| Concrete is an irreplaceable and widely used artificial material in marine engineering construction,and its consumption is huge.The improvement of corrosion resistance of concrete in the harsh marine environment is a key technical support for the national marine strategy.However,the marine environment is extremely complex,and the high concentration of Cl-in seawater can easily cause the deterioration of reinforced concrete structure and performance,seriously endanger the safety,reliability and service life of marine construction projects.The main reason is that the electrochemical reactions between the steel bars and Cl-in reinforced concrete accelerate the corrosion of steel bars,thereby leading to a reduction of the adhesion between steel bars and concrete.Based on this,the development of chloride binding and transport retardation in marine concrete is crucial.At present,the measures to enhance the anti-corrosion and penetration resistances properties of concrete are mainly includes adding modified components into concrete,concrete surface coating,steel surface protection and other aspects.The measures present a certain effect on improving the anti-corrosion and penetration resistances properties of concrete,but the safety and service life of marine engineering construction are still difficult to meet.Layered double hydroxides（LDHs）are a kind of multifunctional 2D nanoflakes with a layered structure.The formula of LDHs can be described as[M2+1-xMx3+（OH）2]x+(An–)x/n·m H2O.LDHs have two unique functions,namely ion exchange properties and“structural memory function”.The unique ion exchange properties and“structural memory function”are used to adsorb Cl-.Meanwhile,the LDH 2D nanoflakes is used to increase the tortuosity of capillary pores in the cement-based matrix,further retarding the water-salt transport and achieving the improvement of anti-corrosion and penetration resistances properties of marine concrete.In this study,two new Fe-based LDHs were synthesized by a co-precipitation method by Ca,Fe,Al and O as the main laminate skeleton of LDHs,and the ion exchange properties and“structural memory function”of Fe-based LDHs are used to adsorb Cl-and retard water-salt transport to improve the anti-corrosion and penetration resistances properties of cement-based matrix.Meanwhile,two composites of LDHs-strongly basic anion exchange resin（SBAER）and LDHs-nanometres（colloidal silica sol（SS）and nano-montmorillonite（NM））were constructed with LDHs as the matrix to further enhance the Cl-adsorption and the anti-corrosion and penetration resistances properties of cement-based matrix.The related studies further revealed the time-sensitive characteristics of LDHs for Cl-adsorption and elucidated the long-lasting mechanism of Cl-adsorption in cement-based matrix.The main research results are as follows:（1）Two new Fe-based LDHs,Ca Fe-NO3（abbreviated as LDHs-CF）and Ca Fe Al-NO3（abbreviated as LDHs-CFA）,were synthesized and their lattice parameters were determined,and the adsorption behavior on Cl-were investigated.Phase analysis showed that LDHs-CF was mainly dominated by[Ca2Fe（OH）6]（NO3）·2H2O with lattice parameters c=0.8636 nm(d-spacing=d001)and a=0.5888 nm(d-spacing=2d110),which belonged to hexagonal crystal system.While LDHs-CFA contained LDHs-CF and two crystalline types of Ca Al-NO3 LDHs,namely[Ca4Al2（OH）12]（NO3）2·4H2O and[Ca4Al2（OH）12]（NO3）1.66（OH）0.34·3.5H2O,and a small amount of Ca Fe Al-LDHs.Fe based LDHs exhibited excellent Cl-adsorption effect in solutions,and their adsorption capacities are proportional to the concentration of chloride ion,and the maximum Cl-adsorption capacities of LDHs-CF and LDHs-CFA in pore solutions reached 40 mg/g and 42 mg/g,respectively.The kinetic data of these two Fe-based LDHs showed that the adsorption kinetics fit well to the pseudo-second-order model,indicating that the adsorption process covered various adsorption modes such as surface adsorption,ion exchange,and internal particle diffusion.Meanwhile,the adsorption of the synthesized Fe-based LDHs exhibited a monolayer adsorption,which could be described by the Langmuir isotherm model,indicating that all adsorption sites are equivalent.