Binding Mechanism Of Chloride Ion By Cement Minerals And Their Hydration Products

The corrosion of reinforced concrete in service often occurs due to chloride ion erosion,which leads to the failure of reinforced concrete structure,especially in Marine engineering.Therefore,it is very important for reinforced concrete to improve its ability to resist chloride ion erosion,which is also important to its service life.The chloride ion in concrete was introduced by the raw material itself on the one hand,such as sand and mixing water.On the other hand,the chloride ion in the external environment can enter into concrete structure during its service by the way of permeation and so on.Reducing the content of chloride ion in concrete materials will fundamentally improve the resistance of reinforced concrete to chloride ion erosion,and effectively avoid the risk of corrosion of steel bars.This paper is based on the"13th Five-Year"national key research projects-The research and application of chloride binding and new resistance technology in sea sand concrete,and starts from the cement component in concrete and studies several typical minerals as well as their hydration products,mainly including tricalcium aluminate?C₃A?,calcium monosulfoaluminate hydrate?AFm?and tricalcium silicate?C3S?.At the same time,the influence of fineness of C3A on chloride binding capacity,the influence of Ba²⁺ion on the chloride binding capacity of AFm,the influence of modified fly ash?M-FA?after added to C₃S on chloride binding capacity and so on are also studied.Furthermore,multiple testing methods are used to investigate their binding mechanism,which provides the theoretical support of chloride ion bound by concrete itself and paves the way for looking for new type of chloride binding materials.The results of this paper show that:?1?At the same age,the chloride binding capacity of C₃A increased with the increase of chloride concentration in solution as a whole.At the same concentration of chloride ion,the chloride binding capacity of C₃A first increased and then stabilized with the extension of curing age.Under the design experimental conditions,the limit of chloride content bound by C₃A reached 0.16 g/g-C₃A.The products of C₃A combined with chlorine ions included Friedel's salt and its solid solution,and the structuralformulaofFriedel'ssaltanditssolidsolutionwas[Ca₂Al?OH?₆]₂·?2Cl?·?4H₂O?and Ca₂Al?OH?₆·?Cl?·?2H₂O?,respectively.The formation mechanism of these two products included chemical adsorption and ion exchange mechanism,and the two kinds of mechanism were independent of each other,where the chemical adsorption dominated.?2?At the same age,the chloride binding capacity of C₃A increased with the increase of its fineness as a whole.Under the same fineness,the chloride binding capacity of C₃A increased as the curing age prolonged.Furthermore,the smaller the fineness of C₃A was,the more the content of Friedel's salt and its solid solution was,and the less the content of unhydrated C3A was.?3?At the same age,the chloride binding capacity of AFm increased with the increase of chloride concentration in solution as a whole.At the same concentration of chloride ion,the chloride binding capacity of AFm increased with the prolongation of curing age.Under the design experimental conditions,the limit of chloride content bound by AFm reached 41.16 mg/g-AFm.The products of AFm after reacting with chloride ion mainly included Friedel's salt and its solid solution,Kuzel's salt and AFt,as well as some unreacted AFm.The SO₄^2-anion between the interlayer of AFm could exchange with the chloride ion in solution,thus forming the Friedel's salt and its solid solution or Kuzel's salt.The chemical formula of Kuzel's salt was C₃A·0.5CaCl₂·0.5CaSO₄·11H₂O,and the interlayer structure could be expressed as[Cl_0.5·?SO₄?_0.25·2.5H₂O]^-.?4?After the introduction of Ba²⁺ion into the solution with the same concentration of chloride ion,the chloride binding capacity of AFm increased with the increase of Ba/S molar ratio as a whole.Under the same Ba/S molar ratio,the chloride binding capacity of AFm increased with the prolongation of curing age.Under the design experimental conditions,the limit of chloride content bound by AFm reached57.57 mg/g-AFm,which was higher than the limit of 41.16 mg/g-AFm without Ba²⁺ion,indicating that under certain conditions the introduction of Ba²⁺ion was beneficial to increase the chloride binding capacity of AFm.The products of AFm after the introduction of Ba²⁺ion mainly included Friedel's salt and its solid solution,Kuzel's salt,AFt and BaSO₄.?5?At the same concentration of chloride ion,the chloride binding capacity of C3S increased as the curing age prolonged.The chloride binding effect of C3S was mainly attributed to the physical adsorption of C-S-H gel which was formed by the hydration of C₃S.At the same dosage of M-FA,the chloride binding capacity of C₃S increased with the extension of curing age.With the dosage of M-FA increasing,the chloride binding capacity of C₃S increased at 3 d and 7 d as a whole,while first increased and then decreased at 28 d and 60 d.After adding M-FA,the chloride binding capacity of C₃S was higher than that without M-FA.?6?When the dosage of M-FA reached 60%,the hydration products of C₃S included C-S-H gel,CH,unhydrated C3S,CaCO3,and Friedel's salt and its solid solution.The active Al2O3 in M-FA reacted with CH,and formed hydrated calcium aluminate,which was converted into Friedel's salt and its solid solution after combined with chloride ion.