Integrated Approach to Modeling Long-Term Durability of Concrete Engineered Barriers in Llrw Disposal Facility

Abstract

This paper describes an integrated approach to developing a predictive computer model for long-term performance of concrete engineered barriers utilized in LLRW and ILRW disposal facilities. The model development concept consists of three major modeling schemes: hydration modeling of the binder phase, pore solution speciation, and transport modeling in the concrete barrier and service environment. Although still in its inception, the model development approach demonstrated that the chemical and physical properties of complex cementitious materials and their interactions with service environments can be described quantitatively.Applying the integrated model development approach to modeling alkali (Na and K) leaching from a concrete pad barrier in an above-grade tumulus disposal unit, it is predicted that, in a near-surface land disposal facility where water infiltration through the facility is normally minimal, the alkalis control the pore solution pH of the concrete barriers for much longer than most previous concrete barrier degradation studies assumed. The results also imply that a highly alkaline condition created by the alkali leaching will result in alteration of the soil mineralogy in the vicinity of the disposal facility.

Thermodynamic equilibrium calculations in cementitious systems

“…The modeled changes of the composition of the solids and liquid phase, of the enthalpy, of the porosity and the amount of pore solution during hydration have been found to agree well with the experimental data determined as a function of time (see e.g., Fig. 1; [9,10,[15][16][17][18]100]). The good agreement between experimental and modeled results indicates that the quasi equilibrium approach used in thermodynamic equilibrium calculations is valid for Portland cement systems.…”

“…With the exception of ettringite, the amount of each hydration product continues to slowly increase with time. At the same time the amount of pore solution and the capillary porosity decreases [10,16,18]. The total of the volume of the solid and liquid phase decreases during hydration (Fig.…”

“…Thermodynamic modeling can be used in cementitious systems to calculate the stable phase assemblages based on the solution composition [7][8][9][10], to model the influence of the composition of the stable phase assemblage [11][12][13][14], to model hydration [9,[15][16][17][18] or, often in combination with transport models, to calculate the interactions with the environment [19][20][21][22][23][24]. Since the early 1990s, thermodynamic modeling has been used to calculate the composition of hydrated Portland cements and to predict the composition of the solution in equilibria [25][26][27][28][29][30][31][32][33][34][35].…”

Thermodynamic modelling: state of knowledge and challenges

“…An experimental set of kinetic equations to describe the dissolution behaviour of the clinker phases based on Avrami's equation can be used to predict quantitatively the amount of hydrates formed during OPC hydration (Guillon et al, 2008;Lee et al, 1995;Lothenbach and Winnefeld, 2006;Lothenbach et al, 2008aLothenbach et al, , 2008b. These papers deal with the calculation of the composition of the solid and liquid phases as a function of time, relative humidity, water/cement ratio, cement composition and/or temperature.…”

Stabilization of in-tank residual wastes and external-tank soil contamination for the tank focus area, Hanford tank initiative: Applications to the AX Tank Farm

“…For the natural equivalent of a marl, the chemical effect of concrete on pore water chemistry is calculated to extend out about 0.2 meters in 12,500 years (Eikenberg and Lichtner, 1992), which suggests that if an unreacted septum of clay is to exist sandwiched in concrete, the barrier should be on the order of 60 cm thick. Clearly, the process needs to be quantified (e.g., Lee et al, 1995) for the conditions relevant to the planned structure.…”