Modeling of pH Elevation Due to the Reaction of Saline Groundwater with Hydrated Ordinary Portland Cement Phases

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-S-H) gel phase.