Research On Modification Mechanism And The Application Of Layered Double Hydroxides For Durability Of Concrete

Concrete structures in ocean engineering are often served under the environmentwith multiple factors including ion erosion. Exploration of causes for durabilitydegradation of concrete and modification of concrete materials to enhance itsdurability performance based on the traditional technologies to prolong its service lifehas important practical significance and social economic benefits. For the existingstrategies to improve the erosion resistance of concrete, different methods alwayspresent some shortcomings, and a lot of methods are proposed for single durabilityfactor. Establishment and development of new strategies to control carbonizationreaction, sulfate attack and chloride ion erosion is the guarantee of high performancefor concrete with high content of mineral admixtures. Based on the research trend andhotspot in recent years, inorganic intercalation material(layered double hydroxides,LDHs) has become a new research direction in the field of building materials, itpresents the new technical prospects to improve concrete durability.Chemical composition and structure of Mg-Al-CO3LDHs were tested andanalyzed. Under different temperature conditions, LDHs were calcined and theprocess of thermal decomposition was analyzed. The thermal decomposition kineticsand thermodynamic properties of LDHs were explored. Setting time, hydration degree,ion concentration of pore solution of cement paste were investigated based on3Dcomputer model as well as porosity. Quantitative study of adsorption ions by LDHsmaterials in pore solution in the process of structure reconstruction were conducted.Morphology and structure of the reconstruction product were analyzed andcharacterized. Influence of modified LDHs on concrete mechanical performance wasstudied and effects of LDHs on durability aspects of concrete were comprehensivelydemonstrated. Finally, application of combination of LDHs and metakaolin toimprove permeability in concrete was explored.The main conclusions are drawn as follows:(1) Solubility of CO32-intercalated hydrotalcite materials is less than the Cl-andNO3-intercalated hydrotalcite materials, which means stability of former is better thanthe latter. These properties are closed related to its larger charge density, strongerlayer board binding and the parallel structure inserted into the layer for stable configuration. Al/Mg ratio=1:2or1:3generates appropriate and stable hydrotalcitephase, and this finding provides selection principle for use of hydrotalcite materials.(2) Before calcination, Mg-Al-CO32-LDHs indicates relatively best crystallinestate, after calcined at600°C and rehydrated with saturated Ca(OH)2solution, thestructure fulfills regeneration, but the crystallinity decreases. Original Mg-Al-CO32-LDHs is obvious layered structure before calcination, calcined product LDOs at600℃still shows layered structure characteristics. When the calcination temperatureincreases to800°C, the layered structure is damaged and results in sintering spinel,and the structure reconstruction could not occur. Microscopic analysis showsagreement with the decomposition thermodynamic of the second stage.(3) LDHs materials presents effects on hydration degree of cement, but theextent is relatively small based on the results of setting time, the content ofevaporation water and hydration heat. Variation law of concentration of Na+in poresolution is similar with K+with increasing hydration time. Concentration of SO42-changes synchronously with Ca2+, after mixed with LDHs materials, changes ofconcentration of SO42-is relatively marked compared with Ca2+. The pH value in thewhole stage is within a certain range and the fluctuation is small, the pH values arenot affected by different types of LDHs.(4) Three dimensional computer simulation shows that the incorporation ofLDHs materials only present water absorption, ion adsorption and exchange reactionin the initial stage of hydration, and the influence is modest, it has no obvious impactson hydration for a long time. The porosity of cement paste decreases to a certainextent after the incorporation of LDHs at early ages, it shows certain optimizationfunction for enhancement of the permeability resistance properties of concrete.(5) Based on the adsorption kinetics in simulated pore solution, in view ofdifferent types of Mg-Al-CO32-LDHs and Mg-Al-NO3-LDHs, calcined productpresents the best adsorption effect on CO32-, SO42-and Cl-. When compared with theFreundlich model, Langmuir isotherm can best fitting the experimental data ofadsorption of SO42-by calcined Mg-Al-CO32-LDHs in the simulated pore solution ofconcrete. In different adsorption kinetics model, Pseudo-second-order kinetics modelconsistent with the experimental results. Adsorption of SO42-by calcinedMg-Al-CO32-LDHs is spontaneous, endothermic and entropy increasing process.(6) Improving effects on durability of concrete by LDHs indicate that in the sameage, carbonation depth of concrete declines to some extent after mixed with LDHs materials. The calcined product has the most marked effect and carbonation depthdecreases obviously. Validation and forecast based on selection of common models ofcarbonation depth shows that the accuracy of the models. For incorporation of LDHssamples, the smaller measured values and calculated values suggests thecorresponding smaller concrete carbonation depth, which means the incorporation ofLDHs materials is still accord with the models. For original and calcined Mg-Al-NO3-LDHs, both present the improving effect on adsorption ability of chloride ion inconcrete. For Mg-Al-CO32-LDHs, it also presents the improving effect on adsorptionability of sulfate. The promotion of durability performance of concrete mainly resultsfrom the ion exchange and adsorption based on structure reconstruction.(7) Influence of LDHs-MK compound modifying agent on early strength ofconcrete is not obvious, but the contribution to the later strength is significantly.Bonding between cement paste and aggregate becomes closer when incorporation ofMK. LDHs-MK compound modifying agent indicate more marked effect oninterfacial transition zone and form the most compact structure. Also, LDHs-MKcompound modifying agent improves the ability of chloride ion penetration resistanceof concrete. Different experimental methods show agreement with each other.The research results will contribute to the development of anion exchangematerials applied in the field of cement chemistry and concrete science, and promotethe establishment of design technology for LDHs-based high efficient concreteperformance modification materials. The results are expected not only to solve theproblem of rapid carbonation of concrete with high volume fraction of mineraladmixture, but also to build an inherent defence system against chemical aggressioninto concrete structures.