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 Effects of Accelerated Ca-leaching on Microstructure and Permeability of Hardened Cement Paste
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Tác giả hoặc Nhóm tác giả: Q.T. Phung, N. Maes, D. Jacques, G. De Schutter, G. Ye
Nơi đăng: Belgian Nuclear Research Centre (SCK•CEN), Belgium; Số: SCK•CEN-BA-56;Từ->đến trang: 48;Năm: 2013
Lĩnh vực: Kỹ thuật; Loại: Báo cáo; Thể loại: Quốc tế
TÓM TẮT
Ca-leaching in cement-based materials is an extremely slow process. Even though leaching is considered as a performance test for long-term durability assessment, especially for concretes used in waste disposal systems. Therefore, accelerated methods are needed to reach a certain degradation stage to study the behaviour of concrete after hundreds of years of placing. Leaching reduces the pH and mechanical properties while it increases the porosity, corrosion, cracking and transport properties of cementitious materials. In the present work, an ammonium nitrate solution of 6 mol/l was used to accelerate the leaching degradation kinetics and examine the changes in microstructure and permeability of leached materials which help to better predict the long-term durability of cement-based materials subjected to chemical degradation. Cement paste samples with different water/powder ratios were immersed into NH4NO3 solution. The change of sample mass over time was monitored by weighing, whereas the amount of calcium ion leached out was monitored by ion chrotomagraphy. A variety of post-analysis techniques like SEM, XRD, MIP and BET were used to characterize the microstructural changes, while the degraded front was determined by phenolphthalein spraying. The effect of accelerated leaching on transport behaviour was studied by measuring changes in the water permeability. Results show that NH4NO3 solution is a reasonable aggressive agent which can be used to accelerate the leaching kinetics for long-term durability studies while still keeping the “nature” of leaching. The square-root-time law of degradation is still applicable in accelerated condition. Both the BET specific surface area and the porosity of the leached samples significantly increase and the “critical” pore size is shifted to larger range. Those changes lead to an increase in water permeability by one order of magnitude.
ABSTRACT
Ca-leaching in cement-based materials is an extremely slow process. Even though leaching is considered as a performance test for long-term durability assessment, especially for concretes used in waste disposal systems. Therefore, accelerated methods are needed to reach a certain degradation stage to study the behaviour of concrete after hundreds of years of placing. Leaching reduces the pH and mechanical properties while it increases the porosity, corrosion, cracking and transport properties of cementitious materials. In the present work, an ammonium nitrate solution of 6 mol/l was used to accelerate the leaching degradation kinetics and examine the changes in microstructure and permeability of leached materials which help to better predict the long-term durability of cement-based materials subjected to chemical degradation. Cement paste samples with different water/powder ratios were immersed into NH4NO3 solution. The change of sample mass over time was monitored by weighing, whereas the amount of calcium ion leached out was monitored by ion chrotomagraphy. A variety of post-analysis techniques like SEM, XRD, MIP and BET were used to characterize the microstructural changes, while the degraded front was determined by phenolphthalein spraying. The effect of accelerated leaching on transport behaviour was studied by measuring changes in the water permeability. Results show that NH4NO3 solution is a reasonable aggressive agent which can be used to accelerate the leaching kinetics for long-term durability studies while still keeping the “nature” of leaching. The square-root-time law of degradation is still applicable in accelerated condition. Both the BET specific surface area and the porosity of the leached samples significantly increase and the “critical” pore size is shifted to larger range. Those changes lead to an increase in water permeability by one order of magnitude.
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