Slag-Based Saltstone Formulations

Abstract

Approximately 400 × 106 liters of low-level alkaline salt solution will be treated at the Savannah River Plant (SRP) Defense Waste Processing Facility (DWPF) prior to disposal in concrete vaults at SRP. Treatment involves removal of Ca+and Sr+2 followed by solidification and stabilization of potential contaminants in saltstone, a hydrated ceramic waste form.Chromium, technetium, and nitrate releases from saltstone can be significantly reduced by substituting hydraulic blast furnace slag for portland cement in the formulation designs. Slag-based mixes are also compatible with Class F fly ash used in saltstone as a functional extender to control heat of hydration and reduce permeability. (Class F fly ash is locally available at SRP.)A monolithic waste form is produced by the hydration of the slag and fly ash. Soluble ion release (NO3−) is controlled by the saltstone microstructure (bulk porosity and pore size and connectivity). Chromium and technetium are 103−104 times less leachable from slag mixes than from cement-based waste forms. Results suggest that chemical stabilization rather than physical entrapment, as in the case of nitrate is responsible for this improved leaching. Reduction of Cr+6 and Tc+7 to Cr+3 and Tc+4 by ferrous iron or Mn+2 in the slag and subsequent precipitation of the relatively insoluble phses Cr(OH)3 and TcO2 are proposed as the stabilizing reactions.

Long-Term Technetium Interactions With Reducing Cementitious Materials

“…What we observe from Serne et al (1992) Langton (1988) reported low (slow) D e values using SRS saltstone (10 -12 cm 2 /s). In Serne et al (1992), the Tc(VII) was added to two cement formulations (T106-AN 47.5% slag and DSSF 47% slag).…”

“…Brodda (1988) and Tallent et al (1988) also did D e measurements of slag-cements and reported that nitrate (indicative of a conservative tracer or of how fast water moves) leached about 100 times faster than Tc. Langton (1988) , respectively. The NO 3 -D e values for the 0%, 15% and 26% slag concentrations were 10 -9 , 10 -7 and 10 -9 , respectively.…”

“…5. A field study conducted at SRS in the 1980s indicated that a slag-saltstone immobilized Tc for at least 2.5 years (Langton 1988). Below background concentrations of Tc leached out of the slag-containing saltstone, whereas Tc leached out of the slag-free saltstone at the rate of nitrate loss.…”

“…There were a few studies involving Tc and slag-cementitious materials (Serne et al (1992), Langton 1988, Kaplan and Coates 2007, Kaplan et al 2009Brodda 1988;Tallent et al 1988) (Table 12). Studies by Serne et al (1992), Langton (1988), Brodda (1988) and Tallent et al (1988) measured effective diffusion, D e , (or its negative logarithm, leachability index) from monoliths of reducing grout using a standardized method, ANSI/ANS-16.1.…”

“…Studies by Serne et al (1992), Langton (1988), Brodda (1988) and Tallent et al (1988) measured effective diffusion, D e , (or its negative logarithm, leachability index) from monoliths of reducing grout using a standardized method, ANSI/ANS-16.1. As such they did not measure K d values.…”