The Effect of Ettringite on Cementitious Waste Form Development for Low- Activity Waste - 17170

The current Cast Stone formulation, with 8% ordinary Portland cement (OPC), 45% fly ash (FA), and 47% blast furnace slag (BFS), proposed for cementitious waste forms (CWFs) at the Hanford site has been shown to be inadequate for secondary liquid wastes sent to the Effluent Treatment Facility (ETF). This is due to the high sulfate concentrations and non-alkaline conditions present in the ETF waste stream that lead to the late formation of ettringite, [Ca₆Al₂(SO₄)₃(OH)₁₂.26(H₂O)], which can consequently cause undesirable swelling and cracks in the CWF that facilitate ⁹⁹Tc release. To overcome this challenge, hydrated lime (HL), Ca(OH)₂, can be used instead of FA to provide the Ca necessary to sequester sulfate early in the cast stone curing process through early formation of ettringite. Early ettringite formation minimizes the risk of localized disruptive action or harmful stress that lead to undesirable swelling and cracking of the CWF. Furthermore, early formation reduces the permeability of the CWF, further reducing the risk of ⁹⁹Tc release and increasing structural stability. Two liquid waste simulants, 1) WTP off-gas condensate (WTP) and 2) Effluent Management Facility (EMF) process condensate combined with low activity waste (LAW) caustic scrubber (labeled EMF/CS), were prepared and tested for ettringite formation effects on HL-containing CWF development and ⁹⁹Tc leachability. After curing for 28 days, X-ray diffraction analysis revealed that ettringite was formed in the early stages of curing for the CWFs prepared using both simulants. However, because of the higher sulfate concentration in the EMF/CS simulant and limited Ca and Al sources, some sulfate did not form ettringite during the early stages of curing, resulting in significant monolith swelling and cracking during subsequent leaching tests. Despite observable cracking in monoliths prepared with the EMF/CS simulant, low ⁹⁹Tc release from HL-based waste forms was measured in later leaching periods tested by the EPA Method 1315 in both simulants. Ettringite-⁹⁹Tc co-precipitation batch experiments support a possible chemical sequestration mechanism that is hypothesized to contribute to the observed low ⁹⁹Tc leach rates, which are likely the result of an additional chemical reaction that sequesters ⁹⁹Tc rather than simply physical factors such as reduced porosity and permeability (or tortuosity). Overall these findings quantitatively support the use of HL to overcome the deleterious effects of sulfate-rich conditions on CWFs through early formation of ettringite and provide evidence for ⁹⁹Tc release rates that fall within low risk levels reported by the United States Department of Energy (US DOE). (authors)