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 Changes in Diffusion Properties of Cement Paste Induced by Leaching and Carbonation Accounting for the Effects of Limestone Fillers and Water/Powder Ratio
Tác giả hoặc Nhóm tác giả: Quoc Tri Phung, Norbert Maes, Elke Jacops, Diederik Jacques, Geert De Schutter
Nơi đăng: 4th International Workshop on Mechanisms and Modelling of Waste/Cement Interactions, Murten, Switzerland; Số: -;Từ->đến trang: -;Năm: 2016
Lĩnh vực: Chưa xác định; Loại: Báo cáo; Thể loại: Quốc tế
TÓM TẮT
To assess the long-term performance of concrete used as engineered barrier for disposal of radioactive waste, the knowledge of the diffusive transport characteristics is important because diffusion plays a key role in most of the chemical degradation processes (governs the transport of reactive species responsible for degradation). Alteration of the microstructure, due to degradation, induces changes in diffusivity. This work quantitatively studies to what extent the diffusivity of cement pastes changes as a result of carbonation and Ca-leaching. The experiments were performed on cement paste samples with different water/powder (w/p) and limestone filler (LS) replacement ratios. Ca leaching was performed using an ammonium nitrate solution of 6 mol/l to accelerate the process. Carbonation was performed by controlled "purging" of the samples with pressurized pure CO2. To investigate the changes in diffusive behavior, a through-diffusion method was applied in which non-reacting dissolved gases were used as diffusing species. To characterise the microstructural changes, different post-analysis techniques including SEM, MIP, and N2-adsorption were used. Higher w/p ratios and LS replacements increased the degree of carbonation and leaching, hence the degree of degradation. Changes in w/p ratios had a more significant effect. The accelerated leaching significantly altered the microstructure, while the alteration extent for carbonation was lower. But both, led to a significant, but opposite, modification in diffusivity depending on the degradation state. After 28-day carbonation, the diffusivity of carbonated zone decreased by a factor of 5. On the other hand, the modification for the leached zone was more pronounced, as the diffusivity increased by more than one order of magnitude.
ABSTRACT
To assess the long-term performance of concrete used as engineered barrier for disposal of radioactive waste, the knowledge of the diffusive transport characteristics is important because diffusion plays a key role in most of the chemical degradation processes (governs the transport of reactive species responsible for degradation). Alteration of the microstructure, due to degradation, induces changes in diffusivity. This work quantitatively studies to what extent the diffusivity of cement pastes changes as a result of carbonation and Ca-leaching. The experiments were performed on cement paste samples with different water/powder (w/p) and limestone filler (LS) replacement ratios. Ca leaching was performed using an ammonium nitrate solution of 6 mol/l to accelerate the process. Carbonation was performed by controlled "purging" of the samples with pressurized pure CO2. To investigate the changes in diffusive behavior, a through-diffusion method was applied in which non-reacting dissolved gases were used as diffusing species. To characterise the microstructural changes, different post-analysis techniques including SEM, MIP, and N2-adsorption were used. Higher w/p ratios and LS replacements increased the degree of carbonation and leaching, hence the degree of degradation. Changes in w/p ratios had a more significant effect. The accelerated leaching significantly altered the microstructure, while the alteration extent for carbonation was lower. But both, led to a significant, but opposite, modification in diffusivity depending on the degradation state. After 28-day carbonation, the diffusivity of carbonated zone decreased by a factor of 5. On the other hand, the modification for the leached zone was more pronounced, as the diffusivity increased by more than one order of magnitude.
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