（2）The adaptability of Fe-based LDHs to cement and their influence on the adsorption capacity of Cl-were determined.The results showed that both Fe-based LDHs shorten the final and initial setting time,increase the compressive strength and dynamic yield stress of cement pastes,while had less effect on the 28 d chemical shrinkage value and plastic viscosity.Both Fe-based LDHs promote early hydration of the cement and increase the amount of hydration products such as C-S-H gels and Ca（OH）2.At 1.5 wt.%LDHs-CFA,the free Cl-content of the mortars was 0.0394%at 7 d and 0.0325%at 28 d,compared with the mortars without LDHs,the values decreases about 36.2%and 32.9%,respectively,showing excellent adsorption effect of Cl-.（3）The effect of LDHs-SBAER composites on the adsorption mechanism of Cl-and the anti-corrosion and penetration resistances properties of concrete were determined.The results showed that SBAER can also adsorb Cl-,but the amount of admixture was not too large,otherwise it would reduce the mechanical properties of hardened cement paste.Based on the response surface methodology analysis,the optimization of LDHs-SBAER composites can further improve their mechanical properties and Cl-adsorption capacities.The results suggest that the addition of LDHs-CF,LDHs-CFA,and SBAER in the range of 0.3～0.6 wt.%,0.2～0.5 wt.%,and 0.2～0.5 wt.%,respectively,were the most optimal for ensuring a excellent performance of mortar.The LDHs-SBAER composites showed that the 28 d rapid chloride migration coefficient(DRCM)and total charge passed decreased by 15.2%and 7.0%compared to those of the control,respectively.The fitted curve of electrochemical impedance spectroscopy（EIS）was consistent with the R（QR）（Q（RW））equivalent circuit,and the corrosion inhibition efficiency of the composites was 19.9%.The micro-scanning electrochemical scanning Kelvin probe（SKP）analysis showed that the steel bars with LDHs-SBAER only had a work function of 349 m V,which is 72.9%lower than that of the control,showing excellent corrosion resistance property.（4）The effect of LDHs-SS-NM composites on the adsorption mechanism of Cl-and the anti-corrosion and penetration resistances properties of concrete were determined.Based on the response surface methodology analysis,when the same content of Fe-based LDHs were added and the addition of SS and NM were in the range of 0.2～0.4 wt.%and 0.2～0.7 wt.%,respectively,the free Cl-content in cement paste decreased significantly.The results indicated that LDHs,SS and NM have a synergistic mechanism to affect the adsorption effect of Cl-.The DRCM and total charge passed of 28d LDHs-SS-NM composites were 14.3×10-12m2/s和7025 C,which are 17.3%and 7.9%lower than the control,respectively.The average corrosion rate of LDHs-SS-NM composites was 1.80×10-3 mm/year and the inhibition efficiency of steel bars reached 42.4%,thus confirming better corrosion resistance property.（5）The effect of calcined LDO-CFA on the adsorption mechanism of Cl-and the anti-corrosion and penetration resistances properties of concrete were determined.The results showed that LDHs-CFA lost the NO3-of interlayer after calcined at 750℃to form Ca O,Ca Al8Fe4O19,Ca2（Al,Fe）O5,Ca2（Al,Fe）2O5,Ca2Fe2O5.The adaptability data of Fe-based LDHs to cement showed that calcined LDO-CFA improved the compressive strength and dynamic yield stress of cement pastes,especially the LDO-750-CFA,but it had less effect on28 d chemical shrinkage value.The Cl-adsorption capacity of LDO-750-CFA in concrete pore solutions reached 68 mg/g,which is 61.9%higher than that of LDHs-CFA.Additionally,the DRCM of the LDO-750-CFA system decreased by 13.9%compared to the Ref.,and the corrosion inhibition efficiency of steel bars reached 23.4%,all of which are higher than that of the LDHs-CFA system.（6）The time-sensitive characteristics of the Cl-adsorption by LDHs and composites were determined.The results showed that the Cl-adsorption of mortars was a slow process.Even after curing for 120 d,the Cl-adsorption capacities of mortars with two protective layer thicknesses（50 mm and 35 mm）still differed significantly,and both were much smaller than the theoretical Cl-adsorption capacities.The time-sensitive characteristics of LDHs and their composites to Cl-are not only related to the thickness of the protective layer of concrete,but also to the concentration of Cl-in solutions.The adsorption time-sensitive characteristics of mortar mixed with LDHs and their composites to Cl-conform to Expassoc kinetic model,which can easily and quickly predict the adsorption trend of mortar mixed with LDHs and their composites for Cl-. |
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