報告書番号

T01007

タイトル（和文）

TRU廃棄物処分環境におけるセメント系材料の化学的変質評価（その1） -― カルシウムシリケート水和物の熱力学的溶解沈殿モデルの提案 ―-

タイトル（英文）

Chemical Alteration of Cement Materials in a Radioactive Waste Repository Environment (1)- Thermodynamic Modelling for the Dissolution and Precipitation of Calcium Silicate Hydrates -

概要 （図表や脚注は「報告書全文」に掲載しております）

TRU廃棄物処分を対象とした超長期の安全評価では、セメント系材料の化学的バリア（核種閉じ込め）性能が特に重要となる。本研究では、化学的バリア性能を決定づける因子であるセメント水和物の溶解現象の長期評価に必要な、主要鉱物であるカルシウムシリケート水和物（C-S-Hゲル）の溶解モデルを提案した。本C-S-Hゲル溶解モデルの特徴は、C-S-Hゲルの再沈殿現象を再現可能とするために、全てのCa/Si比にわたってCa(OH)2とSiO2の非理想的固溶体と仮定したことにある。C-S-Hゲルの溶解・沈殿実験、C-S-Hゲルとエトリンガイトの共存系の溶解実験、さらに普通ポルトランドセメント（OPC）の溶解変質実験の結果をほぼ再現することができたことから、本C-S-Hゲル溶解モデルは処分場におけるセメント水和物の長期的変質の評価精度向上に資するものと考えられる。

概要 （英文）

Cementitious material, a potential waste packaging and backfilling material for the disposal of radioactive waste, is expected to provide both physical and chemical containment. In particular the ability to provide high-pH conditions, which depends on the solubility of the constituent hydrated minerals in cement matrices, is a very important parameter when considering the release of radionuclides from radioactive wastes. For long-term safety assessment, it is necessary to develop a series of predictive calculation models. Several models have been proposed for the incongruent dissolution of calcium silicate hydrate (C-S-H) gel, which is the principal product of hydrated cement phases. In the repository, the re-precipitation of minerals would follow their dissolution when an equilibrated solution moves to the other regions and equilibrates with the constituent minerals there. However, previous models have not yet adequately quantitatively evaluated the re-precipitation or interaction of other minerals with the dissolution of C-S-H gel. In this study, therefore, a thermodynamic dissolution and precipitation model of C-S-H gel is proposed assuming a binary non-ideal solid solution of Ca(OH)2 and SiO2. Dissolution and precipitation experiments on C-S-H gel were performed. C-S-H precipitates were prepared using two techniques. One was based on hydrolysis in a mixture of Ca and Si solutions and produced homogeneous gels. With the other technique, precipitates were prepared by contacting Ca(OH)2 solution with C-S-H gel with low Ca/Si ratios (0.47, 0.65, 0.9). The results were well predicted by the proposed model.Dissolution experiments on C-S-H gel coexisting with ettringite were also carried out. At Ca/Si>1.2 in C-S-H gel, the equilibrated pH and Ca and Si concentrations are nearly the same as those for C-S-H gel alone. At lower Ca/Si, interaction between ettringite and C-S-H dissolution was observed more clearly. The measured pH and Ca and Si concentrations were reasonably well predicted using a calculation based on the proposed C-S-H dissolution/precipitation model, combined with the congruent dissolution of ettringite. The dissolution of Ordinary Portland Cement (OPC), which was assumed to consist of C-S-H gel, Ca(OH)2, ettringite and hydrotalcite, was also reasonably well predicted.

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