Study On Deterioration Mechanism And Performance Evaluation Of Cement-based Materials Under Calcium Leaching And Chloride Attack

Long-termly exposed to the environmental water like seawater,concrete structures aresuffered from not only calcium leaching but also chloride attack.Chloride ion from environmental water diffuses into the surface of steel bar through concrete protective layer to cause the corrosion of steel bars.The leaching of environmental water to concrete results in the increase of porosity and deterioration of microstructure,which can accelerate the chloride diffusion in concrete,while further reduce the strength of concrete,cause the earlier rust of steel bar and the premature failure of concrete structures.Thus,the calcium leaching and chloride attack is one of the important factors for the deterioration of concrete performance,the reduction of structural durability and the shortening of in-service lifetime.However,there are few mechanism studies and evaluation methods on the durability deterioration of cement-based materials under calcium leaching and chloride attack.Aiming at the deterioration mechanism and performance evaluation of cement-based materials under calcium leaching and chloride attack,the experiments on the deterioration mechanism of cement-based materials under calcium leaching and chloride attack were carried out.The diffusion model of calcium ion and chloride ion in cement-based materials and its performance evaluation method are established,which can provide the theoretical basis for the performance evaluation and lifetime prediction of concrete structures permanently in water environments.The main contents and results of this paper are as follows:(1)Leaching characteristics and boundary movement of cement-based materials in the solution with low flowing velocityThe acceleratedly leaching experiment of cement-mortar cylinder specimens in NH₄Cl solution with different flowing velocities has been carried out.The leaching depth,porosity,phase composition,microstructure and hydrated product content were measured to analyze the influence of flowing velocity of NH₄Cl solution on the leaching characteristics of cement mortar.The failure characteristics on the surface of cement-mortar specimens caused by the calcium leaching and flowing abrasion were obtained,and the boundary movement condition on the surface of specimen subjected to the flowing abrasion was determined.Results show that,the flowing velocity of solution is an important factor affecting the leaching process of cement-based materials,and higher flowing velocity of solution results in faster leaching process and surface boundary movement in cement-based materials.(2)Influence of mineral admixtures on the leaching characteristics of cement-based materials and its improvement mechanismThe conventional and accelerated leaching experiments on the binary and ternary cement paste specimens with fly ash and slag were respectively carried out.The change of leaching depth,porosity,p H value of pore solution,phase composition,microstructure and element content in the leaching process were investigated to analyze the influence of mineral admixtures on leaching characteristics of cement pastes and its improvement mechanism.Results show that,the addition of mineral admixture with an appropriate content in cement pastes can improve its microstructure,slow down the microstructural deterioration and leaching process,and improve the leaching resistance of cement-based materials.(3)Solid-liquid equilibrium characteristics and accelerated leaching mechanism of cement-based material in NH₄Cl solutionSolid-liquid equilibrium concentrations of cement-based materials in 1,3 and 6M NH₄Cl solution were tested.The influence of NH₄Cl solution concentration on the accelerated leaching process of cement-based materials was analyzed,and the acceleratedly leaching mechanism of NH₄Cl solution on the cement-based materials has been revealed.The solid-liquid equilibrium equation of cement-based materials in NH₄Cl solution was given.Results show that,the solid-liquid equilibrium curves and leaching mechanism of cement-based materials in NH₄Cl solution are similar to those in deionized water.The NH₄Cl solution concentration has basically no influence on main characteristics of solid-liquid equilibrium curve,which are similar in NH₄Cl solutions with different concentrations.(4)Influence of calcium leaching on chloride binding capacity of cement-based materials and its quantitative characterizationThe isothermal adsorption and desorption experiments of cement paste particles with different leaching degrees were carried out.The influence of leaching degree on the chloride-bound capacity of cement-based materials was analyzed,and the quantitative characterization method of the chloride-bound capacity of leached cement-based materials was established.Result shows that,the calcium leaching in cement-based material can influence its absorptivity on free chloride ions,and higher leaching degree results in weaker absorptivity on the free chloride ions in cement-based materials.(5)Influence of calcium leaching on the chloride diffusivity in cement-based materials and its mechanism.The immersion experiment of cement-paste prism specimens in Na Cl solution,NH₄Cl solution and the mixed solution of Na Cl and NH₄NO₃has been carried out.The leaching depth,pore structure,morphology,phase composition and free chloride ion content were measured to analyze the leaching behavior and its influence on the chloride diffusivity in hardened cement pastes.Results show that,the calcium leaching of cement-based materials reduces its binding capacity of free chloride ions,and accelerates the diffusivity of free chloride ions.Higher leaching degree results in weaker binding capacity and higher diffusion rate of chloride ions in cement-based materials.(6)Calcium and chloride diffusion model and numerical simulation of cement-based materials Combined with the boundary movement condition,solid-liquid chemical equilibrium and chloride-bound capacity equations,the diffusion model of calcium ions and chloride ions in cement-based materials was established by using Fick law and mass conservation law.Meanwhile,this model was verified by the experimental results.Then,numerical simulation were performed to investigate the calcium and chloride diffusion process in the cement-mortar specimens,and the influence of water-cement ratio,slag content and immersion solution on the calcium ion concentration,porosity and chloride ion concentration in cement-based materials was analyzed.Additionally,the in-service performance of cement-mortar-lined ductile iron pipe for water supply was numerically evaluated by using this diffusiion model.