Study On Calcined Layered Double Hydroxide To Improve Durability Of Concrete

Under the coastal environment,the concrete-based infrastructure is exposed to a salt mist-filled atmosphere for a long time,and the corrosion of concrete steel bars corroded by carbon dioxide and chloride ions at the same time causes the failure of the concrete structure,making the service life of the infrastructure much lower than normal.The problem of resistance to chloride and carbon dioxide has attracted much attention in coastal cities.The corrosion of the steel bars in the concrete is determined by the Cl^-/OH^-value on the surface of the steel bars.Most of the traditional ideas are to slow the infiltration of carbon dioxide and chloride ions into the concrete,which has inspired various improvements in the density of the concrete to delay the failure of the concrete structure.A lot of research shows that the method has been applied in engineering applications.However,concrete itself has a certain buffering capacity for acid gases such as carbon dioxide due to its strong alkalinity,and concrete has a certain curing effect on chloride ions.If the concrete’s compactness can be improved,at the same time,the buffering capacity of the coagulation to carbon dioxide and the curing ability of chloride ions can further slow down the rate of their erosion into the concrete,thereby improving the durability of the concrete.LDHs material is also called anionic clay,which has a special two-dimensional layered structure;interlayer anions can be ion exchanged with external anions.The interlayer ion exchangeability of the LDHs material is used to add intruded CO₃^2-and Cl^-in the cement-based material,so as to improve the carbon dioxide buffering ability and the ability to solidify the chloride ions of the cement-based material.In this paper,the Ca-based LDHs material synthesized in the laboratory by co-deposition method is used as the research object.Using the memory effect of LDHs,they are calcined at different temperatures to study their performance in adsorbing chloride ions in aqueous and pore solutions.The potential of carbonate ions,after verifying their structural reconstruction in fresh cement paste,they were added to the cement mortar in different amounts,and their effect on the carbonization resistance of the cement mortar was studied.The experiment as a micro-characterization object explores the mechanism of the carbonization resistance of CLDH reinforced cement-based materials from three aspects:the pore structure of concrete,the alkalinity,and the storage capacity of carbon dioxide.The calcined CLDH was blended into the concrete,and the chloride ion permeability coefficient of the concrete was measured by the rapid chloride ion migration method（RCM）.The performance of combining CLDH with chloride ion in the pore simulation fluid was analyzed based on Fick’s second law.The mutual influence of CLDH’s ability to cure chloride ions after adding concrete to concrete points out the problems of the potential of further releasing LDHs in concrete,and points out the direction for the next research.Based on the above studies,the conclusions of this article are as follows:（1）The X-ray characterization test of Ca-LDHs synthesized in the laboratory shows that the crystallinity is good,and the interlayer distance of LDHs is 0.857 nm.By comparing with the thermal behavior of Mg-LDHs,it is predicted that Ca-LDHs can withstand higher calcination temperatures.The layered structure of LDHs was destroyed after high-temperature calcination.The XRD patterns of CLDH（700CLDH,800CLDH） calcined at 700°C and 800°C were similar to those after calcination at 600°C.The calcination temperature gradually increased,and the SEM photographs of CLDH showed that the layered structure of LDHs was gradually destroyed.In the high-magnification SEM photographs of 700CLDH and 800CLDH,dense pinholes appeared in the lamellae of LDHs,which was beneficial to increasing the specific surface area of CLDH.（2）The isothermal adsorption results of 600CLDH in Na Cl solution show that the maximum theoretical adsorption capacity of 600CLDH for chloride ions is about 155 mg/g.Kinetic tests show that chemical adsorption may be a key factor in controlling ion adsorption by LDHs,and dynamic adsorption will reach equilibrium in about 400 minutes.The strong alkaline environment has a greater impact on CLDH’s adsorption of chloride ions.In the Na Cl solution with p H=13,700CLDH has the highest chloride ion adsorption capacity,about 152 mg/g.The competitive adsorption of CLDH on CO₃^2-/Cl^-indicates that CLDH will preferentially adsorb CO₃^2-.However,the performance of CO₃^2-on CLDH’s adsorption of Cl^-is also affected by the concentration of CO₃^2-.Fresh cement paste experiments show that CLDH calcined at different temperatures can complete structural reconstruction in the cement environment,and the crystal form after700CLDH reconstruction is better.（3）Cement mortar/paste experiments show that CLDH calcined at different temperatures can enhance the carbonization resistance of cement-based materials.The 2%content achieves the best reinforcement effect,and the 2%content of 700CLDH has the best reinforcement effect.The addition of CLDH enhances the carbonization resistance of cement-based materials from three aspects:a)The addition of CLDH increases the alkalinity of cement-based materials,thereby enhancing the neutralizing and buffering performance of cement-based materials against acid gases.b)The addition of CLDH optimizes the pore structure of cement-based materials,reduces the porosity,and reduces the number of pores around 1000nm.c)The addition of CLDH improves the storage capacity of cement-based materials for carbon dioxide.（4）The addition of CLDH calcined at different temperatures can improve the resistance of concrete to chloride ion attack.The chloride ion permeability coefficient of concrete mixed with CLDH measured by RCM has been reduced to varying degrees.The ion permeability coefficient is more sensitive to the amount of CLDH,and the concrete with700CLDH can lower the chloride ion permeability coefficient of the concrete at a lower amount.