Investigation On Binding And Microstructure Of Chloride Ions During Transport In Cement-based Materials

The ma in reason that re inforced concrete structures are dama ged is the corrosionof steel bar caused by chloride ions. There are two kinds of chloride ions ince ment-based materia ls: b ind ing chloride io ns and free ones. Only the free chlorideions are respons ib le for steel bar corrosion. However chloride io ns are essential topredict service life of concrete structures, the two ma in reasons are:1st, it prevents thepotentia l flow o f chloride io ns, that is to say, it can dela y the time of the chloride ionspenetration into the stee l surface,2n d, it ca n form Friedel’s sa lt, whic h can reduce theporosity of ce ment-based materia ls, thus improving the res istance of chloride ionspenetrating into cement-base materia ls. Therefore, when it comes to the durabilitydesign of re inforced concrete structures in chloride enviro nment, the chloride ionbind ing capacity must be taken into consideratio n studying the trans miss io n ofchloride io ns in ceme nt-based materia l. This paper aims to study chloride b ind ing andits effects on the microstructure and properties of cement-based materials. The ma inwork and achievement are as follows：Based on orthogo nal designs, chloride ion b ind ing capacit ies of the mortarssubject to d ifferent factors were determined by the isotherma l absorptio n method. Theresults showed that the chloride ion binding capacity of mortar increased as thewater-cement ratio, curing period and chloride ion concentratio n increase d, decreasedas the soaking temperature and carbonizatio n time increased. As for s lag content,however, whe n the s lag content increased, the chloride ion bind ing capac ity o f mortarincreased at first and then decreased. When sla g content is40%, the chloride ionbind ing capacity of mortar reaches the highest va lue. The chloride ion concentrationis the most outsta nding factor that influences chloride ion b inding capacity o f mortar,as well as water-cement ratio and carbonation.Chloride ion equilibrium concentration of mortar exposed to sodium chloridesolutio ns with different concentrations were determined by the isotherma l absorptionmethod. Results showed that carbonization and low te mperature d id not cha nge thelaw of chloride io n nonlinear bind ing. Carbonization and water-cement ratio affectedchloride ion b ind ing capac ity of mortar obvious ly, but temperature a lmost did notaffect chloride ion binding capacity of mortar.The formation of Friedel’s salt is the main chemical binding, and physical adsorption is ma inly contro lled by C-S-H ge l. This paper determines the c hanges ofmicrostructure after the chloride bind ing of cement-based materia ls by therma lanalys is and vo latile water conte nt method. Results showed that chloride ion bind ingcapacities of the pastes increased as the concentratio n of free chloride ions increased.The porosity increased as the concentratio n of soaking chloride ions increased. Theporosity was proportional to the production of Friedel’s salt.Fina lly, this paper studied the influe nce of mixed chloride ions o n performanceof mortar, inc lud ing the fluidity of fresh mortar, compressive strength, permeabilityand electrochemica l properties.It is shown that chloride io ns can improve the earlycompress ive strength of the mortar, but weakens the long-term strength. Chloridecationic spec ies also affects mortar compressive strength obvio us ly, mixed chloridecan decrease the permeability o f mortar, the Bode angle o f AC impedance decreasedas the content o f mixed chloride ions increases, but more researches for quantitativeanalysis are essential.