Preparation Of Inhibitor Ions Intercalation Modified Hydrotalcite And Its Application In Cement Mortar

In the coastal environment,the reinforced concrete structures are prone to the cracking of passive film and severe corrosion of steel rebars due to the continuous penetration of chloride ions,resulting in durability deterioration of buildings.Among the ways to improve the durability of reinforced concrete structures,the addition of mineral admixtures and inhibitors is widely applied in many practical projects.In recent years,the investigations of layered double hydroxides（LDH）on improving the durability of concrete are gaining popularity due to its micro-filler effect and releasing interlayer inhibitor ions,however,systematic research and practical application is lacking.Therefore,in this paper,the inhibitor anions were intercalated into LDH interlayer to enhance the anticorrosion effect.Besides,the synthesis methods of inhibitor anions intercalated LDH and the practical utilization in the simulated concrete pore solution and mortar concrete were respectively studied in detail.In this paper,the pH values of filtrate after reconstruction of CLDH in the corresponding salt solution were measured and compared,obtaining the affinity consequence between inhibitor ions and chloride ions:CrO₄^2->C₆H₅COO^->Cl^->NO₂^->NO₃^-.Another common method was performed by the FTIR,UV visible spectrums and XRF tests for comparison and the same results verified the feasibility and accuracy of the above method.The experiment results of mass loss of rebar in paste suggested that the addition of A_n-LDH apparently improves the anticorrosion performance of rebar and reduces the risk of corrosion.Electrochemical tests showed that A_n-LDH efficiently strengthened the charge transfer resistance(R_ct)and reduced the corrosion current density(I_corr)of carbon steel in the simulated concrete pore solution.The results of corrosion potential,EIS and polarization curves indicated the following inhibition effect sequence:NO₂-LDH>NO₃-LDH>C₆H₅COO-LDH>CrO₄-LDH,and NO₂-LDH exhibited the long-term protection effect.The NO₂^-intercalated LDH was successfully synthesized by calcined-rehydration（R-LDH）,co-precipitation（C-LDH）and hydrothermal（H-LDH）methods.The effects of different experimental parameters on the intercalation ratio of the synthesized products were investigated to screen out the optimal synthetic conditions.The appropriate calcined temperature in calcined-rehydration method was 500℃.In co-precipitation method,the intercalation ratio was the largest when the stirring time was 3h and the addition amount of NaOH was 2g.Hydrothermal temperature was 140℃ and the time was 5d in hydrothermal method.The properties of the LDH prepared by different methods were further explored and compared by XRD,FTIR,SEM,TG-DSC,elemental analysis and equilibrium isotherms analysis,these results demonstrated that C-LDH possesses the highest intercalation ratio.The EIS tests showed the inhibition efficiency of C-LDH,H-LDH and R-LDH for carbon steel in the simulated carbonated concrete pore（SCCP）solution after immersion for 8h was 99.29%,98.81%and93.44%,respectively.Polarization curves also indicated that the I_corr value of carbon steel in the solution with C-LDH decreased by more than two orders of magnitude compared with the reference sample.Based on the above conclusions,NO₂^-intercalated Mg-Al-LDH and Ca-Al-LDH were synthesized by co-precipitation and hydrothermal methods.The effects of synthetic LDH on flow ability,mechanical properties,water absorption,chloride diffusion coefficient and the durability of mortar concrete were investigated.The flow ability tests suggested that synthetic LDH had no obvious negative effects on flow ability of fresh mortar and its fluidity decreased slightly with the increase of LDH content.The addition of synthetic LDH simultaneously improved the mechanical properties and chloride ions resistance performance,2%H-C-LDH in mortars exhibited the best improvement effect.Compared to the reference sample,the compressive and flexural strength were improved by 25.2%and 12.6%,respectively.The water absorption and chloride diffusion coefficient reduced by 8.9%and 22.5%,respectively.Electrochemical tests revealed that the addition of synthetic LDH could enhance the corrosion potential and charge transform resistance of carbon steel in mortars,as well as reduce the corrosion current density.The H-C-LDH held the greatest protection effect.The analysis of the concentration of different anions in the pore solution of the paste and the XRD patterns of paste powder further confirmed that LDH could reduce the concentration of free chloride ions and release the inhibitor ions through the ion-exchange.Besides,the synthetic LDH facilitated the formation of C-S-H gel and Fridel’s salt,which enhances the immobilization amount of chloride ion in paste.The above conclusions demonstrated the synthetic LDH could ameliorate the corrosion resistance of carbon steel in mortars simultaneously via three approaches of physical barrier,chloride ion fixation and inhibitor ions release,and H-C-LDH held the best improvement effect.