Observation of Diffusion Behavior of Trace Elements in Hardened Cement Pastes by LA-ICP-MS
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Observation of Diffusion Behavior of Trace Elements in Hardened Cement Pastes by LA-ICP-MS Taiji Chida and Daisuke Sugiyama Central Research Institute of Electric Power Industry, 2-11-1, Iwado-Kita, Komae, Tokyo 201-8511, Japan Abstract Diffusion coefficients of iodine (I), cesium (Cs) and strontium (Sr) in solid cement blocks of ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) are measured by in-diffusion experiment. By using “laser ablation inductively coupled plasma mass spectrometry”, the penetration profiles of trace elements by diffusion were obtained quantitatively in cement solids. The apparent diffusion coefficients of the near-surface of the cement samples were estimated to be about 10-13 m2 s-1for I, Cs and Sr. Lower diffusion coefficients were observed for FAC than those obtained for OPC. Introduction For the performance assessment of radioactive waste disposal, the diffusion of radionuclides in cementitious materials, which are used as an engineered barrier [1], is an important factor. However, there is much to clarify in the diffusion of radionuclides because it is difficult to observe trace-element distribution in solid materials in detail. In particular, because elements like iodine (I) hardly sorb on cementitious materials, it is important to clarify the diffusion of these elements in an engineered barrier. Recently, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used to analyze trace elements in rocks, metals, and other materials [2-4]. The notable feature of LA-ICP-MS is to determine trace elements in solid quantitatively without special preparation. In this study, the diffusion behavior of cesium (Cs), strontium (Sr) and I in hardened cement pastes was examined by an in-diffusion technique. The detailed penetration profiles of trace elements in hardened cement pastes were successively observed by LA-ICP-MS, and diffusion coefficients were estimated from the penetration profiles.
Experimental Materials Solid block samples of ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) were prepared for the diffusion experiments. The chemical compositions of OPC and FAC used in this study are shown in Table 1. The cements were mixed with deionized water in water/cement clinkers (w/c) of different mixing ratios of 0.35, 0.70 and 1.0, in order to give different diffusivities in solid. To prevent bleeding (separation of water at the top of the paste) at high w/c ratios of 0.7 and 1.0, the cement pastes were repeatedly mixed before setting. The cement pastes were allowed to set for 24 h at 30oC, and cured in water for 91 days at 50oC. The porosity of the hardened cement pastes was measured by mercury intrusion porosimetry, and these data are shown in Table 2. Hardened OPC and FAC were cut in the form of 20 × 20 × 20 mm using a diamond cutter, and were covered with epoxy resin except on one surface, for the block samples in the in-diffusion experiments. Cement-equilibrated solutions were prepared by p
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