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Abstract. The Cementitious Barriers Partnership (CBP) is focused on reducing uncertainties in current methodologies for assessing cementitious barrier performance and increasing the consistency and transparency in the assessment process. One important set of US Department of Energy challenges is assessing the integrity and closure of the high-level waste (HLW) tanks that currently store millions of gallons of highly radioactive wastes. Many of these tanks are decades past their design lives, have leaked or been overfilled, and must be emptied and closed to satisfy regulatory agreements. Carbonation-induced corrosion has been identified as a primary degradation and possible failure mechanism for the HLW tanks prior to closure. After closure the impact of carbonation (and concurrent oxidation) may be to increase the release and short-range transport of contaminants of concern. HLW tanks may be significantly empty for many years (and possibly decades) prior to closure; the performance of the closed tank over centuries, if not millennia, must be assessed to evaluate the potential release of residual radionuclides to the environment.CBP is developing models to evaluate a representative HLW tank closure scenario including the potential impacts of carbonation on waste tanks prior to and post closure. CBP modeling tools, including LeachXS™/ORCHESTRA, are being used to simulate waste tank carbonation, major constituent leaching, and contaminant releases to evaluate the source term and near-field conditions. Simulations presented here include sensitivity analysis for uncracked concrete to varying input parameters including composition, effective diffusivities, and thermodynamic parameters.

While the influence of paste properties on concrete performance has been extensively studied and in many cases reduced to quantitative relationships (e.g., Abram's law), that between aggregate characteristics and concrete performance has not been investigated in detail. Based on previous research that demonstrated significant strength differences for two similar concrete mixtures, one prepared with limestone aggregates and the other with siliceous gravel, a joint study between the National Institute of Standards and Technology (NIST) and the Federal Highway Administration (FHWA) was initiated to explore in detail the influence of aggregate source, mineralogy, and material properties on concrete performance. Eleven aggregates of differing mineralogy were identified and obtained both for bulk characterization and for incorporation into two concrete mixtures. The first concrete mixture was based on a 100 % ordinary Type I/II portland cement (OPC), while the second consisted of a ternary 60:30:10 volumetric blend of this cement with 30 % of a Class C fly ash and 10 % of a fine limestone powder. This latter sustainable mixture had exhibited exemplary performance in a previous study. Aggregates were characterized with respect to mechanical and thermomechanical properties, geometrical characteristics, and surface energies. For the prepared concretes, mechanical, thermomechanical, and electrical properties were measured at different ages out to 91 d and microstructural examinations were conducted to examine the interfaces between aggregates and cement paste. Concrete performance varied widely amongst the different aggregates, with the (range/average) ratio for 28-d compressive strength being 0.32 for the OPC concretes and 0.37 for those based on the ternary blend binder. With the exceptions of relating concrete modulus to aggregate modulus and concrete coefficient of thermal expansion (CTE) to aggregate CTE, weak correlations were generally obtained between a single aggregate characteristic and concrete performance properties. Models to predict 28-d compressive strength based on the aggregates' CTE (and aggregate absorption in the case of the ternary blend mixtures) provided predictions with a relative standard error (standard error/mean) of about 7 %. It is suggested that aggregate and binder characteristics control the bond between aggregates and paste. Then, for most properties, concrete performance is primarily controlled by the level of this bonding, a characteristic that was only assessed in an indirect manner in the present study. Research using non-linear ultrasonic measurements to better assess this bonding in specimens remaining from the present study is currently underway.

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