PH=2,PH=3 and PH=4)as the immersion solution,in order to remove the boundary layer more thoroughly,based on the immersion test of cement mortar,a method of regularly cleaning the surface of the specimen to remove the attachment and replace the immersion solution is added.The sulfuric acid consumption of each concrete specimen at different time was obtained through experiments.On the basis of that,the apparent diffusion coefficient conversion coefficient of concrete material at different time was calculated with the dissolution reaction model.The surface morphology and phase composition of the sulfuric acid corroded concrete are further studied by using the X diffractometer,the electron microscope scanner and the energy spectrometer.The specimens of the apparent diffusion coefficient decreases exponentially in time,but by changing the way of soaking liquid,will reduce the impact of boundary layer effects,in the use of replacement soak regularly and brush the specimen surface to remove attachments way immersion conditions,can basically eliminate the influence of boundary layer effect.There is a close relationship between the surface layer effect and the pH value of the soaking solution.The boundary layer effects caused by the material for CaSO4 and SiO2 after 2H2 O.Calcium sulfate will not be precipitated at the early stage of the reaction.At this point,silica alone is used as a surface to reduce the erosion rate of sulphuric acid to concrete.In the middle and late reaction,the corrosion rate of sulphuric acid to concretewas reduced by the joint surface layer of silica and calcium sulfate.