Researches On Modification And Mechanism Of LDHs-MK Compound Defence System In Concrete

The durability of concrete is of importance for service life extension and reliability of concrete structure, which is a major part of concrete performance design. Most of the corrosion reactions of concrete are relevant to the invasion of anions from external environment. The invasion process in concrete includes the transmission, penetration, movement, absorption, etc. Reducing the penetration rate of anions from external to internal environment and the anion concentrations of pore solution in concrete is an intergral component of strategies to enhance the durability of concrete. Specifically, improving the density of concrete and absorbing the anions in the invasion process are effective means to control the penetration rate and reduce the aggressive anion concentrations in the pore solution, and finally, to enhance the durability of concrete.Layered Double Hydroxide (LDHs) is a new type of functional material of the interlayer anion exchanging ability and structure memory effect. The aggressive anion in concrete could be absorbed by LDHs, which leads to the control of anion transmission in concrete. Metakaolin (MK) is a new type of high quality concrete mineral admixture of pozzolanic activity and filling effect, which optimize the pore structure remarkably to improve the permeability performance. An additive effect system could be built based on the combination of LDHs and MK, which can both improve the pore structure and control the transmission of anion, in order to enhance the durability of concrete comprehensively.The compound additive effect of LDHs and MK in concrete is investigated in this thesis. The chemical composition, molecular structure and granule morphology of LDHs is characterized. The anion binding capacity of different LDHs is analyzed and the durability of concrete containing LDHs is studied. Based on above researches, the LDHs-MK based compound modifier is developed and a LDHs-MK compound defense system is established, in which the performance and application technology is studied systematically.The main research outputs of the thesis are concluded as:(1) The structure and morphology of LDHs with different interlayer anion is analyzed after calcined and rehydration. The characteristic diffraction peaks show obvious difference when different interlayer anion. The layered structure of LDHs is destroyed after calcined and rebuilt while CO32-and OH-inserting into the interlayer in the alkaline condition.(2) The CO32-binding capacity of LDHs (including calcined product and rehydrated product) and the mechanism is studied. The CO32-binding process of uncalcined and rehydrated product is mainly a reaction of interlayer anion exchanging, while of calcined product (LDOs) is a reaction of structure reconstruction. The carbonation reaction of concrete containing different LDHs is studied in the experiment, which shows that the addition of LDHs has delayed the transmission of CO2and enhanced the carbonation resistance of concrete.(3) The chloride ion binding capacity of Mg-Al-NO3LDHs and LDOs in the chloride environment and the mechanism is studied, which shows that the Mg-Al-Cl layered structure is built by NO3-exchanging or inserting into the interlayer. The chloride ion from external environment is absorbed by LDHs in concrete, which leads to an obvious improvement of the chloride ion resistance.(4) The sulfate resistance of concrete containing different LDHs is studied. The addition of LDOs shows remarkable improvement to the sulfate resistance, while the SO42-binding capacity of uncalcined and rehydrated LDHs is much weaker.(5) MK based modifier is prepared while the reaction mechanism in concrete is studied. The combination of MK, fly ash and limestone powder has compensated the workability loss caused by the high water requirement of MK. MK based modifier leads to the reduction of concrete porosity, the enhancement of strength, chloride ion resistance and volume stability.(6) On the basis of the MK based modifier, the LDHs-MK compound defense system is established and the anion transmission mechanism in concrete under the LDHs-MK combined condition. The compound additive effect both optimizes the pore structure and delays the anion transmission, which improves the impermeability furthermore.(7) The volume stability of concrete under the action of LDHs-MK compound system. The influence of water-storage effect of MK and LDHs on the concrete shrinkage in the cement hydration process is analyzed. The addition of LDHs-MK compound system reduces the volume deformation and optimizes the grain composition in concrete, thus improves the volume stability obviously.The study in this thesis shows that LDHs-MK compound defense system is a comprehensive and efficient concrete durability enhancement technology. The system has improved the density and strength of concrete, controlled the aggressive anion transmission in concrete and enhanced the ability to defend different anions. Therefore, LDHs-MK compound defense system is one of the approaches to enhance the durability and service life of concrete comprehensively.