Research And Application Of Chloride Ion Immobilization And Anti-migration Materials For Sea Sand Concrete

With the implementation of China’s marine strategy,the construction of offshore islands and reefs is developing rapidly.The sand used for concrete construction are mainly from inland,with long transportation distance and high cost.The utilization of marine resources such as sea sand and coral sand has become a trend.But the direct use of marine resources introduces a large amount of chloride ions,which could induce the corrosion of steel bars and bring potential risks to the durability of concrete.The use of cement-based materials to immobilize chloride ions is an effective technical way to reduce the risk of steel corrosion induced by chloride ion.Based on the National Key Research and Development Program of the 13th Five-Year Plan“Research and Application of Chloride Ion Immobilization and New Anti-migration Technology in Sea Sand Concrete”（2016YFC0701003-5）,this dissertation put forward a new model based on the combination of in-situ immobilization,efficient anti-migration,and dynamic capture of inner chloride ion,designed in-situ immobilization materials,efficient anti-migration materials,and dynamic capture materials,explored the mechanism of these materials,forming an design method of inner chloride ion immobilization and anti-migration materials system.The innovative achievements are as follows:（1）Aiming at the inner chloride ion in marine concrete,a model of chloride ion immobilization and anti-migration in cement-based materials with the combination of in-situ immobilization,efficient anti-migration and dynamic capture is proposed.For the free chloride ion on the surface of the sea sand,the in-situ immobilization material is designed to promote the formation of chloroaluminate salts to realize the in-situ immobilization of chloride ion.The efficient anti-migration material is designed to refine the pore structure,block the migration channel,and prevent the chloride ion migration.For the chloride ion in pore solution,the dynamic capture material is designed to slowly release barium ion,promote the exchange of sulfate ion and chloride ion in calcium sulphoaluminate to form calcium chloroaluminate,to realize the dynamic capture of chloride ion.On the basis,a new technology of chloride immobilization and anti-migration was formed by combining in-situ immobilization based on active aluminum phase promoting the formation of chloroaluminate salts,efficient anti-migration based on the nano-micron silicon-aluminum materials modifying pore structure,and dynamic capture based on the barium-containing sustained-release materials.（2）The in-situ immobilization material,based on the principle of aluminum-chloride reaction in the early stage of cement hydration,facilitated the formation of chloroaluminate salts to chemically immobilize the chloride ion by increasing the content of active aluminum phase and adjusting the distribution of aluminum.The pretreated fly ash（PFA）introduced active aluminum phase,and at the dosage of 40%increased the immobilization rate of chloride ion by 9.25%at 7 d.The XRD-TG and other microscopic test results indicate that PFA had strong pozzolanic reactivity to release amounts of aluminum phase,which facilitated the formation of chloroaluminate salts.The inorganic aluminum-rich additive AAH at the dosage of 4%increased the immobilization rate of chloride ion by 3.65%at 7 d.Tests such as ²⁷Al NMR reveal that AAH increased the content of active aluminum in the cement system to promote the formation of chloroaluminates.The organic TIPA additive enhanced the activity of aluminum phase and increased the immobilization rate of chloride ion by 11.40%at 7d with the dosage of 0.05%.The results of ion dissolution test show that TIPA could accelerate the dissolution of aluminum from mineral phase to increase the formation of chloroaluminate salts.Based on the study of in-situ immobilization materials,the chloride ion in-situ immobilization technology was formed that the formation of chloroaluminate salts was promoted by increasing the content of aluminum phase and improving the reactivity of aluminum phase.（3）In the study of efficient anti-migration materials,it is proposed that the chloride ion migration channel could be blocked by refining the pore structure,to achieve the efficient anti-migration.The nano-Si O₂（NS）at the dosage of 0.5%increased the immobilization rate of chloride ion by 4.44%and 5.80%respectively in28 d and 90 d.The results of ²⁹Si NMR and MIP imply that NS could accelerate the hydration process of cement-fly ash system to produce hydrates and refine pore structure,which blocked the chloride ion migration channel.The nano-Al₂O₃（NA）at the content of 4%promoted the immobilized chloride ratio by 2.57%and 7.63%at 28d and 90 d.The results of ²⁷Al NMR and MIP reveal that the hydration of NA could increase the content of hydrates to refine the pore structure and reduce the migration rate of chloride ion.The nano-micron silicon-aluminum material UFA increased the chloride ion immobilization rate by 13.24%at 7 d.Tests such as SEM show that UFA had high pozzolanic activity to produce a large amount of hydration phases,increase the compactness,and reduce the rapid channel of chloride ion migration.Based on the study of efficient anti-migration,the chloride ion efficient anti-migration technology was formed that nano-micron silicon-aluminum materials blocked the migration channel of chloride ion by refine the pore structure.（4）Dynamic capture is based on the reaction principle of barium-sulfate-aluminum-chloride after the formation of cement paste structure.It is proposed that barium ion chemically modifies sulphoaluminate to form chloroaluminate.Barium-containing sustained-release materials were designed and prepared.The barium-containing sustained-release ALG material at the content of 3%increased the immobilization rate of chloride ion by 12.15%at 90 d,indicating that the ALG material released Ba ions to capture chloride ion.The barium-containing sustained-release EP material increased the chloride ion immobilization rate by 13.10%at 240 d,implying the stable improvement at a long age.Based on the study of dynamic capture materials,the chloride ion dynamic capture technology was formed that barium-containing sustained-release material converted sulphoaluminate to chloroaluminate.（5）According to the research results of chloride ion immobilization and anti-migration,the chloride ion anti-migration and immobilization admixture was prepared by optimizing the combination of in-situ immobilization materials and efficient anti-migration materials.Under the condition that the chloride content of sand used in the demonstration project of China-Maldives Friendship Bridge was 0.13%,the chloride ion anti-migration and immobilization rate of modified concrete was higher than 60%.The admixture could extend the service life of the engineering structure.This research provides important technical reference for the construction of offshore islands and reefs in China.