The Compatibility Of Oxides In Cementitious Materials Under Chloride Attack And The Mechanism Of Interaction Between Oxides And Eroded Ions

Cementitious materials are the most widely used building materials in the world.However,under the environment of ocean,deicing salt and salt lakes,the corrosion of concrete reinforcement caused by chloride erosion is one of the most important reasons for the failure of concrete structures.At present,the linear relationship between the aluminum content and the chloride binding capacity of cementitious materials has been recognized.It is expected that the chloride binding capacity can be estimated by the oxide content of cementitious materials.However,in the engineering environment,chloride often coexists with sulfate,and the influence of sulfate ion on chloride erosion is very complex.Therefore,to study the mechanism of chloride corrosion on cementitious materials in chloride solution and chloride-sulfate composite solution,and to reasonably design the components of cementitious materials resistant to chloride corrosion,as well as to reduce the chloride corrosion caused by marine environment and salt lake brine on cementitious materials meets the major needs of Chinese"Marine Strategy"and"Sustainable Development Strategy".In this paper,the main oxide content of cementitious materials is replaced by two mineral admixtures（slag and fly ash）.The chloride binding amount of the samples was determined by isothermal adsorption method,the morphology of the corrosion products was observed by field emission environmental scanning electron microscope（ESEM）,and the corrosion products were quantitatively characterized by X-ray diffractometer（XRD）and the synchronous thermal analyzer（TG-DSC）.The quantitative relationship and microscopic mechanism between the oxide content and the chloride binding amount of cementitious materials,as well as the interaction between sulfate ion and chloride ion were explored.First of all,it is found that the chloride binding capacity of cementitious materials is positively related to the Al₂O₃and SiO₂content,but inversely related to the CaO content under the attack of chloride solution.Moreover,when the same sample with the same oxide content is immersed in different concentrations of chloride solution,the ability of physically adsorbing chloride ions with high salt concentration is enhanced,and the ability of chemically combining chloride ions with high salt concentration is slightly reduced.Secondly,the effect of sulfate on the ability of hardened cementitious material to bind chloride ions was studied.The results show that the presence of sulfate will reduce its ability to bind chloride ions.When the concentration of sulfate is low（0.5%Na₂SO₄）,the amount of chemically bound chloride ions is not affected,but the total amount of chloride ions is slightly reduced,there is no obvious competition between the two ions.When the sulfate concentration is increased to 5%Na₂SO₄,the binding capacity of chloride ions decreases significantly,which proves that the competitiion between the two ions is significant,but the existence of sulfate does not affect the correlation between the oxide content and the chloride binding capacity of the cementitious materials.The ternary linear equation of the chloride ion binding capacity of cementitious materials with 5%NaCl solution and 5%NaCl+5%Na₂SO₄solution was established.It can be used to calculate the chloride ion binding capacity of cementitious materials with different oxide content compatibility.Finally,for the cementitious material system with strong chloride binding capacity,the isotherm model simulation is used to compare with the test data,and it is found that when the standard curing age is 28 days,the relationship between the bound chloride concentration and the free chloride concentration of sample is more consistent with Freundlich isotherm.Moreover,the model calculation data of the main hydration products of cementitious material on the chloride binding amount is close to the measured data,it is proved that this model can not only predict the chloride binding amount of cementitious material in different concentration of salt solution,but also put forward the oxide in cementitious material resistant to chloride corrosion according to the predicted chloride binding amount compatibility.