PERFORMANCE PROPERTIES OF SALTSTONE PRODUCED USING SWPF SIMULANTS
The overwhelming majority of waste to be immobilized at the Saltstone Production Facility will come from the waste stream exiting the Salt Waste Processing Facility (SWPF). These SWPF batches are salt solutions that result from pretreatment of the High Level Waste (HLW) supernate by an Actinide Removal Process followed by Caustic Side Solvent Extraction. The concentration of aluminate within these streams will vary and be determined by (1) the concentration in the incoming salt waste stream, (2) the degree of aluminum leaching from the HLW, (3) the method for introducing the aluminate into the waste stream (continuous or batch) and (4) and any operational or regulatory limitations. The overall Performance Assessment outcome for the Saltstone Disposal Facility will depend significantly on the performance properties of the SWPF Saltstone grouts. This report identifies and quantifies, when possible, those factors that drive the performance properties of the projected SWPF grouts. Previous work has identified aluminate concentration in the salt waste stream as a key factor in determining performance. Consequently, significant variation in the aluminate concentration to a maximum level of 0.65 M was investigated in this report. The SWPF baseline grout is a mix with a 0.60 water to cementitious ratio and a premix composition of 45 wt % slag, 45 wt % fly ash and 10 wt % portland cement. The key factors that drive performance of the SWPF mixes were determined to be (1) the time/temperature profile for curing, (2) water to cementitious materials ratio, (3) aluminate concentration in the waste stream, and (4) wt % slag in the premix. An increase in the curing temperature for mixes with 45 wt % slag resulted in a 2.5 times decrease in Young's modulus. The reduction of Young's modulus measured at 60 C versus 22 C was mitigated by an increase in the aluminate concentration but was still significant. For mixes containing 60 wt % slag, the reduction in Young's modulus between these two curing temperatures was significantly lessened. The importance of curing conditions can not be overemphasized. The gain realized in performance by, e.g., a higher level of aluminate or wt % slag or a reduction in w/cm ratio, can be offset by the effects of a higher curing temperature. In fact, the final performance properties of a mix cured at 60 C can be lower than the initial values before any of the performance enhancing changes are introduced. Control of the time/temperature curing profile can be managed by pour schedules and other temperature control measures. The reduction in performance at higher curing temperatures is consistent with results obtained in a separate study. Although preliminary, results from this task on the measurement of hydraulic conductivity at MACTEC showed that curing of a Saltstone mix at 60 C increased the hydraulic conductivity by several orders of magnitude. The permeability data are based on only one mix but, were consistent with a measured reduction in Young's modulus for these same samples. Therefore, it is recommended that impact of curing temperature on performance properties be further investigated. An increase in dynamic Young's modulus (indicator of performance) is observed as the water to cementitious materials (w/cm) ratio decreases. The w/cm ratio is a process parameter which can be adjusted to improve performance as long as the processing properties of the grout are still within an operational window that will lead to successful placement. The same conclusions apply to wt % slag in the premix. That is, an increase in the wt % slag at the expense of fly ash in the premix increases Young's modulus and performance. An increase in wt % slag (as with a decrease in w/cm ratio) increases viscosity and yield stress and a final mix design must be balanced such that acceptable processing properties are obtained. The performance properties of SWPF mixes show a non-linear dependence on aluminate concentration. As the aluminate concentration is increased from 0.1 M to 0.25 M, the Young's modulus and compressive strength increase and the porosity is reduced. Consistent with this improvement in performance is an increase in the heat of hydration, reflective of an increase in the degree of hydration. Higher degrees of hydration in mixes generally lead to better performance. In the region of 0.25 to 0.45 M aluminate, very little additional change is observed in any of the properties. A further increase in the aluminate concentration from 0.45 M to 0.65 M leads to a slight reduction in Young's modulus. It was observed however, that the values of Young's modulus for samples cured at 22 C decreased after approximately 50 days. This was the first time a reduction in Young's modulus was observed as a function of time and is most likely correlated with the high levels of aluminate present in these mixes. This reduction can be accounted for by cracking of the Saltstone samples with time.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 973190
- Report Number(s):
- SRNL-STI-2009-00810; TRN: US1001905
- Country of Publication:
- United States
- Language:
- English
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