Experimental Study On The Corrosion Performance Of Mineral Admixture Concrete Under Soft Water Conditions

For concrete structures exposed to soft water environment for a long time,there occurs a high dissolved calcium concentration difference between the environmental water and pore solution,which can cause the dissolution of calcium hydroxide and the decalcification of C-S-H gel in the hydration products,which is called the calcium leaching.The calcium leaching can cause the porosity increase,microstructural deterioration and mechanical properties decrease,and consequently leads to the durability deterioration of concrete structures and reduces its in-service lifetime.Calcium leaching is one of the important factors affecting the durability of concrete structures.Thus,it is of great engineering significance to improve the leaching resistance of concrete for the durability and in-service lifetime of concrete structures in water environments.In this paper,the slice cement paste specimens instead of cement-based materials are used as the research object,and the conventional leaching experiment of specimens with fly ash and slag in deionized water was carried out to analyze the influence of mineral admixtures on the leaching performances of blended cement pastes.Additionally,the accelerated leaching experiments of cement paste specimen in 1M NH₄Cl,1M NH₄NO₃ and 1.5g/100mL Acetic Acid solutions were carried out to analyze the influence of different corrosion media on the leaching behaviors of cement paste.The main contents and conclusions are as follows:?1?The slice cement paste specimens with fly ash and slag singly instead of cement based materials were used as the research objects,and the conventional leaching experiments of binary cement pastes with different content of fly ash and slag were carried out.By using Ethylene Diamine Tetraacetic Acid?EDTA?titration,saturation-drying weighing,Scanning Electron Microscopy?SEM/EDS?and X-ray diffraction analysis?XRD?methods,the change of dissolved calcium ion concentration,porosity,microstructure,calcium-silicon ratio?Ca/Si?and phase composition was investigated to analyze the influence of mineral admixtures on leaching characteristics of cement paste and its improvement mechanism on leaching resistance.Results show that,the addition of mineral admixtures with appropriate content in cement paste can improve its microstructure and phase composition,slow down its microstructural deterioration and leaching process,and effectively improve its leaching resistance in deionized water.?2?The ternary cement paste specimens with fly ash and slag were used as the research object,and the leaching experiments of ternary cement paste specimens in deionized water were carried out.By using EDTA titration,saturation-drying weighing,SEM/EDS and XRD methods,the change of dissolved calcium ion concentration,porosity,microstructure,Ca/Si and phase composition of blended cement paste specimens was investigated to analyze the influence of partial replacement of fly ash with slag on leaching characteristics of binary cement paste specimens and its improvement mechanism on leaching resistance.Results show that,The partial replacement of fly ash with slag in the binary cement paste specimen can improve its microstructure,and slow down its microstructural deterioration and leaching process,and improve the leaching resistance of blended cement paste in deionized water.?3?The deionized water and 1M NH₄ Cl,1M NH₄NO₃ and 1.5g/100 mL Acetic Acid solutions were used as corrosion mediums to carry out conventional and accelerated leaching experiments on the cement paste slice specimens,respectively.By using EDTA titration,saturation-drying weighing,SEM/EDS and XRD methods,the dissolved calcium ion concentration,porosity,microstructure morphology,Ca/Si and phase composition of the specimens immersed in four corrosion solutions have been studied to analyze the corrosion mechanism of cement paste in conventional and accelerated leaching experiments.Results show that,compared to the leaching behavior of cement paste slice specimens in deionized water,the leaching rate,characterized by the increasing porosity and the decreasing Ca/Si ratio,of cement paste slice specimens immersed in 1M NH₄Cl,1M NH₄NO₃ and 1.5g/100 mL Acetic Acid solutions has a significant increase.Thus,1M NH₄Cl,1M NH₄NO₃ and 1.5g/100 mL Acetic Acid solutions can greatly accelerate the leaching process of blended cement paste without any alteration to its phase microstructure characteristics and composition in the process of calcium leaching,and it can obtain the same leaching characteristics of blended cement paste in deionized water at a short period of time.This indicates that NH₄Cl,NH₄NO₃ and Acetic Acid solutions can be used as corrosion mediums for accelerating leaching experiment of cement paste.