Modeling of pH Elevation Due to the Reaction of Saline Groundwater with Hydrated Ordinary Portland Cement Phases
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Modeling of pH Elevation Due to the Reaction of Saline Groundwater with Hydrated Ordinary Portland Cement Phases A.Honda1 , K.Masuda1, H.Nakanishi1, H.Fujita2, and K.Negishi2 1 Japan Atomic Energy Agency, 4-33, Muramatsu, Tokai-mura, Naka-gun, Ibaraki, 319-1194, Japan 2 Taiheiyo Consultant Co., Ltd., 2-4-2, Osaku, Sakura-shi, Chiba, 285-8655, Japan ABSTRACT Thermodynamic calculations of the reaction between hydrated OPC phases and saline groundwater indicate an elevated pH > 13, which is not associated with the well known initial release of the alkalis. Instead, the pH elevation is attributed to the generation of OH– accompanying the precipitation of Friedel’s salt (Ca3Al2O6·CaCl2·10H2O; AFm-Cl2) from the reaction of portlandite (Ca(OH)2; CH) and hydrogarnet (Ca3Al2O6·6H2O; C3AH6) with chloride ions from the saline groundwater. If such a reaction mechanism were to occur in the context of the geological disposal of radioactive wastes, the impact of a hyper-alkaline plume on other barrier components, such as the host rock or bentonite buffer, could be significant. Experimental investigations were therefore conducted using only CH and C3AH6 to represent hydrated OPC and NaCl solution to represent a saline groundwater. The pH elevation was confirmed and showed good agreement with thermodynamic calculations. The experiments were repeated using hardened OPC paste to confirm this reaction mechanism in the presence of other hydrated OPC phases. In this case, however, the pH elevation was not as high as expected. This deviation can be explained by the residual aluminum, after being partially consumed by ettringite (Ca6Al2(OH)12(SO4)3·32H2O; AFt), monosulfate (Ca4Al2O6SO4·12H2O) and/or monocarbonate (Ca4Al2O6CO3·11H2O; AFm-CO3) phases, not being wholly assigned to C3AH6. A better agreement between the thermodynamic calculations and the experimentally measured results can be made assuming a fraction of aluminum is incorporated into the calcium silicate hydrate (C-SH) gel phase. INTRODUCTION Current proposals for the geological disposal of TRU waste in Japan involve the construction of a purpose-built repository based on the combined performance of natural and engineered barriers [1]. Using cementitious materials in the engineered barrier, however, will almost certainly produce a hyper-alkaline plume as they are subjected to leaching by invading groundwaters. A hyper-alkaline plume will have detrimental effects on the performance of the host rock and bentonite backfill, which are known to be thermodynamically unstable at high pH [2]. The pH and composition of hydrated OPC during leaching in pure water can be subdivided into three regions (Region I, Region II and Region III) [3]. Region I is characterized by a high pH (>13), which is attributed to the release of the completely soluble alkalis (Na and K). Region II has a pH ≈ 12.5, which is controlled by equilibrium with CH. Following the loss of CH, the pH of Region III is largely controlled by the incongruent dissolution of C-S-H gel.
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