Seepage/Cement Interactions

The Development Plan (CRWMS M andO 1999a) pertaining to this task defines the work scopes and objectives for development of various submodels for the Physical and Chemical Environment Abstraction Model for TSPA-LA. The Development Plan (CRWMS M andO 1999a) for this specific task establishes that an evaluation be performed of the chemical reactions between seepage that has entered the drift and concrete which might be used in the repository emplacement drifts. The Development Plan (CRWMS M andO 1999a) then states that the potential effects of these water/grout reactions on chemical conditions in the drift be assessed factoring in the influence of carbonation and the relatively small amount of grout. This task is also directed at: (1) developing a conceptualization of important cement/seepage interactions and potential impacts on EBS performance, (2) performing a screening analysis to assess the importance of cement/seepage interactions. As the work progresses and evolves on other studies, specifically the Engineered Barrier System: Physical and Chemical Environment (P andCE) Model (in progress), many of the issues associated with items 1 and 2, above, will be assessed. Such issues include: (1) Describing the mineralogy of the specified cementitious grout and its evolution over time. (2) Describing the composition of the water before contacting the grout. (3) Developing reasonable upper-bound estimates for the composition of water contacting grout, emphasizing pH and concentrations for anions such as sulfate. (4) Evaluating the equilibration of cement-influenced water with backfill and gas-phase CO2. (5) Developing reasonable-bound estimates for flow rate of affected water into the drift. The concept of estimating an ''upper-bound'' range for reaction between the grout and the seepage, particularly in terms of pH is based on equilibrium being established between the seepage and the grout. For example, this analysis can be based on equilibrium being established as long as any given reactant is present, for example, calcium hydroxide. Such an analysis does not account for any channelized flow within the grout, along, for example, carbonated fracture surfaces. In the event of channelized fracture flow, residual calcium hydroxide might still exist within the grout matrix but would be prevented from direct contact with the seepage due to armoring by a reaction salvage of calcite. Calcite can be included as a reactant, to accommodate limited carbonation, but this calcite is presumed to not impede dissolution of more soluble cementitious components. Another premise that can be used is that the seepage would not interact with the in drift atmosphere until after reaction with the grout, at which time a decrease in pH would be measured and precipitation of certain phases would occur. The specific purpose of this analysis is to predict the time required for carbonation of the calcium hydroxide present within the grout used to anchor rockbolts. This analysis estimates the time required for carbonation to take place at three different partial pressures of carbon dioxide: 1 ppmv, 100 ppmv, and 1,000 ppmv, expressed as parts per million on a volume basis (the CO2 partial pressure in an ideal gas mixture is equal to the total pressure multiplied by the volume fraction of CO2). The basis for the range of values used in this analysis stems from the ambient carbon dioxide concentration of 1,000 ppmv in the unsaturated zone being used in another study (Engineered Barrier System: Physical and Chemical Environment (P andCE) Model) (in progress) (CRWMS M andO 2000d, p. 1 of 1). This value was used to establish an upper bound. It is recognized that boiling would cause a decrease in the air-mass fraction and thereby a decrease in the concentration of carbon dioxide. Essentially this analysis assesses the rates of carbonation predicted at air-mass fractions of 0.001, 0.1 and 1.0