Study On The Mechanism Of Chloride Binding Of LDHs And Its Effect On The Corrosion Behavior Of Steel Bars

Chloride-induced corrosion of steel bars is the most critical factor leading to premature failure of reinforced concrete structures in marine environments.Layered double hydroxides（LDHs）can enter the interior of concrete in aggressive media（chloride ions,etc.）to achieve effective binding.The structure is similar to that of cement hydration products.It has become a research and application hotspot in the field of building materials.However,the current understanding of the mechanism of chloride binding in LDHs is not deep enough.For example,the structural composition of the LDHs（the type of divalent metal cations,the ratio of divalent to trivalent cations）will affect the ability to chloride binding to a certain extent,and the application of reinforced concrete in the actual environment has carbonation problems.In addition,the LDHs has the problems of easy agglomeration,few surface functional groups.Therefore,it is the focus of this study to further explore its mechanism of chloride binding in cement-based material systems while enhancing the active functional groups on the surface of LDHs to provide more sites for chloride ion adsorption.In this project,three types of LDHs（MgAl-LDH,CaAl-LDH,ZnAl-LDH）were synthesized by hydrothermal synthesis.Based on the interlayer ion exchangeability of LDHs materials,compared the ability of different types of LDHs to consolidate chloride ions in different solutions（deionized water,ordinary simulated concrete pore solution,carbonized simulated concrete pore solution）,the effects of the laminate structure of LDHs on the abilities of chloride binding were studied,and the mechanism of chloride binding by LDHs was discussed in depth.In addition,the LDHs was modified with sodium lignosulfonate.The ability to bind chlorides before and after modification and the inhibition efficiency of steel bars were compared,it was explored to be added to cement as an additive,and to explore the appropriate amount of additives as additives to cement-based materials.Main results can be summarized as follows:（1）The three types of LDHs（MgAl-LDH,CaAl-LDH,ZnAl-LDH）prepared by hydrothermal synthesis method.had a typical hexahedral layered structure.Compared with MgAl-LDH and CaAl-LDH,ZnAl-LDH had the largest interlayer spacing and the highest ability to bind chlorides.As the proportion of the divalent to trivalent metal ion ratio increased,the charge density of the laminate decreased,and chloride binding capacity also decreased.（2）The mechanism of chloride binding of LDHs included two mechanisms of ion exchange and surface adsorption and mainly based on ion exchange mechanism.The chloride binding capacity of LDHs in simulated concrete pore solution was slightly lower than that in deionized water,while simulated carbonation of the simulated concrete pore solution resulted in debonding of the chloride bond by ion exchange.However,the chloride bond by surface adsorption was not affected by carbonation.It was consistent with the adsorption law in the aqueous solution of Na Cl,that was,the saturation adsorption capacity of Zn（2）Al-NO₃ was the largest.（3）Compared with MgAl-LDH and CaAl-LDH,the addition of Zn-LDH would seriously inhibit the hydration of cement in ordinary Portland cement systems.When the curing age was 28 days,the compressive strength of mortar with 2%,4%,and 6%Zn-LDH was only 7 MPa,6.4 MPa,and 6.3 MPa.The incorporation of Ca-LDHs would promote the hydration of cement,which was conducive to the development of strength.Considering both the amount of consolidated chloride ions and the impact on the basic properties of cement-based materials,the addition of Ca-LDH had the best performance on cement-based materials.（4）Compared to unmodified Ca-LDH,Ca-LDH modified by sodium lignosulfonate（Ca-SLS-LDH）had a smaller average particle size and better dispersion performance.The specific surface area after modification was increased from 1.367 m²/g to 84.428m²/g,and the saturated adsorption capacity of chlorides was increased by 0.433 mmol/g.Under the same dosage,the critical chloride concentration value of the steel bars in the simulated concrete pore solution could be effectively improved,and with the increased of the dosage,the corrosion resistance of the steel bar was stronger.（5）The low content of Ca-LDHs had a certain improvement effect on the pores of cement-based materials.The increase of hydration products was beneficial to the development of the compressive strength of mortar.And when the amount was 2%,the improvement of the performance of cement-based materials was optimal,but excessive incorporation would lead to increasement in porosity and strength.Ca-LDH modified by sodium lignosulfonate（Ca-SLS-LDH）had a slight retarding effect on cement-based materials under the action of lignosulfonate when the dosage was 2%.However,with the increase of the dosing amount,the water absorption rate was high due to the large specific surface area of Ca-SLS-LDH.Compared with the reference group,the setting time was shortened,and excessive addition would also produce fine ettringite（AFt）,which was not conducive to strength development.Therefore,the content of Ca-LDHs in cement-based materials should be controlled at 2%to 4%of cementitious materials.