Mechanism Of Chloride Binding Of LDHs And Its Application In Cement-based Material

Modern marine concrete is always exposed to sea water in a long period of time, which easily leads to the corrosion of steel bars in concrete mainly through chloride penetration and thus causes degradation of concrete durability. Therefore, it is beneficial for prolonging concrete service life and protecting environment as well to enhance the concrete resistance of chloride penetration. The key to improving the resistance of chloride penetration is to reduce the transmission velocity of outer chloride ion diffusing into the inner concrete and drop the concentration of chlorides in pore solution. Only when the chlorides concentration reaches a certain degree could corrosion of rebar happen. Hence, if part of the chlorides could be bound through the process of penetration and meanwhile the density of concrete could be increased, consequently the penetration speed can be controlled. Therefore, measures can be taken from these two aspects to limit the chloride concentration in pore solution, and in turn achieve the purpose of improving the durability of concrete.Ca Al-NO3 LDHs is chosen as an investigated object in this study and is added as a chloride absorbent in cement paste to increase the resistance of chloride penetration. Ca Al-NO3 LDHs, which belongs to the family of LDHs(Layered Double Hydroxides) have the ability of anion exchange and can absorb chloride ion in cement-based materials. This paper describes the synthesis and characterization of LDHs and its structural characteristic. Then chemical change of LDHs in simulated pore solution and the interaction rule of LDHs and cement paste are investigated in view of cement matrix system. The mechanism of LDHs for improving the resistance of concrete is explained. The results of this paper show that:(1) Ca Al-NO3 LDHs synthesized in laboratory by co-precipitation methods is good in crystalline state and belongs to hexagonal crystal system. XRD evidence suggests that crystal parameter is a=b=0.305 nm, c=2.587 nm. The interlayer space is 0.827 nm. Ca Al-NO3 LDHs has the ability of anion exchange. Chloride can substitute nitrate in the interlayer with a decrease to 0.7827 nm in interlayer space owing to a smaller anion radius of chloride ion(0.168nm) than nitrate ion(0.200nm).(2) The chloride adsorption ability of Ca Al-NO3 LDHs is quantitatively analyzed in aqueous solution and simulated pore solution. Results show that the adsorption isotherm fits Langmuir model and the maximum adsorption quantity is 3.38 m M/L, which is in accordance with the theoretical maximum adsorption ability(2mol/mol). What's more, the adsorption behavior in cement paste is the same as in aqueous solution.(3) The main products after interaction between Ca Al-NO3 LDHs and simulated pore solution is Ca Al-(NO3, OH) LDHs solid solution, which has an irregular morphology and mess interlayer structure. After simulation of chloride penetration, the main product is Ca Al-(Cl, NO3, OH) LDHs, in which chloride occupy the most adsorption sites. The morphology has a hexagonal structure with a better crystalline.(4) Initial setting time and final setting time are shortened with the addition of Ca Al-NO3 LDHs. Early strength is increased as well but with no significant increase in long-term strength. Ca Al-NO3 LDHs can improve the chloride resistance of cement paste to some degree, but excessive amount of Ca Al-NO3 LDHs addition will lead to a decrease in density of cement paste and a rise in total pore volume. Therefore, the premium addition of Ca Al-NO3 LDHs should be around 1-2%.(5) LDHs-MK composite admixture can dramatically improve the chloride resistance ability of concrete. Ca Al-NO3 LDHs can bind chloride through chemical adsorption, while metakaolin can improve the density of concrete through secondary hydration. The combination of these two admixtures can build a double defense system, which has a better performance in improving the durability of concrete.