Silica Fume Slurry Lagoon Closure: Thixotropic Material Characterization and Solidification

This paper reports on the evaluation and stabilization/solidification (S/S) of a thixotropic, silica-rich slurry sediment (SS) media from a 14.2-ha (35-acre) unlined lagoon that exhibited flash setting behavior when contacted with low doses (≤5 wt%) of portland cement (PC), transforming it into sticky, tacky peanut butter-like solid in less than 20 s. This presented a significant challenge for material handling. Meanwhile, the high initial unconfined compressive strength (UCS) at 3 and 7 days decreased by 50% by 28 days, indicating metastability. Numerous mix designs that increased calcium content were explored to mitigate the flash setting behavior and strength loss. Higher PC-only doses could not be effectively homogenized in the lab. Mix designs with hydraulic lime, agricultural lime, or calcium chloride (road salt) brine solutions were mixable, and the latter two maintained high UCS. EPA Leaching Environmental Assessment Framework (LEAF) tests (Methods 1313, 1315, and 1316) were used to characterize leaching from the SS media, which contained 400–800 µg/L arsenic (As) in its natural liquefied state. Despite the high concentrations in the SS, dissolved As levels in two groundwater wells immediately downgradient of the lagoon were below the regional groundwater criterion for As of 10 µg/L. EPA 1313 testing on the untreated SS media revealed that As leaching was independent of pH, and EPA 1316 testing indicated that As was available-content controlled, making dilution effects fairly predictable. EPA 1315 (extended to 120 days) and 1316 testing on select intact and crushed S/S mix designs, respectively, showed that the long-term EPA 1315-As water bath concentrations and those of crushed (simulated reworked surface) ISS samples were ≤10 µg/L. It was thought that the leaching of calcium and silica would be lower from high-strength mix designs since calcium silica hydrate compounds are responsible for high strength. On the contrary, silica leaching (100–200 mg/L as SiO2) was regulated by amorphous silica and independent of UCS. Calcium concentrations ranged from 20 to 30 mg/L and were likewise similar for all mix designs.