Study On The Migration And Bonding Of Chloride Ions In Seawater And Sea Sand Concrete

The offshore ports,docks,and reef buildings have developed rapidly with the implementation of China’s strategy of marine development.The scarcity of raw materials such as river sand and freshwater resources is deepened by the sharply increased demand for building concrete.The shortage of resources can be alleviated and the transportation distance of raw materials can be reduced,if the traditional river sand and freshwater can be replaced by abundant reserves of sea sand and seawater to prepare concrete.However,the reinforcement corrosion will be inevitably induced by the high content of chloride ions in seawater and sea sand concrete（SWSSC）,which is the biggest obstacle to its wide application.Therefore,the effect of seawater and sea sand on the basic properties of concrete,and the migration of chloride ions in the interior of SWSSC should be studied.Besides,it is very necessary to explore an effective way to improve the chloride binding capacity of SWSSC and a corrosion inhibitor suitable for SWSSC.Hence,in this study,the main raw materials（freshwater and river sand）were replaced by seawater and sea sand to prepare concrete and mortar.The basic properties of SWSSC and the migration rule（leaching and migration to adjacent concrete）of chloride ions in the interior of SWSSC were studied.Chloride binding by calcined layered double hydroxides（CLDH）and alumina-rich cementitious materials（metakaolin（MK）,fly ash（FA）,and calcium aluminate cement（CAC））in mortar mixed with seawater and sea sand,and the effect of layered double hydroxide（LDH）intercalated with inhibitor on the corrosion of steel bars at the simulated pore solution of SWSSC were also carried out,as shown in the following text.（1）The concrete or mortar mixed with seawater and sea sand was prepared,and three supplementary cementitious materials（fly ash,grinding fine grained blast furnace slag,and silica fume）were added to the mixtures.The effects of seawater and sea sand on the workability,compressive strength,splitting tensile strength,capillary absorption,and drying shrinkage properties of concrete were studied.The results show that the hydration of cement was accelerated by seawater and sea sand,inducing that its 3 d and7 d compressive strength increase by 34.8%and 54.6%,respectively,compared with freshwater river sand concrete（FRSC）.However,the long-term strength was the same as that of FRSC.The water absorption performance of concrete was not significantly affected by seawater and sea sand,whereas the workability of concrete was decreased,and the dry shrinkage performance of the mortar was increased by seawater and sea sand for that its dry shrinkage is 1.43 times higher than that of FRSC.Experiments prove that dry shrinkage can be decreased by 60%by adding an expansion agent of magnesium oxide with a dosage of 5%.Besides,the mechanical properties and durability of SWSSC can be improved by adding 0.5%calcium nitrite or 5%silica fume,and its workability can also be improved by the addition.（2）The seawater and sea sand concrete mixed with different strength grades were prepared,and it was soaked in the static water（no replacement of the soaking solution）and dynamic water（regular replacement of the soaking solution）environment after curing for the different age.The numbers of chloride ions dissolved in the soaking solution and the concentrations of chloride ions at different depths in concrete at different ages of soaking were measured by Potentiometric titration.The results show that the leaching of chloride ions in SWSSC is a relatively slow process,which is similar to that of calcium in cement-based materials.The chloride ions were slowly migrated from the inside of concrete to the surface area,and the phenomenon of surface enrichment was formed in the area（0～5 mm）,when the concrete was soaked in the still water environment.The content of chloride ions in the solution of leaching increased rapidly in the early stage and gradually slowed down in the later stage.In the dynamic water environment,the phenomenon of surface enrichment in the surface of chloride ions has still appeared,whilst the amount of enrichment was reduced.The concentration of chlorides in the aqueous solution and leaching rate are both increased.The leaching of chloride ions in SWSSC can be delayed by improving the strength of concrete,which can control the leaching amount maintaining only 0.116～0.158 mg Cl/gconcrete.（3）The FRSC with different strength grades was cast on the substrate of seawater and sea sand concrete,and the conjunct concrete was separated from the contact surface after curing different ages.The reverse migration rule of chloride ions in seawater and sea sand concrete was obtained by measuring the change of chloride ions content in the concrete surface.The results show that the chloride ions slowly migrated from the interior of SWSSC to the surface and gathered in the surface about 2mm,with the growth of combining age of that two concrete.Only a part of chloride ions migrated into the FRSC,and the depth of migration reached 5mm in the FRSC after combining90 d.The simulation results of COMSOL show that more chloride ions（0.076%）were migrated to the FRSC and the depth of migration reached about 20 mm after combining1 year.（4）The effect of admixtures on the chloride binding capacity of concrete was analyzed by testing the free and total chloride ion content of seawater and sea sand mortar mixed with CLDH and alumina-rich cementitious materials（calcium aluminate cement,fly ash,and metakaolin）.The results show that the maximum adsorption amount of chloride ion can be obtained by CLDH calcined at 500℃,and its structure can be reconstructed by adsorbing Cl^-or SO₄^2-in the solution.CLDH has priority to adsorb sulfate ions and its adsorption capacity is about 1.5 times that of Cl^-,leading to that the adsorption capacity of chloride ions in diluted seawater solution was about 10mg Cl/g CLDH,which is only 1/9 of that of in sodium chloride solution.Both CLDH,FA,MK,and CAC alone can effectively improve the chloride binding capacity of mortar by 65%,35%,65%,and 200%,when the optimal dosages are 7.5%,30%,30%,and 100%,respectively.Moreover,molecular dynamics simulation shows that the radius correlation function（RDF）value of Cl-Ca ionic pairs is cut down at the condition that has CLDH,which reveals that the adsorption of chloride ions by CLDH occurs in the mortar is confirmed.（5）The layered double hydroxide（LDH）intercalated with nitrite inhibitor was prepared by a method of calcination-reconstruction,and the effect of it and CLDH on the corrosion of steel bars at the pore solution of SWSSC was studied.The results show that nitrite antirust anions can be intercalated between layers of LDH by the method of calcination-reconstruction,and its loading rate of the rust inhibitor was close to 9%.Rust inhibitor anions in LDH-NO₂ were released quickly to the SWSSC pore solution after the reaction of 3 d,and its release rate of rust inhibitor can reach 99%.The corrosion of steel bars occurred rapidly in the SWSSC pore solution,and it was controlled by adding CLDH or LDH-NO₂.CLDH not only reduced the content of free chloride ions in the pore solution but also improved the basicity of it by adsorbing Cl^-and releasing OH^-,inducing that the corrosion rate of steel bars was delayed.The formation of the passivation film on the surface of steel bars was promoted through the adsorbing of Cl^-and the releasing of rust inhibitors and OH^-by LDH-NO₂,which can provide long-term corrosion protection for steel bars.