MEASUREMENT OF SPECIFIC HEAT CAPACITY OF SALTSTONE
One of the goals of the Saltstone variability study is to identify (and quantify the impact of) the operational and compositional variables that control or influence the important processing and performance properties of Saltstone grout mixtures. The heat capacity of the Saltstone waste form is one of the important properties of Saltstone mixes that was last measured at SRNL in 1997. It is therefore important to develop a core competency for rapid and accurate analysis of the specific heat capacity of the Saltstone mixes in order to quantify the impact of compositional and operational variations on this property as part of the variability study. The heat capacity, coupled with the heat of hydration data obtained from isothermal calorimetry for a given Saltstone mix, can be used to predict the maximum temperature increase in the cells within the vaults of the Saltstone Disposal Facility (SDF). The temperature increase controls the processing rate and the pour schedule. The maximum temperature is also important to the performance properties of the Saltstone. For example, in mass pours of concrete or grout of which Saltstone is an example, the maximum temperature increase and the maximum temperature difference (between the surface and the hottest location) are controlled to ensure durability of the product and prevent or limit the cracking caused by the thermal gradients produced during curing. This report details the development and implementation of a method for the measurement of the heat capacities of Saltstone mixes as well as the heat capacities of the cementitious materials of the premix and the simulated salt solutions used to batch the mixes. The developed method utilizes the TAM Air isothermal calorimeter and takes advantage of the sophisticated heat flow measurement capabilities of the instrument. Standards and reference materials were identified and used to validate the procedure and ensure accuracy of testing. Heat capacities of Saltstone mixes were {approx} 55% higher than the previous measurement of specific heat capacity on a reference Saltstone mix in 1997. Values of mixes prepared using Deliquification, Dissolution and Adjustment (DDA), Modular Caustic Side Solvent Extraction Unit (MCU) and Salt Waste Processing Facility (SWPF) simulants and premix at 0.60 w/cm ratio were {approx} 1.95 J/g/{sup o}C and were equivalent within experimental error. The simple law of mixtures was used to predict the heat capacities of the Saltstone and the results were in excellent agreement with experimental data. This simple law of mixtures can therefore be used to predict the heat capacities of Saltstone mixes in those cases where measurements have not been made. The time dependence of the heat capacity is important as an input to the modeling of temperature increase in Saltstone vaults. The heat capacity of a mix of MCU and premix at 0.60 w/cm ratio was measured immediately after initial mixing and then periodically up to times greater than 100 days. Within experimental error, the heat capacity did not change with time. Therefore, the modeling is not complicated by requiring a time dependent function for specific heat capacity. The water to cementitious material (w/cm) ratio plays a key role in determining the value of the heat capacity. Both experimental and predictive values for SWPF mixes as function of the w/cm ratio were obtained and presented in this report. Predictions of the maximum temperatures of the Saltstone mixes were made using the heat of hydration data from previous isothermal measurements and the newly measured heat capacities for DDA, MCU and SWPF mixes. The maximum temperature increase ranged from 37 to 48 C for these mixes. The presence of aluminate at 0.33 M produced a temperature increase of 68 C which is close to the adiabatic temperature rise of 74 C observed by Steimke and Fowler in 1997 for a mix containing 0.35 M aluminate. Aluminum dissolution of the sludge will increase the aluminate in the DSS which in turn will result in a larger temperature increase in the Saltstone vaults during the curing period. The initial temperature of the Saltstone mixes in the Saltstone Production Facility (SPF) can be predicted using the specific heat capacities of Decontaminated Salt Solutions (DSS), the specific heat capacities of cementitious materials, and the initial temperatures of the DSS and the premix. This initial mix temperature is dominated by the DSS temperature due to its high heat capacity relative to the cementitious materials. Therefore, controlling the initial temperature of the DSS to lower temperatures will lead to lower maximum temperatures in the vault for a given batch of Saltstone.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 945000
- Report Number(s):
- SRNS-STI-2008-00102; TRN: US0900806
- Country of Publication:
- United States
- Language:
- English
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